i hc nng trng i hc s phm

PGS.TS. l t hi

2009 Nng - 2009

Chng 1 dung dch cht in li v thuyt in ly arrhnius 1.1. Khi nim cht in li Cht in li l nhng hp cht ho hc c kh nng phn li (hon ton hay mt phn) trong dung dch thnh nhng ht mang in tri du nhau c gi l cc ion (cation v anion). 1.2. Nhng bng chng thc nghim v s tn ti ca cc ion trong dung dch cht in li Quan nim v s tn ti cc phn t tch in, cc ion, trong dung dch cc cht in li khng phi c khng nh ngay trong in ho, v giai on pht trin u tin ca mnh mn khoa hc in ho cha c quan nim ny. Bng chng v s tn ti ca cc ion trong dung dch c khng nh trn c s nhng d kin thc nghim v mt s thuc tnh nhit ng sau y ca dung dch in li. 1. p sut thm thu: T thuyt dung dch suy ra rng, trong cc dung dch long, p sut thm thu lin h vi nng mol/l theo phng trnh: = C.R.T (1.1)

y: C l nng mol/l; R l hng s kh; T l nhit tuyt i. i vi cc dung dch cht khng in li, v d nh dung dch ng trong nc, phng trnh (1.1) ph hp tt v cc s liu thc nghim. i vi cc dung dch in li, v d dung dch NaCl, gi tr thc nghim ca ln hn gi tr tnh theo (1.1). gii thch mt cch hnh thc hin tng ny, VantHoff a vo h s ng trng Vant-Hoff (i) v biu thc p sut thm thu trong cc dch dch in li l: = i.C.R.T (1.2)

y i > 1. T (1.1) v (1.2) suy ra > . iu ny dn ti ngha rng tng s cc phn t trong dung dch in li cao hn trong dung dch khng in li c cng nng mol. 2. p sut hi trn dung dch: Cht tan lm gim p sut hi ca dung mi trn dung dch so vi dung mi nguyn cht (P). Trong dung dch khng in li long th: P =P0 M 0 .C m 1000

(1.3)

4

vi P0: p sut hi trn dung mi nguyn cht; M0: phn t gam dung mi; P: gim p sut hi bo ho ca dung mi trn dung mi nguyn cht so vi dung mi trn dung dch; Cm: nng molan cht tan. i vi dung dch in li, cc gi tr thc nghim ca P ln hn gi tr tnh theo (1.3). Do vy, gi thit rng s tiu phn trong cc dung dch in li ln v a vo (1.3) h s i > 1 gii thch s mu thun ny. iu ny c ngha l trong cc dung dch in ly, cc phn t cht tan b phn ct cho s phn t trong dung dch tng ln so vi dung dch khng in ly. 3. h bng im v tng im si ca dung dch: Ngi ta thy rng cc dung dch in li c nhit si cao hn v nhit ng c thp hn cc dung dch khng in li c cng nng C. iu ny chng t s cc phn t trong dung dch in ly ln hn trong dung dch khng in ly c cng nng . 4. Hiu ng nhit ca phn ng trung ho: Hiu ng nhit ca phn trung ho axit mnh bng baz mnh l mt gi tr khng i, khng ph thuc vo bn cht ca axit v baz. HCl + NaOH NaCl + H2O HNO3 + KOH KNO3 + H2O H1 = H2 = -57,3 kJ/mol ( 250C) iu ny chng t rng phn ng trung ho trn l phn ng gia ion H+ v OHdo axit v baz phn li ra: H+ + OH- H2O (c) H1 H2 (a) (b)

5. S lin quan gia tc dng xc tc ca cc axit v dn in ca chng: nng cho, dn in ca axit cng ln, hiu ng xc tc ca n cng mnh v nh hng n qu trnh thu phn este. iu ny cho thy, trong dung dch cc axit phn li ra ion H+.

Grothus (1805) v Faraday (1833) cho rng, s phn li ra cc ion l do tc dng ca in trng. T khi nim cht in ly do Faraday ngh mang ngha l b phn ly bi in. Cc ion c sinh ra do s phn ly nh vy chy v cc in cc di tc dng ca in trng. Tuy nhin, cc th nghim v s tn ti ion trong dung dch, nh s lin quan gia tc dng xc tc ca axit v dn in cho thy phn ng thu phn ca este xy ra khi khng c mt ca in trng. Kayander th cp n s tn ti trong dung dch axit v kim nhng phn t b ct khng ph thuc vo s c mt ca dng in. Song Kyander cha a ra c l thuyt in ly gii thch s in ly ca cc cht in ly.

1.3.1. Cc gi thuyt trc Arrhenius

Arrhenius 1.3. Thuyt in li Arrhenius

1.3.2. Thuyt in li Arrhenius

5

Trong khong thi gian 1883-1887 Arrhenius a ra thuyt in li da trn c s nhng lun in c bn sau: 1. Cc phn t axit, baz, mui khi ho tan th b phn li thnh cc phn t mang in (ion). V d: HCl NaCl H+ + ClNa+ + ClNaOH CH3COOH Na+ + OHCH3COO + H+

Cc ion c th cu to bi cc nguyn t hay nhm nguyn t. Trong dung dch cc ion khng tng tc g vi nhau v c tc dng nh cc phn t kh l tng. 2. Trong dung dch, s phn li cc phn t thnh cc ion l khng hon ton, tc l khng phi tt c cc phn t cht in li, m ch mt phn no ca chng, gi l in li, phn li thnh cc ion; phn cc phn t cn li khng phn li bng (1-). =n' n

(1.4)

n: tng s phn t cht tan; n : s phn t phn li. Gi C l nng mol/l ca dung dch; 1 phn t cht in li phn li ra ion th tng s tiu phn trong h l: C + (1- ) C = C [1 + ( - 1)] (1.5)

Biu thc [1 + ( - 1)] cho thy nng phn t chung ca cc tiu phn trong dung dch tng ln bao nhiu ln do s in li gy ra v c ngha vt l tng ng vi h s ng trng i ca Vant-Hoff. Do : i = 1 + ( - 1) (1.6)

V > 1 v < 0, nn i > 1 v phng trnh (1.6) cho php gii thch hp l cc s liu thc nghim v p sut thm th, s thay i p sut hi ca dung mi trn dung dch; cng nh s h nhit ng c v s tng nhit si ca dung dch cht in li. 3. S phn li c coi nh mt phn ng ho hc v tun theo nh lut tc dng khi lng. Kho st cht in phn 1-1 : MA M + + AK [M+] = [A-] = C ; [MA] = (1-)C Ta c[ M +][ A] K = = [ MA]

2

C

1

(1.7)

6

(1.7) suy ra =

4C K ( 1+ - 1) 2C K K C

(1.8)

Nu nh (1- ~ 1) th : =

(1.9)

Nu thay C = 1/V : pha long, th (1.9) tr thnh: =K .V

(1.10)

Phng trnh (1.10) gi l nh lut pha long Ostwald.

1.3.3. ng dng ca thuyt in li Arrhenius

Thuyt in li ca Arrhenius gii thch tt mt s tnh cht ca dung dch in li nh p sut thm thu, p sut hi ca dung mi trn dung dch, h bng im, tng nhit si .. . Trn c s thuyt in li Arrhenius hnh thnh khi nim axit v baz u tin v gii quyt vn cn bng trong dung dch in li cng vi mt s tnh cht ca h dung dch in li. Theo thuyt Arrhenius th axit HA l hp cht phn li thnh cc ion hidro v gc axit: HA H+ + A v baz MOH l hp cht phn li cho cation kim loi v anion hidroxyl : MOH M+ + OH-

V vy phn ng trung ho lun lun dn n s tng tc ca cc ion H+ v OH. Do , hiu ng nhit ca phn ng gia axit mnh v baz mnh trong cc dung dch long c gi tr khng i v khng ph thuc vo bn cht ca axit v baz. Thuyt Arrhenius c s dng rng ri gii thch cc cn bng axit v bazo v s in li ca nc. S in li ca nc xy ra nh sau: H2 O Hng s in li ca nc: K = H+ + OH[ H + ][OH ] [ H 2 O]

(1.11)

V in li ca nc rt nh, do nng H2O c th c xem nh khng i. (1.11) K. [H2O] = [ H+] [ OH-] = Kw (1.12)

7

Kw c gi l tch s ion ca H2O. 250C Kw = 10-14. Do , [ H+][ OH-] = 10-14. Hng s Kw ph thuc rt ln vo nhit , khi nhit tng th Kw tng. t, 0C Kw.1014 20 0,681 30 1,470 60 9,610 70 15,800 80 25,10 90 38,000 100 55,000

Do vy, gi tr pH = 7 dung dch s c tnh axit nu t0 < 250C v c tnh kim nu t0 > 250C.

1.3.4. Nhng thiu st ca thuyt Arrheniussau:

Bn cnh nhng u im xt, thuyt Arrhenius c cc nhc im ch yu

1- Thuyt ny khng tnh n tng tc ca cc ion vi lng cc (dipol) nc hoc ca cc dung mi khc tc l tng tc ion - dipol. Tng tc ny l c s vt l to thnh cc ion trong dung dch khi ho tan cc cht in li. V khng tnh n tng tc ion - dipol nn khng gii thch c qu trnh to thnh ion v bn vng ca cc h ion. Thuyt arrhenius khng cp n nguyn nhn ca s in li. 2- Thuyt Arrhenius xem ion nh nhng phn t c lp, khng tnh n tng tc ion - ion do lc Coulomb gy ra. Vic b qua tng tc ion - ion hon ton khng th chp nhn c v phng din vt l. S b qua lm ph v cc h thc nh lng ca thuyt Arrhenius v do khng th gii thch c s thay i ca hng s phn li K theo nng cht in li. S thay i ca K xut hin r nht trong cc dung dch cht in li mnh, c in li biu kin gn bng 1. Nhng ngay c trong dung dch axit axetic yu, s ph thuc K vo nng CH3COOH vt ng k sai s ca php o. Gi tr hng s phn li K ca cc dung dch nc KCl v CH3COOH 250C : CKCl (mol/l) K(KCl) mol/l C(CH3COOH) mol/l K(CH3COOH) .105 mol/l 0,0001 0,0128 0,001 1,751 0,001 0,0456 0,02 1,740 0,01 0,1510 0,10 1,700 0,1 0,5349 0,20 1,653

3- Theo thuyt Arrhenius th l i lng c trng, c gi tr khng i d o bng phng php no v < 1. Tuy nhin, thc nghim cho thy i vi mt dung dch nht nh, nu o bng phng php khc nhau th s c gi tr khc nhau, thm ch > 1. Gi tr ca dung dch axit HCl theo phng php o dn (1) v sc in ng (2) nh sau:

8

C (mol/l ) 0,003 0,080 0,300 3,000 6,000 16,000

1 0,986 0,957 0,903 -

2 0,990 0,880 0,773 1,402 3,400 13,200

S khc nhau gia cc gi tr o c bng cc phng php khc nhau s tng khi nng dung dch tng. c bit, gi tr in li o c bng phng php sc in ng ln hn 1 khong nng cao. Do , trong trng hp ny khi nim in li theo l thuyt Arrhenius khng cn ngha vt l. Nh vy, l thuyt in li ca Arrhenius ch c p dng vi dung dch long ca cht in li yu. Cn i vi dung dch in li yu c nng cao v dung dch in li mnh bt c nng no th l thuyt ca Arrhenius khng th p dng c v mt nh lng. . I. Mendeleev cho rng, nhc im chnh ca thuyt in li Arrhenius l khng tnh n tng tc gia cc phn t cht tan vi nhau cng nh tc ng gia cc phn t cht tan vi cc phn t dung mi. ng cho rng i vi dung dch khng ch c qu trnh phn li, m cn c qu trnh to thnh hp cht mi vi s tham gia ca cc phn t dung mi. Quan im ny c xem l ng n v c nhiu nh Bc hc (.P.Cnvalp, J.A.Cablucp) pht trin trong cui th k 19 v u th k 20. . Cu hi v bi tp 1. Hy nu nhng bng chng thc nghim chng minh s tn ti ca ion trong dung dch cht in ly. 2. phn ly ca mt cht in ly yu trong dung dch nc c thay i khng v thay i nh th no khi tng nhit ? 3. Hng s phn li Kc ca mt cht in phn yu s thay i nh th no khi thay dung mi nc bng ru metylic? B qua tng tc ho hc gia cht tan v dung mi. Hng s in mi nc > hng s in mi ru metylic. 4. 25oC hng s phn li ca NH4OH bng 1,79.10-5. Hi nng no phn li ca NH4OH bng 2%. Tnh nng OH- trong dung dch ny. 5. Hng s phn li ca axit benzoic bng 6,3.10-5, ca axit axetic bng 1,79.10-5. Xc nh t s nng ion H+ trong hai dung dch axit trn c cng nng . 6. Hng s phn li nhit ng ca axit -clopicric 298K bng 1,47.10-3. Tnh phn li ca axit ny trong dung dch nng 0,01M. 7. Trnh by ni dung c bn ca thuyt Arrhenius, u v nhc im ca thuyt .

9

Chng 2 Tng tc ion - lng cc dung mi trong cc dung dch in ly 2.1. Nguyn nhn ca s in li v tng tc ion - lng cc dung mi Thuyt in li Arrhenius c ng l s to thnh cc ion trong dung dch ch xy ra khi cc phn t trung ho ca cht tan b phn hu. Song thc t cc ion tn ti ngay c trc khi ho tan. Ngi ta chia cht in phn ra lm hai loi: - Cht in phn tht l cht in phn trng thi phn t tn ti lin kt ion. V d: NaCl, KCl .. . - Cht in phn tim nng l cht in phn trng thi phn t cha tn ti ion. V d: HCl ... i vi cht in phn tht th qu trnh in li bao gm: - Ph hu mng li tinh th do tng tc ca cc ion trong mng li tinh th vi cc dipol (lng cc) ca dung mi. - Qu trnh solvat ho ion to thnh bi cc phn t dung mi. i vi cht in phn tim nng th qu trnh in li bao gm: - Tng tc ho hc gia phn t vi dung mi b gy lin kt phn t to ion. - Qu trnh solvat ho ion. V d: khi ho tan kh HCl vo nc to thnh cht in li mnh, axit clohidric. Song s tht y khng phi l s phn li phn t HCl m l s tng tc ho hc ca chng vi cc phn t nc. HCl + H2O H3O+ + Cl y proton chuyn t phn t HCl n phn t H2O to thnh ion hydroxoni. Ngoi ra, cc ion H3O+ v Cl- c to thnh tham gia vo tng tc ion - dipol vi cc phn t H2O d. V vy, khi to thnh dung dch axit clohidric, nng lng cn thit b gy lin kt ho hc H - Cl (khong 432kJ/mol), c b tr bi nng lng lin kt ca proton (H+) vi phn t nc trong ion H3O+ v nng lng hydrat ho ca cc ion H3O+, Cl-. y l hai c ch c bn to thnh cc dung dch in ly. 2.2. Nng lng mng li tinh th chng minh qu trnh hnh thnh dung dch cht in ly theo c ch trn i vi cht in phn tht, ta cn so snh nng lng cn thit ph v mng li tinh th to ra ion vi nng lng solvat ha cc ion bi cc phn t dung mi.10

Nng lng mng li tinh th l cng cn tiu tn ph v mng li. Nng lng mng li tinh th c tnh cho 1 mol cht nghin cu. tnh nng lng ny, ngi ta dng m hnh ca M. Born. Xt hp cht ion c to bi 2 ion tch in tri du c in tch tuyt i l Z1e v Z2e, v nm cch nhau mt khong r. Lc tng tc tnh in ca hai ion ny c xc nh theo nh lut Coulomb l:Z1Z 2 eo 2 Fc = 4 o r 2

(2.1)

y o l thm in mi trong mi trng chn khng. th biu din nng lng tng tc ca hai ion trnh by hnh 2.1.Fd U FC U

ro

r

Hnh 2.1: th biu din nng lng tng tc gia hai ion C th gi thit rng ion th nht c gn v tr r = 0 v lc Fc tc dng ln ion th hai theo hng ca ion th nht (hnh 2.1). Do hng ca lc ny trng vi hng ca trc r cho nn Fc < 0. ng thi lc y F tc dng ln cc ion, lc ny do tng tc ca cc lp v electron quyt nh v khng cho php cc ion tin li gn nhau mt khong cch bt k c. Gi thit rng lc F cng t ln ion th hai (hnh 2.1). Do hng ca lc ny trng vi hng ca trc r cho nn F > 0. Lc y c th biu din di dng:Fd = B r n+1

(2.2)

Trong B v n l cc hng s, n > 1. Vy lc tng tc tng cng ca cc ion l:

11

F = Fc + Fd =

Z1Z 2eo 2 B + n+1 2 4 o r r

(2.3)

Theo nh ngha F = -dU/dr, vi U l th nng ca h. V vy khi tch phn i lng F t khong cch cn bng (ro) gia cc ion trong mng li tinh th n r = ta c th tch c th nng U ng vi s tch hai ion c xt n khong cch v cng ln (hnh 2.1).U = (ro

Z1Z 2eo 2 ZZe2 B B + n+1 )dr = 1 2 o n 2 4 o r r 4 o ro nro

(2.4)

Hng s B c th tnh theo phng trnh (2.3). V ti trng thi cn bng, khi r = ro v F = 0; nn ta c:B= Z1Z 2 eo 2 ro n 1 4 o

(2.5)

Phng trnh (2.4) c vit li:U= Z1Z 2 eo 2 1 (1 ) 4 o ro n

(2.6)

Cng thc (2.6) c tnh cho mt cp cc ion. tnh nng lng mng li tinh th cho mt mol cht cn nhn U vi NA (s Avgadro) v tnh tng tc gia cc ion ny. S tng tc c xc nh bi s phn b tng h ca chng trong th tch mng li. S b chnh cui cng tng ng vi vic a vo mt phn t A (gi l hng s Madelung), n c xem l tng ca chui v ph thuc vo kiu mng li tinh th. V d vi phn t NaCl th gi tr ca A = 1,7476. V vy, m hnh Born i vi mng li tinh th dn ti cng thc:U = N A . A. Z1Z 2 eo 2 1 (1 ) 4 o ro n

(2.7)

Phn tch cu trc Rngen cho php xc nh hnh hc ca cc tinh th v xc nh cc thng s A v ro. Hng s n c tnh t cc s liu v ln ca tinh th theo cng thc:n= 1 + 18ro 4 Aeo 2

(2.8)

12

y l h s nn. i vi NaCl ta thu c n = 7,5. i vi cc mui khc, ga tr ca n dao ng t 5 n 12. Khi tnh nng lng mng li tinh th NaCl theo cng thc (2.7) ta c ga tr sau: GNaCl 762 kJ/mol Gi tr GNaCl thu c trn c th so snh vi s thay i entanpi trong qu trnh ph v mng li tinh th NaCl (HNaCl). i lng HNaCl c th xc nh nh chu trnh sau:NaCltt H Na+(khi) +e H5 H3 Nar + 1/2Cl2 H1 + Cl-(khi) -e H2

Na(khi)

Cl(khi) H4

Vi: H 1 = - 496kJ/mol; H2 = 365 kJ/mol; H3 = 109 kJ/mol; H4 = -121 kJ/mol v H5 = - 411 kJ/mol. HNaCl = - (H1 + H2 + H3 + H4 + H5) = 772 kJ/mol

Ngoi ra, ga tr T.S ca NaCl 25oC gn bng 15 kJ/mol. V vy, i vi nng lng mng li tinh th NaCl ta thu c: G = H - TS = 757 kJ/mol. Gi tr ny ph hp tt vi ga tr c tnh t (2.7). Gi tr i lng thay i entanpi khi ph v mng li tinh th ca dy cc halogenua kim loi kim c trnh by bng 2.1. Bng 2.1: Gi tr H ca cc mng li tinh th ca cc halogenua kim loi kim

25oC

Cation Na+ K+ Rb+ Cs+ F911 810 780 744

H (kJ/mol) Cl 774 702 -

Br 741 678 658 -

I637 621 604

13

Nh vy, s to thnh cc ion khi ph v mng li tinh th i hi phi tiu tn mt nng lng rt ln. Nu nng lng ny khng c b chnh bi nng lng hydrat ha, th kh nng phn ly ca cht tan bng khng. 2.3. Nng lng solvat ha Nng lng solvat ha l nng lng thu c khi chuyn mt mol ion nghin cu t chn khng vo dung mi cho. y gi thit rng, trong nng lng ny khng cha tng tc tnh in ca cc ion, tng tc ny khng th trnh khi khi a vo dung mi ion th hai v mi mt ion tip theo. trnh s phc tp ny, dung dch vn c gi trung ho in, tc l a vo dung dch lng thch hp cc ion nghch du v s pha long dung dch ln lm trit tiu tng tc ht gia cc cation v anion. M. Born a ra m hnh n gin nht tnh nng lng solvat ha nh hnh 2.2.W1 O W2 = 0 W3 O

2.2: Hnh 2.2: S tnh nng lng solvat ha theo m hnh Born Theo m hnh ny, ion c xem nh qu cu tch in c bn knh ri v dung mi l mi trng ng nht lin tc c hng s in mi . Qu trnh chuyn cc qu cu tch in t chn khng vo mi trng c chia thnh 3 giai on: 1- S phng in qu cu trong chn khng. 2- S chuyn cc qu cu khong tch in t chn khng vo dung mi. 3- S tch in ca qu cu trong mi trng. y gi thit rng cng ca giai on 2: W2 = 0, v tnh cng ca cc giai on 1 v 3 (W1 v W3) ta s dng cc nh lut tnh in c bn v ta thu c cc biu thc sau:W1 = Zi eo 2 8 o ri

v

W3 =

Z i eo 2 8 o ri

(2.9)

Vi l hng s in mi ca mi trng v ri l bn knh qu cu i.

14

Nu xem rng nng lng t do solvat ha GS = NA Wi th ta c c cng thc c bn sau ca m hnh Born:G = N A Z i eo 2 1 (1 ) 8 o ri

(2.10)

Mt khc, ta c phng trnh Gibb - Helmholt:H = G T d (G ) dT

(2.11)

T (2.10) v (2.11) ta c biu thc entanpi solvat ha:H S = N A Z i eo 2 T d (1 . ) 8 o ri dT

(2.12)

Phng trnh (2.12) gi l phng trnh Born - Berrum. Cc kt qu tnh theo phng trnh (2.10) v (2.12) i vi s hydrat ha cc cation kim loi kim v cc anion halogen c trnh by trong bng 2.2. Bng 2.2: Nng lng t do v entanpi hydrat ha tnh theo m hnh Born 25oC Ion Li+ Na+ K+ Rb+ Cs+ FCl Br Iri(theo Pauing) 0,60 0,95 1,33 1,48 1,69 1,36 1,81 1,95 2,16 - GS (kJ/mol) 1143 722 515 463 406 504 379 352 317 - H (kJ/mol) 1163 734 524 471 412 513 386 358 323

Nng lng hydrat ha ca phn t l tng nng lng hydrat ha ca cc ion trong phn t v kt qu cho thy nng lng hydrat ho phn t tnh theo m hnh Born kh ln v hon ton c th m bo ph v mng li tinh th khi to thnh dung dch in ly. y l kt lun quan trng nht v mt nh tnh t thuyt Born, n cho thy s solvat ha cc ion l nguyn nhn ch yu to thnh v bn vng ca cc dung dch in ly. Hiu ng nhit hydrat ha ca phn t mui cng c th c tnh theo chu trnh nhit ng. V d i vi mui natri clorua:

15

NaCltt

HNaCl

Na+(khi) Hs +

+

Cl-(khi) Hs -

-

Hht Na+(aq) Cl-(aq)

y Hht l hiu ng nhit ho tan c ngoi suy n dung dch mui rt long. T chu trnh ny ta thu c hiu ng nhit solvat ho i vi NaCl nh sau: HS(NaCl) = HS+ + HS - = Hht - HNaCl = -768 kJ/mol Bng phng php tng t ta tnh c HS vi cc mui khc. Gi tr HS thu c t nhit ng v theo m hnh Born i vi cc mui clorua kim loi kim c trnh by bng 2.3. Bng 2.3: HS khi hydrat ho cc clorua kim loi kim 25oC Phng php tnh T chu trnh nhit ng Theo m hnh Born LiCl -883 -1549 NaCl -768 -1120 HS (kJ/mol) KCl RbCl -685 -664 -910 -857 CsCl -639 -798

Bng 2.3 cho thy, cc gi tr HS tnh theo nhit ng v chu trnh Born c s sai lch tng i ln. iu ny cho thy c trng hon ton gn ng ca cc gi thit c a ra trn c s m hnh Born. V vy, ngi ta a vo m hnh Born cc hiu chnh chnh xc ho thuyt solvat ho ca Born. V d m hnh ca Uebe a vo s hiu chnh v s gim hng s in mi ca dung mi gn ion (do s nh hng nghim ngt ca cc dipol trong ion) v hiu chnh v s gim cng nn dung mI khi a cht in ly vo n (hiu ng in gii). Nhng m hnh phc tp hn cho php loi tr s khc nhau gia gi tr HS theo nhit ng v theo m hnh solvat ha.

16

Chng 3 Tng tc ion - ion trong dung dch cht in ly Tng tc ion - dipol v mt vt l cho php gii thch s to thnh v bn vng ca cc dung dch in li. Song m t nh lng cc tnh cht ca nhng dung dch ny cn phi tnh n tng tc ion - ion. 3.1. Hot v h s hot Trong cc dung dch long, khi cc ion cch xa nhau, tng tc ca chng v c bn l tng tc tnh in. Tu theo s gn nhau ca cc ion khi tng nng dung dch m bt u c s xen ph gia cc lp v solvat ca cc ion ring bit v tng tc tnh in gia cc ion b phc tp ho v tr nn quan trng. Ngoi ra, khi cc cation v anion li gn nhau cn xy ra s kt hp, khi lc tng tc gia cc ion khng th xem l thun tu tnh in. Trong dung dch cht in ly to thnh cc ion phc v cc phn t khng in ly cn xut hin tng tc khng c tnh cht tnh in mnh hn nhiu. m t mt cch hnh thc tt c cc t hp tng tc xut hin trong cc dung dch in ly, Lewis a ra i lng hot (a) thay cho i lng nng trong cc phng trnh nhit ng. y, cng nh trong cc dung dch khng in ly, gi thit rng tt c cc h thc nhit ng u c biu din dng cc phng trnh ca dung dch l tng, nhng thay nng bng hot . V vy, tt c cc dng tng tc gia cc phn t ca dung dch khng tnh n bn cht ca chng dn ti s sai lch ca hot tm c bng thc nghim vi gi tr nng tng ng. y l phng php m t cc tng tc c dng cho cc dung dch in ly. Ta c biu thc i vi th ho hc ca phn t i c dng:i = (G )N N i ji ,T ,P

(3.1) (3.2)

Hay i = i0 + RTlnai

xc nh hot ca cc ion v hot ca phn t, ta xt s phn li ca cht in phn M+ A- : M + A vi + + - = + M z + + -Az -

Ta c biu thc th ho hc ca cc phn t v ion nh sau: s = s0 + RTlnas

17

+ = +0 + RTlna+ - = -0 + RTlna0 l th ho hc tiu chun; as, a+, a- l hot ca phn t v cc ion. Ta c: s = + + + - v s0 = + 0+ + - 0-

Suy ra: RTlnas = + RTlna+ + - RTlna as = a++. a-(3.3)

m t cc tnh cht ca dung dch in li, ngi ta a vo khi nim hot ion trung bnh: a =

aS

=

a + + .a

(3.4)

Hot ca cc ion ring bit l tch s ca nng ion vi h s hot : a+ = + m+ ; a- = - m(3.5)

m+ , m- : nng molan ca cation v anion. +, - : h s hot ca cation v anion. Ta c : a = . m = ( ++. --)1/ m = ( m++.m--)1/ m = m. vi = (++.--)1/ Vi cc thang nng khc nhau ta c: ac = c.C ; aN = N . N Trong tt c cc thang nng , h s hot trung bnh pha long v cng ca dung dch tin ti n v, do khi tng tc ion- ion tin ti khng v dung dch c nhng tnh cht nh dung dch l tng. (3.6) (3.7) (3.8) (3.9)

18

C mt vi phng php xc nh hot v h s hot cht in li nh p sut hi ca dung mi trn dung dch, phng php nghim si v nghim lnh, phng php p sut thm thu. Ngoi ra, ngi ta cn s dng phng php o hiu th ca mch in ho cn bng. Phng php ny da trn cc nh lut nhit ng hc in ha. Hot v h s hot thu c bng cc phng php khc nhau trng vi nhau trong gii hn chnh xc ca th nghim. iu ny cho thy rng phng php nhit ng m t tng tc trong cc dung dch in ly l ng n v t ph hp (chng ta nh rng, in ly c xc nh bng cc phng php khc nhau, mu thun vi nhau). Debye3.2. Thuyt Debye-Huckel

3.2.1. Nhng gi thuyt ban u ca Debye-HuckelPhng php hot cng nh h s hot ng gp rt quan trng vo m t nhng thuc tnh nhit ng ca dung dch cht in li. Tuy nhin, nhng quy lut v hot v h s hot cn mang tnh kinh nghim. m t tng tc ion - ion cn phi bit s phn b ca ion trong dung dch v bn cht ca lc tng tc gia cc ion. Do cc ion v cc dipol ca dung mi chuyn ng Brown v cc ion c th to thnh cc cht lin hp, cc phc cht v cc phn t khng phn li, cho nn dng tng qut bi ton v s phn b cc ion l cc k phc tp. c th tnh ton l thuyt tng tc ion-ion, Debey v Huckel xy dng thuyt tnh in vi cc gi thuyt sau: 1. Trong dung dch cht in phn phn li hon ton, = 1. 2. Ion c xem l cc in tch im khng c kch thc vt l. 3. Tng tc gia cc ion ch n thun l tng tc Coulomb. 4. S phn b ion trong dung dch tun theo nh lut phn b thng k Boltzmann. 5. Chp nhn hng s in mi ca dung mi nguyn cht l hng s in mi ca dung dch. Debye-Huckel b qua s thay i hng s in mi ca mi trng gy ra bi in trng ca ion trong dung dch.

3.2.2. M hnh dung dch in li ca Debye-HuckelDebye-Huckel a ra m hnh v dung dch cht in li nh sau: Trong dung dch in li gm c nhng ion trung tm, xung quanh ion trung tm lun c bao bi mt kh quyn ion hnh cu. Trong kh quyn ion th xc sut tm thy ion tri du vi ion trung tm ln hn xc sut tm thy ion cng du vi ion trung tm. Cc ion trong kh quyn khng c nh m lun lun i ch cho nhau. Chuyn ng ca ion trn kh quyn l chuyn ng tnh tin ch khng phi dao ng nh cc ion nt ca mng tinh th. Tt c cc ion trong dch u bnh ng. Mi mt ion trong dung dch va l ion trung tm, va l ion trong kh quyn ion. M hnh kh quyn ion ca Debye-Huckel da trn hai hiu ng: + Hiu ng tnh in (c tnh trt t) + Hiu ng chuyn ng nhit ca cc ion (c tnh v trt t) Vic tn ti kh quyn ion nh vy theo Debye-Huckel l du hiu ni ln rng dung dch thc khc dung dch l tng.

19

Hnh 3.1: M hnh kh quyn ion Theo l thuyt nhit ng hc, th nhit ng G ca dung dch thc cn phi ln hn th nhit ng G ca dung dch l tng mt i lng G* (nng lng b sung): G = G + G* Nh vy, nng lng G* phi phn nh hiu qu ca lc tng tc ion, c th y l tng lc tng tc ion ca cc ion nm trong kh quyn i vi cc ion trung tm. Trn c s m hnh ca mnh, Debye-Huckel tnh nng lng tng tc b sung G* v t tnh h s hot theo cng thc:G * kTlni = g = ( ) P,T, n J ni* i

(3.10)

gi*: th nhit ng ng p ring phn ni : s lng cc phn nh ca cu t i nj : s lng cc phn nh ca cc cu t khc ngoi i. Nh vy, ni dung ch yu ca l thuyt Debye-Huckel l xc nh h s hot , mt khi nim nh lng rt quan trng ng dng trong dung dch in li (dung dch thc). 3.3. Nng lng tng tc gia ion trung tm v kh quyn ion Nng lng tng tc gia ion trung tm v kh quyn ion l dng in nng; v vy nng lng ny phi l mt hm cu mt in tch v th trung bnh ca kh quyn ion. Chn tu mt ion dng lm ion trung tm v chn mt th tch nguyn t dv = dx.dy.dz ca dung dch nm tng i gn ion trung tm mt khong cch r nh sau:

20

Z

dx dz r dy x

y Th ca mt in trng xung quanh mt ion no gm th 0 xc nh bi ion trung tm v th a ca kh quyn ion. = 0 + a - Ta xt th xc nh bi ion trung tm: Theo nh lut Coulomb, th chnh l nng lng cn thit chuyn mt ion dng t v cc n ion trung tm v c ga tr: 0 = -

r

fdr = -

Dr

r

Zie2

dr =

Zie D.r

(3.11)

Zie : in tch ion trung tm; D: hng s in mi. - Ta xt th ti mt im bt k xung quanh ion trung tm: Th ny c xc nh bi mt in tch ti im cho v c to nn bi ion trung tm v kh quyn ion. Theo phng trnh Poisson ta c: = : ton t Laplace =4 . D

(3.12)

2 2 2 + + x 2 y 2 z 2

: mt in tch. Mt in tch ca dung dch l tng s in tch dng v m trong mt n v th tch. Gi dn+, dn- l s ion (+) v (-) trong th tch dv n+, n- l s ion (+) v (-) trung bnh trong mt n v th tch. p dng nh lut phn b thng k Boltzmann ta c:

21

dn+ = n+ . e dn- = n- e

eZ + dv KT

eZ dv KT

Mt in tch s bng: = eZ-n- e eZ eZ + - e.Z+n+ e KT KT

(3.13)

Thay vo (3.12) ta c: = eZ i 4 ZiniN e D KT

(3.14)

Phng trnh (3.14) l c s ca thuyt Debye-Huckel. Hm s m: e eZ i Z e 1 Z e 2 1 Z i e 3 =1- i + ( i ) ( ) + .. . KT 1! KT 2! KT 3! KT

Nu nng lng tnh in rt b so vi chuyn ng nhit, ngha l Zie 0 Du

- Du ca th in cc l m nu E < 0 Du

Nu s dng cc in cc khc lm in cc so snh nh in cc calomen, bc-clorua bc . . . th ta o th in cc bng cch lp mt h in ho gm in cc nghin cu v in cc so snh. Ni in cc so snh vi u m ca my o sc in ng v in cc nghin cu vi u dng cu my o sc in ng. Sc in ng E = + - - , E c th > 0 hoc < 0. Khi xc nh th vi in cc so snh no th ta phi ch r in cc so snh. o sc in ng E ngi ta c th dng phng php xung i hay cn gi l phng php Poggendorff nh sau: - a kho K vo v tr 1, iu chnh con chy C trn dy AB n v tr C no sao cho kim in k G ch s 0. Ta c:EX AX = E AB

- a kho K vo v tr 2, iu chnh con chy n v tr C no sao cho G ch s 0. Ta c:EN AN = E AB AX AN

Ex = EN .

( EN l sc in ng ca pin chun bit).

5.10 10: Hnh 5.10: S o sc in ng

60

Ngoi ra o sc in ng ngi ta s dng von k c in tr ni R> 1012 . Bng 5.1: Th in cc chun ca mt s qu trnh in cc trong mi trng nc 250C in cc Li+/Li K+/K Ba2+/Ba Ca2+/Ca Mg2+/Mg Al3+/Al Ti2+/Ti Mn2+/Mn Zn2+/Zn Fe2+/Fe Cd2+/Cd Cu2O/Cu Ni2+/Ni Pb2+/Pb H+/H2 Cu2+/Cu+ Cu2+/Cu O2/OHCu+/Cu I2(r)/IFe3+/Fe2+ Hg22+/Hg Ag+/Ag Br2(l)/BrO2/2H2O Cl2/ClMnO-4/Mn2+ F2/FPhn ng in cc Li+ + e = Li K+ + e = K Ba2+ + 2e = Ba Ca2+ + 2e = Ca Mg2+ + 2e = Mg Al3+ + 3e = Al Ti2+ + 2e = Ti Mn2+ + 2e = Mn Zn2+ + 2e = Zn Fe2+ + 2e = Fe Cd2+ + 2e = Cd Cu2O + 2e + H2O = 2Cu + 2OHNi2+ + 2e = Ni Pb2+ + 2e = Pb H+ + e = 1/ 2H2 Cu2+ + e = Cu+ Cu2+ + 2e = Cu O2 + 4e + 2H2O = 4OHCu+ + e = Cu I2 (r) + 2e = 2IFe3+ + e = Fe2+ Hg2+2 + 2e = 2Hg Ag+ + e = Ag Br2(l) + 2e = 2BrO2 + 4H+ + 4e = 2H2O Cl2(k ) + 2e = 2ClMnO4- + 8H+ + 5e = Mn2+ + 4H2O F2 + 2e = 2F0(V) -3,045 -2,925 -2,900 -2,870 -2,363 -1,660 -1,630 -1,180 -0,763 -0,440 -0,403 -0,358 -0,250 -0,126 0,000 0,153 0,337 0,401 0,521 0,536 0,771 0,789 0,799 1,065 1,229 1,360 1,507 2,870

Mt s khng t nhng gi tr th in cc chun trong bng ny thu c trn c s tnh ton nhit ng lc hc da vo nhng d kin nhit ho hc, v d nhng gi tr in th cu cc kim loi kim hoc kim th l nhng kim loi c phn ng mnh lit vi nc. Theo h thng du c chp nhn cho th in cc, in th cng m th kh nng chuyn t dng kh sang dng oxi ho cng mnh v ngc li in th cng dng th kh nng chuyn t dng oxi ho sang dng kh cng mnh. Nh vy, kim loi c th m hn c th dng lm cht kh i vi cation c th dng hn.

61

V d: Zn + Cu2+ = Zn2+ + Cu Nh bng th in cc tiu chun chng ta c th d dng thnh lp cc phng trnh phn ng ho hc khc nhau, gii quyt vn chiu ca nhng phn ng ny v tnh hon ton ca phn ng. Tuy nhin, bng th in cc chun ch cho thng tin v kh nng ca cc qu trnh ny; trong cc iu kin thc, c lc no nhng qu trnh ny c th khng quan st c do chng xy ra vi tc rt b.

5.3.1. Nguyn t galvani

5.3. Nhit ng hc v nguyn t galvani

Nguyn t galvani hay nguyn t in ho cn c gi l ngun in ho hc (pin) l mt h in ho cho php bin i nng lng ho hc trn in cc thnh nng lng in. V cu to nguyn t galvani gm hai in cc bng kim loi nhng vo dung dch in phn. Trong nguyn t galvani in cc no c th cn bng m hn (t dng hn) s l cc m ca nguyn t. in cc no c th cn bng dng hn (t m hn) s l cc dng ca nguyn t. Electron s chuyn t cc m sang cc dng qua dy dn kim loi. Kt qu ca hai phn ng in ho trn hai in cc cho ta dng in chy trn dy dn kim loi. V d in hnh cho nguyn t in ho l nguyn t Daniel - Jakobi: (-) Zn / ZnSO4 (1M) // CuSO4 (1M) / Cu (+) in cc Zn l in cc m ca nguyn t, v th tiu chun ca Zn m hn ca Cu ( Zn = - 0,763V ; 0Cu = + 0,340V).0

5.11: Hnh 5.11: S pin in ha Daniel - Jakobi Trn in cc Zn c phn ng ho tan Zn:

62

Zn - 2e

Zn2+

(qu trnh oxi ha)

Trn in cc Cu c phn ng: Cu2+ + 2e Cu (qu trnh kh)

Phn ng chung ca hai phn ng in cc ca nguyn t galvani: Zn + Cu2+ Zn2+ + Cu

5.3.2. C ch xut hin sc in ng ca nguyn t galvaniXt trng hp ca nguyn t Daniel-Jakobi, sc in ng E l hiu th gia in cc Cu v dy dn bng Cu ni vi cc Zn. S ca nguyn t Daniel-Jakobi c vit nh sau: (-) Cu / Zn / ZnSO4 // CuSO4 / Cu (+) 1 2 3 4 1 E = 1 - 1 = (1 - 4) + (4 - 3) + (3 - 2) + (2 - 1) Nh vy, sc in ng E chnh l tng cc bc nhy th ranh gii pha. Nu b qua bc nhy th ranh gii hai pha lng, th E bng tng ba bc nhy th: E = 1 - 1 = (1 - 4) + (3 - 2) + (2 - 1 ). hay E = 1 - 1 = 41 + 32 + 12 (5.35)

Nhng i lng gi l th galvani. Th galvani khng xc nh c bng thc nghim, do o E ca mt nguyn t, ngi ta phi s dng mt loi in cc so snh m gi tr th ca n c chp nhn theo mt qui c no . Gi s xc nh sc in ng E ca nguyn t: (-) M1 / L / M2 / M1 (+) (A)

bng cch s dng in cc so snh M0, ta lp c hai mch nguyn t sau: M0 / L / M1 / M0 M0 / L / M2 / M0 (B) (C)

i vi nguyn t (B) ta c: E1 = M1M0 + LM1 + M0L i vi nguyn t (C ) ta c : E2 = M2M0 + LM2 + M0L 63

Ga thit E2 > E1 : E2 - E1 = M2M0 + LM2 + M0L - M1M0 - LM1 - M0L V M2M0 - M1M0 = M2M0 - M0M1 = M2M1 Nn E2- E1 = LM2 + M1L + M2M1 Biu thc ny l sc in ng ca nguyn t (A), do vy: E = E2 -E1 = M2M1 + LM2 + M1L (5.36)

Biu thc (5.36) chng t sc in ng E ca nguyn t in ho bng hiu gia cc th in ring 2 v 1. V 2 > 1 nn 2 l th ca in cc dng, cn 1 l th ca in cc m. Trong trng hp chung ta c: E = + - -

5.3.3. Quan h gia sc in ng vi cc i lng nhit ngNguyn t galvani l nguyn t in ho lm vic thun nghch, khi cng in m h thc hin c cho bn ngoi l cng cc i A = Amax. Nu h in ho tin hnh T,P = const, th theo nguyn l 2 ca nhit ng hc, gim th ng p s bng cng cc i Amax G = -Amax = - nFE Ta c phng trnh Gibbs-Helmholtz : G = H + T [ hay : E = G ]P T

(5.37)

H dE +T nF dT

(5.38)

i lng dE/dT l h s nhit ca sc in ng V d(G)/dT = -S nn dE/dT = S/nF (5.39)

Biu thc (5.39) cho thy, h s nhit ca sc in ng c trng cho bin thin entropi S ca phn ng tin hnh trong nguyn t galvsni.

53.4. Cc yu t nh hng n sc in ng5.3.4.1. nh hng ca nng .3.4.1.

64

Xt nguyn t galvani: ant: M1- ne catt: M2n+ + ne M1 + M2n+ K= Ta c A = - nFE[ M 1n + ] n [M 2 + ]

(-) M1/ M1n+ // M2n+/ M2 (+) M1n+ M2 M1n+ + M2

M A = - G = RTlnK - RTlnRT lnKnF RT lnK nF0

[ M 1n + ] n [M 2 + ]

Suy ra: E =

RT [ M 1n + ] ln n nF [ M 2 + ]

Ta c E0 =

[ M 1n + ] RT Do E = E ln n + nF [M 2 ]

(5.40)

Xt c th phn ng: Zn + Cu2+ Cu + Zn2+ E = E0 RT [ Zn 2+ ] ln nF [Cu 2+ ]

5.3.4.2. nh hng ca p sut p sut c nh hng n sc in ng ca nguyn t galvani c cc in cc kh tham gia. T nhit ng hc ta c: ( Ta li c: G = - nFE Suy ra: (E ) T = - V/nF P G ) T = V P

(5.41)

Xt nh hng ca p sut hyro n phn ng: H2 + 2AgCl 2HCl + 2Ag65

B qua s bin thin th tch ca tng lng v rn, th V = -RT/P (v th tch ca h gim). Thay gi tr V vo (5.36) ta c: (E RT 1 )T = + . P 2F P

(5.42)

Ly tch phn (4.52) ta c: E = E0 +RT ln PH2 2F

(5.43)

5.3.4.3. nh hng ca nhit .3.4.3. nh T biu thc (5.38) ta c : E = -

H dE +T nF dT

Biu thc ny cho thy s ph thuc ca E vo nhit . Cng c th vit (5.38) di dng: E=H S + T nF nF

(5.44)

5.4. Cc loi pin Trn c s ca cc phn ng oxi ho-kh v s chnh lch v nng lng khi chuyn t dng tn ti ny sang dng tn ti khc, ngi ta xy dng cc loi pin (ngun in ho) khc nhau.

5.4.1. Cc qui c trong pin- Trong pin in cc no c th dng hn ng vai tr in cc dng (gi l in cc catt) ; trn in cc dng xy ra qu trnh kh. in cc no c th m hn ng vai tr in cc m (gi l ant) ; trn in cc m xy ra qu trnh oxi ha. - in cc (-) c t bn pha tay tri v in cc dng t bn pha tay phi ca pin in ha. - Sc in ng ca pin in ha bng th in cc dng tr th in cc m : E = (+) - (-) (- Khi pin hot ng th dng electron chuyn t in cc m sang in cc dng v dng in chuyn t in cc dng sang in cc m.

5.4.2. Pin vt l.4.2 5.4.2.1. Pin trng lc: Pin trng lc gm hai in cc lng ca cng mt kim loi c chiu cao khc nhau v c nhng vo dung dch mui ca cation kim loi .

66

V d pin trng lc Hg:

(-) Hg / Hg2A2 / Hg (+) h1 h2 ( h1 > h2)

Hg2 :

2+

in cc c chiu cao h1 c nng lng t do ln, nn b ho tan to ion v Hg - e 1 /2Hg2+2 1/ 2Hg22+ + e Hg (h2)

Phn ng chung ca pin l s chuyn Hg t in cc cao hn sang in cc thp hn Hg (h1) Hg (h2) Qu trnh ny xy ra cho n khi chiu cao ca hai in cc bng nhau. Nh vy, trong pin trng lc nng lng c hc do s khc nhau v trng lc ca in cc c chuyn thnh nng lng in. Sc in ng ca pin trng lc ph thuc vo s khc nhau v chiu cao ca hai in cc, (h1-h2) cng ln th E cng ln. Sc in ng ca pin trng lc thng b, h = 100cm , E ~ 2.10-5 V. 5.4.2.2. Pin th hnh: L pin gm hai in cc ca cng mt kim loi nhng c th .4.2 hnh khc nhau (M, M) c nhng vo dung dch hoc mui nng chy ca hp cht dn in ion ca kim loi . nhit xc nh, ch mt dng th hnh ca kim loi l bn vng, cn dng th hnh khc nm ttrng thi bn bn vng. in cc kim loi bn bn vng (M ) c nng lng t do ln, nn ng vai tr in cc m ca pin v b ho tan to ion: M = Mn+ + ne in cc kim loi bn vng ( M) l cc dng ca pin: Mn+ + ne = M Qu trnh chung ca pin l : M M T s thay i nng lng t do ng vi s thay i th hnh, ngi ta c th tnh sc in ng E ca pin v ngc li. Pin th hnh cng c th c to ra trong trng hp hai in cc c kch thc tinh th khc nhau hoc c ng sut ni khc nhau. in cc c kch thc ht nh hay ng sut ni ln ng vai tr cc m ca pin v ngc li. Sc in ng ca pin th hnh thng nh.

67

5.4.3. Pin ho hc5.4.3.1. Pin ho hc n gin: Pin ho hc n gin l h in ho trong in cc .4.3 th nht thun nghch vi cation ca cht in li, cn in cc th hai thun nghch vi anion. Mt s pin ho hc n gin: - Pin hyro - oxi: M, H2 / H2O / O2, M Cc phn ng in cc: H2 - 2e 1/ 2O2 + 2e + H2O Phn ng chung: Sc in ng E = E0 + 250C H2 + 1/ 2O2RT ln PH2 .P1/ 2O2 2F

2H+ 2OH-

H2 O (5.45) (5.46)

E = 1,24 + 0,03 lgPH2.PO21/ 2

Pin hyro-oxi c gi l pin nhin liu, nng lng in c hnh thnh t nng lng ho hc ca phn ng gia hyro v oxi. V H2O c dn in km, do ngi ta thay H2O bng dung dch KOH hoc NaOH 27%: M, H2 / KOH / O2, M Ngy nay pin nhin liu H2 - O2 l mt hng nghin cu c cc nh khoa hc quan tm, v n khng gy nhim mi trng. - Pin chun Weston: Cd, Hg / CdSO4 / Hg2SO4 , Hg Phn ng in cc: Cd -2e Hg2SO4 + 2e Sc in ng E = E0 Cd2+ 2Hg + SO42(5.47)

RT RT lnaCd2+.aSO42- = E0 ln a CdSO4 2F F

Pin chun Weston c E n nh v h s nhit nh, nn c s dng lm pin chun o sc in ng trong phng php b chnh Poggendorff. - c qui ch: (-) Pb, PbSO4/ H2SO4 / PbO2, Pb (+) Phn ng in cc: Pb- 2e + SO42PbSO4 PbO2 + 2e + 4H+ + SO4268

PbSO4 + H2O

Phn ng chung:

Pb + PbO2 + 2 H2SO40

2PbSO4 + 2H2O (5.48)

2 4 2 a H + a SO 4 a H + a SO 4 RT RT 0 Sc in ng E = E + ln 2 =E + ln 2F F a H 2O a H 2O

hay E = E0 +

a RT ln H 2 SO 4 a H 2O F a H 2 SO 4 a H 2O

(5.49)

250C : E = 2,04 + 0,059lg

(5.50)

c qui ch l ngun in th cp, c th phng np nhiu ln. Ngoi c qui ch ta cn c mt s c qui khc nh c qui st - niken, niken cadmi, bc - km. 5.4.3.2. Pin ho hc phc tp .4.3 Mt s pin ho hc phc tp in hnh nh pin Daniel-Jakobi v pin Leclanch. - Pin Daniel-Jakobi: (-) Zn / ZnSO4 // CuSO4 / Cu (+)

- Pin Leclanch: (-) Zn / ZnCl2, NH4Cl / MnO2 , C (+) Phn ng in cc: Zn- 2e = Zn2+ 2MnO2 + 2 H+ + 2e = Mn2O3 + H2O Phn ng chung ca pin: Zn + 2NH4Cl + 2MnO2 = Zn(NH3)2Cl2 + Mn2O3 + H2O Pin Leclanch cn gi l pin kh, khng c kh nng dng li bng cch ti np in.

5.4.4. Pin nng Pin nng l h in ho gm hai in cc ging nhau v bn cht song khc nhau v hot ca mt hay nhiu cht tham gia phn ng in cc. in nng ca h l s sang bng nng ca cc cht khi pin hot ng. Pin nng gm hai loi: pin nng c ti v pin nng khng ti. ti: 5.4.4.1. Pin nng khng ti Gm 3 loi: .4.4 a. H in ho c hai in cc ging nhau nhng vo hai dung dch ging nhau v bn cht nhng khc nhau v nng ; hai in cc ny khng trc tip tip xc vi nhau.

69

V d: (-) Zn / ZnCl2 (a2 ) / Hg2Cl2, Hg, Hg2Cl2 / ZnCl2 (a1 ) / Zn (+) Phn ng in cc : (-): Zn + Hg2Cl2 = ZnCl2 ( a2 ) + 2Hg (+): ZnCl2 (a1 ) + 2Hg = Zn + Hg2Cl2 Phn ng chung xy ra trong pin: ZnCl2 (a1 ) ZnCl2 (a2 ) in nng thu c l do s san bng nng ZnCl2. Sc in ng: E = E- - E+ = (3/2).0,059lg(a1 / a2 ) (5.51)

b. H in ho c hai in cc hn hng cng bn cht song khc nhau v nng v c tip xc vi mt dung dch in phn. M, Hg/ MA/ M, Hg (a2) (a1) V d: a2 > a1

(-) Cd, Hg / CdSO4 / Cd, Hg (+) a1 a2 Phn ng in cc: (-) Cd (a2)- 2e = Cd2+ (+) Cd2+ + 2e = Cd (a1) Phn ng chung: Cd (a2) Cd (a1) (5.52)

Sc in ng: E = 0,059/2 lg a2/a1

c. H in ho vi hai in cc kh cng bn cht nhng khc nhau v p sut kh trn in cc, tip xc vi mt dung dch in phn. V d: Pt, H2 / HCl / H2, Pt (P1) (P2) (-) H2 (P2)- 2e = 2H+ (+) 2H+ + 2e = H2 (P1) Phn ng chung: H2 (P2) H2 (P1)

Phn ng in cc:

70

Sc in ng E = (0,059/2)lg (P2/P1)

(5.53)

5.4.4.2. Pin nng c ti: L h in ho gm hai in cc ging nhau c nhng .4.4.2. .2 vo hai dung dch cng bn cht nhng khc nhau v nng . Hai dung dch ny tip xc nhau qua mng xp hn ch s pha trn. Ti ranh gi hai dung dch xut hin th khuch tn D do s khc nhau v linh cation v anion ca cht in phn trong qu trnh khuch tn t ni c nng cao v ni c nng thp. V d: (-) Ag / AgNO3 AgNO3 / Ag (+) a2 a1 NO3(-) > Ag(+)

Ti cc (-) : +1Ag+ - t+ Ag+ + t- NO3- = + t- Ag+ + t- NO3(+) : -1Ag+ + t+ Ag+ - t- NO3- = -t- Ag+ - t- NO3t- AgNO3 ( a1 ) t- AgNO3 (a2 ) hay: t- Ag+ (a1 ) + t- NO3- (a1 ) t- Ag+ (a2 ) + t- NO3- (a2 ) t a tAg + ( a1+ ) a NO 3( a1 ) RT Sc in ng E = E + ln t t F a Ag + ( a 2 + ) a NO 3( a 2 )

0

(5.54)

v E0 = 0 do hai in cc ging nhau ni ngc chiu nhau, nn: E = tRT a 1 a 1 ln + 2 2 F a+ a

(5.55)

hay

E = 2t-

a1 RT ln 2 F a

(5.56)

5.5. ng dng ca php o sc in ng .5.

5.5.1. Xc nh h s hot ion trung bnhH s hot ion trung bnh ca cht in phn c xc nh chnh xc bng phng php o sc in ng ca mch ho hc khng ti. V d: (-) Zn / ZnCl2 / Hg2Cl2, Hg (+) E = E0 - 3/2 . 0,059 lgaZnCl2

71

E = E0 - 3/2 .0,059lg . m.

( = ++ . -- ) (5.57)

Hay E + 3/2 . 0,059lgm. = E0 - 3/2 . 0,059lg

Phng trnh (5.57) cho thy ti mt gi tr m c hai i lng E0 v cha bit. Ta xc nh E0 bng cch v th ( E + 3/2 . 0,059lg m. ) - m . (v s ph thuc ca lg vo m l ng thng) Ngoi suy phn tuyn tnh ca d th ti m = 0, n ct trc tung ti i lng E0, v khi m = 0 th = 1 suy ra lg = 0.(E + 3/2lgm

E

0 1/2 m

Bit E0 v E ti m no , ta xc nh theo (5.57).

5.5.2. Xc nh s ti xc nh s ti ngi ta xc nh sc in ng ca pin nng c ti. V d: Zn / ZnCl2 a1 E = t2 3RT a ln 1 2F a

ZnCl2 / Zn a2 (5.58)

t+ = ( 1- t-) Nu nh hot ca cc cht cha bit th xc nh s ti ngi ta tin hnh o E ca pin khng ti v c ti nh sau: - Pin nng khng ti: Zn / ZnCl2 (a1 )/ Hg2Cl2,Hg, Hg2Cl2 / ZnCl2 (a2 ) / Zn

72

Ta c : E1 = 3/2. - Pin nng c ti : Zn / ZnCl2 a1

a2 RT ln 2F a1

(a)

ZnCl2 / Zn

a2a2 RT ln 2F a1

Ta c: E2 = t- 3/2. T (a) v (b) suy ra

(b)

E2 / E1 = t-

(5.58)

5.5.3. Xc nh pH ca dung dchNguyn tc ca php o l lp mt nguyn t galvani gm mt in cc so snh (thng l in cc calomen) v mt in cc c th ph thuc vo pH ca dung dch c gi l in cc ch th ( thng l in cc hiro, in cc quinhiron hay in cc thu tinh). a- in cc ch th l in cc hiro (-) Pt , H2 / H+ , A- // KCl / Hg2Cl2, Hg (+) E = Cal + 0,059pH pH = 1/0,059 . (E - Cal ) Khong o pH t 1- 8 b- in cc ch th l in cc quinhiron (-) Hg, Hg2Cl2 / KCl // H+ , C6H4O2, C6H4 (OH)2 / Pt (+) E = 0QH - 0,059pH - Cal pH = (0QH - Cal - E )/ 0,059 (5.61) (5.59) (5.60)

Gi hn pH t 1- 8 v dung dch nghin cu khng cha cht oxi ho hoc kh. c- in cc ch th l in cc thu tinh. (-) Hg, Hg2Cl2 / KCl // dung dch cha H+ nghin cu/ C thu tinh (+)73

E = t.t - Cal = 0t.t - 0,059pH - Cal pH = (0t.t - Cal - E )/ 0,059 (5.62)

Gi tr pH c th o c n 12. Hin nay ngi ta s dng cc my o pH vi in cc ch th l in cc thu tinh v my c chun ho sao cho c th c trc tip gi tr pH trn mn hnh.

5.5.4. Xc nh hng s bn ca phc cht

V d hng s bn ca phc cht Ag(CN)2Lp nguyn t in ho: (-) Ag / Ag(CN)2-, KCN // AgNO3 / Ag (+)

- cc m:

Ag - e Ag+ + 2CN-

Ag+

1 Kb

Ag(CN)2-

1 = 0Ag+a Ag ( CN ) 2 a Ag + a

/ Ag

+

RT lna Ag+ F

M Kb =

2 CN

a Ag+ =

a Ag ( CN ) 22 aCN K b

1 = 0Ag /Ag +

RT/F ln aAg(CN)2

- RT/F lnKb.aCN2

1 = 0Ag+ / Ag + RT/F ln aAg(CN)2 - RT/F lnKb - RT/F lnaCN2 - cc dng: Ag+ + e = Ag 2 = 0Ag / Ag + RT/F ln aAg Sc in ng: E = 2 - 1 = 0Ag / Ag + RT/F lnaAg - (0Ag+ / Ag + RT/F ln aAg(CN)2 - RT/F lnKb - RT/F lnaCN2) E = RT/F lnaAg + RT/FlnKb - RT/F lna Ag(CN) + RT/F lnaCN2 T (5.63) ta xc nh c hng s Kb. (5.63) 2

5.5.5. Xc nh hng s tan ca cht it tanXc nh hng s tan ca AgCl.74

Lp nguyn t in ho (-) Ag, AgCl / HCl / Cl2, Pt (+) - cc (-): Ag - e Ag+ AgCl 1 T.t-1

Ag+ + Cl-

1 = 0Ag+ /Ag + RT/F lna Ag+ M: aAg+. aCl- = T.t aAg+ = T.t / aClSuy ra: 1 = 0 Ag + / Ag + RT/F lnTt - RT/F lnaCl- cc dng: 1/ 2 Cl2 - e 2 = 0Cl/ Cl 2

Cl-

2

- RT/F lnaCl-

Sc in ng E = 2 - 1 = 0Cl - / Cl2 - RT/F lnTt + RT/F lnaClhay E = 0Cl - / Cl 2 - 0 Ag + /Ag - RT/F ln T.t (5.64)

T (5.64) ta xc nh gi tr T.t

5.5.6. Xc nh hng s cn bmg ca phn ng oxi ho -khGi s c hai cp phn ng oxi ho kh: oxh1 + ne oxh2 + ne kh1 1 kh2 2

Phn ng c th xy ra theo hai kh nng: aboxh1 + kh2 oxh2 + kh1 oxh2 + kh1 oxh1 + kh2 Ga = - nFEa Gb = - nFEb

Phn ngs xy ra theo cch no m G < 0 , do : E >0 Ta c: Ea = 1 - 2 > 0 1 > 2 Eb = 2 - 1 > 0 2 > 1 Do vy:75

Nu 1 > 2 th phn ng xy ra theo aNu 2 > 1 th phn ng xy ra theo bTm li phn ng oxi ho kh xy ra theo chiu no c E >0 - Hng s cn bng ca phn ng oxi ho kh: Xt phn ng aoxh1 + kh2 oxh2 + kh1 Ga = -nFEa Khi phn ng t cn bng th G = 0 E = 0 hay 1c.b = 2c.b 1c.b = 10 + RT/nF ln[a ox1 ] [a kh1 ] [a ox 2 ] [a kh 2 ]

2c.b = 20 + RT/nF ln 250C : lg

[a ox 2 ][a kh1 ] n = (10 - 20 ) [a ox1 ][a kh 2 ] 0,059

K=

[a ox 2 ][a kh1 ] [a ox1 ][a kh 2 ]n (10 - 20 ) 0,059

lgK =

(5.65)

5.5.7. Xc nh nng ion

Tng t nh cch s dngin cc thu tinh xc nh nng ion H+, y xc nh nng ca mt dng ion xc nh nh Li+, Na+ , K+ , Ag+, Pb2+, Cl-, F-, S2... ngi ta ch to cc loi in cc mng ion m th in cc ch ph thuc mt dng ion xc nh. in cc mng c th dng rn hay lng. Phng php xc nh l lp mt h in ho gm in cc mng v in cc so snh c th bit trc. o sc in ng ca h, t tnh c th ca in cc mng v xc nh nng ca ion cn xc nh. V d: Xc nh nng ion Ca2+ trong dung dch. CaR2 (hu c) 2R- (hu c) + Ca2+ (nc) (5.66)

(mng) = 0 - RT/F ln a Ca 2+

Cu hi v bi tp76

1. Cho pin in vi s : (Pt) H2 (p= 1at) / HCl (a = 0,15) / Hg2Cl2, Hg O 298K (Cl- / Hg2Cl2,Hg) = 0,2681. Hy tnh sc in ng E. 2. 298K s.. ca pin Zn/ ZnCl2 (0,05M) / AgCl, Ag bng 1,015V. H s nhit ca sc in ng bng -0,000492V.K-1. Vit cc phn ng in ha v tnh cc i lng G, H v S ca phn ng xy ra trong pin 298K. 3. 298K th chun ca h redox Ce3+, Ce4+ / Pt v Fe2+, Fe3+ / Pt bng 1,61 v 0,77V. C th c kt lun g v cc d kin trn i vi dung dch cha c hai h redox ny? 4. 298K th chun ca in cc Ag l 0,799V, ca in cc bc-clorua bc l 0,222V. Xc nh tch s tan ca AgCl v tan ca n trong nc nguyn cht. 5. Hng s khng bn ca phc Cu[(NH3)]2+ bng 7,1.10-5. Th chun ca in cc ng bng 0,337V. Hi th ca in cc ng nhng vo dung dch c to bi s pha trn cng th tch ca dung dch amoniac v ng sunphat c nng nh nhau bng 0,1M l bao nhiu? 6. Cho phn ng tng qut xy ra trong nguyn t [ Ag (NH3)2]+ 2NH3 + Ag+ Hy thit lp nguyn t in ha v tnh hng s khng bn ca phc [Ag(NH3)2]+. Bit rng i vi cc na nguyn t 298K + O Ag + e = Ag = 0.7996V + [ Ag(NH3)2] + e = Ag + 2NH3 O = 0,373V. 7. 25OC sc in ng ca pin: Cd, Cd(OH)2 / NaOH 0,001M / HgO, Hg bng 0,940V. O Bit (Cd2+ / Cd) = -0,400V ; O (OH- / HgO, Hg) = 0,114V. a- Vit cc phn ng in cc v phn ng tng qut. b- Tnh tch s tan ca Cd(OH)2. 8. a- Thit lp nguyn t ganvani da trn phn ng sau: Pd + 2HCl = PdCl2 + H2 b- Vit cc phn ng in cc v phn ng tng qut xy ra trong nguyn t sau: Cd / Cd(OH)2 / NaOH / H2. 9. a- Th in cc l g? Bng thc nghim ngi ta xc nh th in cc nh th no? b- Cho bit qui c v cch vit s , v du ca sc in ng v th in cc. 10. Thit lp nguyn t ganvani da trn cc phn ng sau:

77

Cd + CuSO4 = CdSO4 + Cu Ag+ + I= AgI Zn + Hg2SO4 = ZnSO4 + 2Hg 11. Th no l in cc loi 1, 2, 3? Cho v d v vit phng trnh tnh th in cc. a- Trnh by c ch xut hin bc nhy th ranh gii kim lai v dung dch 12. cht in phn. b- Phn bit in cc oxi ha kh v in cc ca nguyn t ganvani thng thng (v d cc in cc ca nguyn t Daniel-Jakobi). Cho v d v vit phng trnh th in cc. 13. sau: a- Vit cc phn ng in cc v phn ng tng qut xy ra trong cc nguyn t Ag, AgCl / KCl / Hg2Cl2, Hg (Pt) H2 / H2SO4 / Hg2SO4 , Hg b-Chng minh rng sc in ng ca pin (Pt) H2 (1at)/ NaOH (m1), NaCl (m2) / AgCl, Ag c xc nh bng cng thc: E = EO - RT ln KW.aCl F a(OH-) 14. Sc in ng ca nguyn t Cd / CdCl2 / AgCl, Ag bng 0,67V 25OC. H s nhit sc in ng bng -6,5.10-4V.-1. a- Vit phng trnh phn ng in cc v phn ng tng qut. b- Tnh G, S, H 298K i vi phn ng tng qut. 15. Tnh sc n ng ca nguyn t sau 298K Zn / Zn2+ // Cu2+ / Cu Vi a (Zn2+) = a (Cu2+). Bit O(Zn2+/Zn) = -0,76V ; O(Cu2+/Cu) = 0,34V. C th thay i nng Cu2+ v Zn2+ i chiu dng in trong nguyn t c khng? 16. Tnh tch s tan ca AgI 25OC. Bit rng 25OC khi thit lp mt pin gm in cc hidro tiu chun v mt n cc Ag trong KI 0,001M bo ha AgI th pin ny c sc in ng l 0,033V v dng in bn ngoi i t in cc Ag sang in cc hidro. Cho th oxy ha kh tiu chun ca Ag+ / Ag l 0,8V. 17. Da vo phn ng : 5Fe2+ + MnO4- + 8H+ = 5Fe3+ + Mn2+ + 4H2O Ngi ta ch to mt pin in nh sau: Nhng mt in cc P t vo mt cc ng dung dch KMnO4 c axit ha bng dung dch H2SO4. Mt in cc P t th hai nhng vo dung dch cha FeSO4. Hai cc ny ni vi nhau bng mt cu mui. Ni hai in cc P t vo vn k. a) Vit phn ng xy ra trn in cc m v dng ca pin. b) Cho bit chiu chuyn dch electron mch ngoi.

78

c) Cho bit chiu chuyn dch ion trong dung dch d) Sc in ng ca pin 18. Cho bit 0 ca Cd2+/Cd = - 0,403 V v I- /I2 = + 0,54V. a- Lp s mch in ha vi hot Cd2+ = 1 v hot I- = 1. Vit cc phn ng xy ra trong pin. Ch r in cc m v dng. b- Tnh sc in ng ca pin. 19. Nc oxy gi H2O2 c th b kh thnh nc vi th chun l o(H2O2/H2O) = 1,77V. Xt dung dch H2O2 nng 1mol/l. a- Vit na phn ng oxy ha kh ng vi s kh oxy gi thnh nc. b- Bit rng pH trong khong t 0 n 2, cc ion Fe2+ v Fe3+ tn ti trong dung dch v (Fe3+/Fe2+) = o(Fe3+/Fe2+) = 0,77V. Chng t rng ti pH = 1, ion Fe2+ b oxy ha bi H2O2. Vit phng trnh phn ng. 20. Cho bit th chun 25oC ca cc cp oxy ha kh sau: o1 = 1,233V MnO2 + 4H+ + 2e Mn2+ + 2H2O + MnO4 + 4H + 3e MnO2 + 2H2O o2 = 1,690V a- Tnh th chun 25oC (o3) ca cp MnO4-/Mn2+. b- M t th nghim xc nh o3. c- T th chun ca MnO4-/Mn2+, vit phng trnh phn ng ca ion MnO4trong dung dch c pH = 0 v tnh Go ca phn ng. Cho bit th chun ca cp O2/H2O l o(O2/H2O) = 1,23V.

79

Chng 6 Lp in kp trn ranh gii in cc dung dch 6.1. S hnh thnh lp in kp Khi nim lp in kp c s dng m t s phn b cc in tch hay cc lng cc trn ranh gii tip xc gia hai pha, hay nh hng trn b mt tip xc gia hai pha. Mc ch nghin cu lp in kp ng dng trong vic xc nh tc phn ng in ho v c ch ca cc phn ng , lm phng tin nghin cu ng hc cc qu trnh in ho. Lp in kp trn ranh gii pha in cc dung dch c th c hnh thnh trong cc trng hp sau: a- S trao i ion gia hai pha in cc v dung dch in li: Nu ho th ca ion kim loi trong kim loi ln hn ho th ca ion kim loi trong dung dch th ion kim loi t kim loi s chuyn vo dung dch, do b mt kim loi s tch in m v ht v pha mnh cc ion kim loi t dung dch v to thnh lp in kp. Ngc li, nu ho th ca ion kim loi trong kim loi nh hn ho th ca iom kim loi trong dung dch th ion kim loa s chuyn t dung dch ln b mt kim loi lm cho b mt kim loi tch in dng, do ht v pha mnh cc anion t dung dch v to thnh lp in kp. Nu ho th ca ion kim loi trong kim loi bng ho th ca ion kim loi trong dung dich th s chuyn ion t tng ny sang tng kia khng xy ra. B mt kim loi s khng tch in. Dung dch c gi l dung dch khng. b- Lp in kp cng xut hin do kim loi hp ph mt loi ion ca vht in li khc. Chng hn trn b mt cu in cc thu ngn ngm trong dung dch KI s c cc ion I- b hp ph. Cc anion I ny s ht li gn mnh cc cation K+ v to thnh lp in kp. c- Lp in kp cng xut hin do s hp ph nh hng cc phn t phn cc, cc cht hu c phn cc ... ln b mt kim loi. + + + + + + + + + + + + + + + + -

80

Hg

I-

K+ + + + + + + + +

+

Hg

Ngoi ra, ta cng c th dng dng in bn ngoi to thnh lp in kp. Cho kim loi vo dung dch cht in li cha ion kim loi ny, chng hn cho Hg vo dung dch KCl, nh dng in bn ngoi, b mt Hg s tch in m. Di tc dng ca lc ht tnh in cc cation K+ s b ht li gn b mt in cc v to thnh lp in kp. 6.2. Cc thuyt v cu trc lp kp

6.2.1. Thuyt HelmholtzHelmholtz a ra m hnh t in phng m t ranh gii gia in cc v dung dch (lp in kp). Theo thuyt Helmholtz, cc ion ngc du b ht mnh v lp in tch trn b mt kim loi, v vy lp kp l t in phng hon chnh c to thnh t hai bn in tch tra d . - Khong cch gia hai bn rt b, xp x ng knh phn t nc. - Th trong lp kp thay i tuyn tnh t bn in tch ny sang bn in tch khc.

Ion i solvat

Lp Helmholtz ngoi

Hnh 6.1: S phn b in tch ca lp kp theo Helmholt v s bin thin th

81

Mt in tch ca in cc: q = C. vi C: in dung lp kp C= 4d : hng s in mi ; d: khang cch hai bn : bc nhy th gia hai bn Suy ra: q= 4d

(6.1) (6.2)

(6.3)

Phng trnh (6.2) v (6.3) khng gii thch c trong thc t in dung C v mt in tch q ph thuc vo nng dung dch v th . Nh vy, m hnh ca Helmholtz ch l s gn ng th thin, v chuyn ng nhit khng cho php cc ion tri du phn b mt cch cng nhc i vi nhau.

6.2.2. Thuyt Gouy-Chapmann (thuyt khuch tn)Theo thuyt ny cc ion nghch c phn b khuch tn trong lp in kp. Lp kp c xc nh bi hai yu t: - in trng ca b mt in cc. - S chuyn ng nhit ca ion nghch. Kt qu l nng ca cc ion nghch s gim t b mt in cc vo su trong lng dung dch, ti n bng nng ca ion cng loi.Lp Helmholtz Ion nghch

Lp kp khuch tn

Hnh 6.2: S lp in kp theo Gouy - Chapmann

82

Ngc li, gn in cc nng ion cng du vi in cc bng khng v cng xa in cc nng ion ny cng tng. S bin i nng ny tun theo phng trnh Boltzmann: C+ = C.exp (-F/RT) C- = C.exp (F/RT) C+ , C- l nng cation v anion tng ng; C l nng chung ca dung dch; l th iin cc. Da vo nh lut thng k Boltzmann v phng trnh Poisson, GouyChapmann a ra biu thc v in tch lp kp khuch tn. QS = - q M = B dy lp khuch tn: d=RT 8CF 2

RT C i (e 2

Z i F RT

1)

(6.4)

(6.5)

Cc phng trnh ny dn n vic tnh ton in dung lp khuch tn: C=F RTZF ZF RT . Ci ( e 2 RT + e 2 RT ) 2

(6.6)

Cc phng trnh (6.4), (6.5), (6.6) m t s ph thuc ca cc i lng in tch, dy lp khuch tn v in dung vo nng C ca dung dch v bc nhy th . Tuy nhin, m hnh lp kp theo Gouy - Chapmann ch cho kt qu ph hp vi thc nghim vng nng thp ca dung dch; khi nng tng th s sai lch gia cc gi tr in dung l thuiyt v thc nghim cng ln. Nguyn nhn l do thuyt Gouy - Chapmann cho rng cc ion l nhng phn t c kch thc im, v vy chng c th li v cng gn b mt in cc. Ngoi ra, thuyt Gouy - Chapmann khng tnh n s hp ph c bit ca cc ion. V vy khng gii thch c hin tng i du in ca b mt.

6.2.3. Thuyt SternTheo thuyt Stern cc ion c kch thc xc nh; do tm im ca cc ion khng tin n st b mt in cc mt khong cch b hn bn knh ion. Gia cc ion v b mt in cc c mt tng tc c bit khng c tnh cht in. Tng tc l do trng lc phn t gy ra ti mt khong cch rt b cch b mt in cc. Do vy,83

lp ion trong dung dch u tin b ht vo b mt in cc bi in trng v trng lc hp ph to ra t in phng nh trong thuyt Helmholtz, nn th gim nhanh theo ng thng. Phn ion cn li c phn b khuch tn to nn lp khuch tn ca lp kp nh thuyt Gouy-Chapmann, do th gim t t. Nh vy, m hnh cu trc lp in kp ca Stern l m hnh tng hp ca hai thuyt Helmholtz v Gouy-Chapmann.Lp Helmholtz Ion i

Lp kp khuch tn B mt Helmholtz

Hnh 6.3: S lp in kp v s bin i th theo m hnh Stern M hnh lp in kp nh vy c coi nh gm hai t in ghp ni tip, ng vi in dung CH v Cd. Cng thc tnh in dung tng qut: v in tch: qS = -qM = qH + qd1 1 1 = + C CH Cd

(6.7) (6.8)

Vi qH , qd l in tch bn Helmholtz v bn khuch tn. Thuyt Stern m t ng ng cong in m qun v ng cong in dung thc nghim, gii thch mt cch nh tnh hin tng i du in ca b mt in cc v gii thch ng s ph thuc ca th khuch tn vo nng . 6.3. Phng php nghin cu lp kp

84

Mc ch ca cc phng php nghin cu cu trc lp in kp l xc nh s ph thuc ca in tch q ca b mt in cc v in dung C ca lp kp vo th . C ba phng php ch yu c p dng nghin cu cu trc lp kp l phng php ng cong in mao qun, phng php xc nh sc cng b mt trn in cc rn v phng php o in dung lp kp.

6.3.1. Phng php ng cong in mao qun. l phng php da vo s o sc cng b mt () ti ranh gii pha in cc Hg - dung dch ph thuc vo th in cc v nng dung dch. S my o hay cn gi my o in mao qun Gouy nh sau:

Hnh 6.4: S my o in mao qun Ngi ta thng nghin cu lp in kp trn in cc Hg v n l mt in cc phn cc l tng, th ca in cc Hg t bin thin theo dng ngoi v b mt in cc Hg ng nht v thnh phn ho hc. Ngm Hg vo dung dch Na2SO4, khi cha cho dng in ngoi vo in cc, b mt Hg mang in dng v cng tch ion Hg2+2 khi Hg ln hn cng kh hydrat ho ca n. in tch dng b mt Hg s ht anion SO42- vo gn mnh. Cc in tch dng trong Hg s y nhau, cc anion SO42- trn b mt Hg cng y nhau lm cho b mt Hg b cng rng ra, din tch b mt ln ln. Trong khi sc cng b mt ca Hg c khuynh hng lm cho b mt Hg gim i. B mt in cc ln ln lm cho sc cng b mt gim i. Nu by gi ta truyn cho in cc mt lng in tch m no , th in tch dng ca in cc s gim i , cc anion SO42- trn b mt cng gim i. V th, lc y gia cc in tch cng d gim lm cho din tch b mt Hg gim v sc cng b mt tng. C nh vy sc cng s tng tip nu in cc cn c truyn in tch m vo b mt in cc, lc ny Hg mang in tch m v s ht cc cation Na+ vo gn mnh. Th m truyn vo cng nhiu, in tch m trn Hg cng ln, cc ion Na+ cng

85

vo gn b mt cng nhiu. Cc in tch cng du cng y nhau nhiu. V th, din tch b mt in cc Hg cng tng v sc cng cng gim nu th m cng tng. Trn ng cong - min Hg mang in tch m, ng cong i xung. Khi ta t vo in cc Hg mt th m (phn cc catt) th sc cng b mt thay i v ng thi chiu cao h ca ct Hg cng thay i. Gia v h lin h nhau qua h thc: = k.h (6.9)

k l hng s my. Nh vy, bng cch o chiu cao h cc gi tr th p t khc nhau vo in cc v tnh ton theo (6.9) ta s xy dng th ng cong in mao qun biu din s ph thuc ca vo nh hnh 6.5 :

q>0

q0, do q >0 b mt in cc tch in dng. - nhnh phi d EI=0 . Nh vy, cng sut thc ca ngun in IEI b hn cng sut l thuyt cc i ca n IEI = 0 ; cn khi xy ra s in phn, cng sut tiu tn IEI ln hn cng sut cn thit v mt l thuyt. Do , h s tc dng hu ch khi h in ho lm vic nh hn 100%. kho st cc qu trnh in ho, ng hc in ho vn dng nhng qui lut chung nht ca ng ho hc nh khi nim tc phn ng, nng lng hot ho, nh hng ca nhit n tc phn ng. . . Song in ho hc cng c nhng qui lut ring ng gp vo vic nghin cu tc phn ng in ho. Khc vi cc phn ng ho hc ng th bnh thng, chng xy ra mt im trong lng dung dch, ni c cht phn ng; phn ng in ho xy ra trn ranh gii gia in cc v dung dch, tc l phn ng d th. Nh vy, mt qu trnh in cc lun lun gm hng lot cc giai on ni tip nhau: 1- Giai on chuyn ion (hoc phn t) t trong lng dung dch n mt ngoi ca lp kp, chuyn ion qua lp kp khuch tn. Ti y ion s tham gia vo bn dung dch ca lp kp Helmholtz, nm cch b mt in cc mt khong bng bn knh ion. Giai on ny xy ra ch yu l do s khuch tn ion t ni c nng cao (trong lng dung dch) n ni c nng thp (trn b mt in cc). b mt in cc cc ion c nng thp v chng b mt i trong qu trmh phn ng in ho. 2- Giai on phn ng in ho thun tu. Trong giai on ny cc ion tham gia trong lp kp Helmholtz s b mt lp v solvat v b thay i in tch. Trn catt ion s nhn electron ca in cc; trn ant ion nhng electron cho in cc. 3- Giai on to thnh tng mi , to thnh sn phm cui cng ca phn ng in ho. Nu sn phm l cht kh th giai on 3 ny c th chia thnh 3 giai on nh: - Cc nguyn t kh hp ph trn in cc s lin kt vi nhau to thnh cc phn t. - Cc phn t tp hp vi nhau to thnh bt kh. - Giai on tch bt kh khi b mt in cc. Nu sn phm l cht rn, giai on 3 l giai on to mng li tinh th. Nu sn phm l cc cht cn nm trong dung dch th giai on 3 l giai on hnh thnh phn t cc cht v tch khi b mt in cc i vo dung dch.

88

Giai on th nht v giai on 3 c cng qui lut v c gi l qu trnh chuyn cht. Cc giai on chuyn cht v phng in ion ho c mt trong tt c khng loi tr cc qu trnh in cc. Tuy nhin, ngoi nhng giai on chnh ny, th cc qu trnh in cc i khi b phc tp ho bi cc phn ng ho hc xy ra trc hoc sau phn ng in ho. Trong tin trnh ca phn ng in ho c th xy ra s chuyn dch cc phn t theo b mt (giai on khuch tn b mt) t cc trung tm xy ra s phng in n nhng v tr khc thun li hn v mt nng lng i vi cc sn phm ca phn ng. Nu nh b mt in cc mang in tch cng loi vi in tch ca cc phn t phn ng, th in trng ca lp kp s ngn cn s hp ph cc phn t ny v cn phi tnh n giai on a cc phn t phn ng vo lp kp. Theo ng ho hc th tc ca qu trnh gm hng lot giai on ni tip nhau c xc nh b giai on chm nht. Do , nu tc ca giai on in ho l chm nht th tc qu trnh c xc nh bi tc phn ng in ho. Nu giai on khuch tn l giai on chm nht th tc ca qu trnh c xc nh bng tc khuch tn. ng hc ca qu trnh do tc khuch tn quyt nh c gi l ng hc khuch tn. Qu 7.2. S phn cc in cc - Qu th S phn cc in cc c gy nn bi nhiu nguyn nhn nh s thay i nng cht phn ng gn b mt in cc, hoc bi cc giai on chm quyt nh tc phn ng. . . Nu bn cht ca giai on chm quyt nh tc phn ng c bit th khi khi nim s phn cc c thay bng kha nim qu th. Nh vy: Qu th l s phn cc in cc c gy nn bi giai on chm quyt nh tc phn ng in cc. Kha nim qu th i khi cng c s dng km vi tn ca mt phn ng in cc nghin cu nh: qu th hiro, qu th oxi. . .

7.2.1. Qu th khuch tn

Ta xt phn ng in ho kh ion kim loi hidrat Mn+.mH2O. Phn ng tng qut xy ra theo s sau: Mn+ .mH2O + ne M + mH2O

Qu trnh ny gm mt s giai on ni tip nhau: giai on khuch tn ion ti in cc, giai on phng in ca ion Mn+, giai on kt tinh. Nh ni trn, tc ca ton b qu trnh s c quyt nh bi giai on no c tc chm nht. Nu giai on chm nht l giai on khuch tn ion Mn+.mH2O n in cc th s phn cc s do giai on ny quyt nh; nng ion Mn+ b mt in cc s thay i khi c dng in chy qua. V th in cc b khng ch bi nng ion gn b mt in cc, nn s bin thin nng cht tham gia vo phn ng in cc lm cho th in cc b thay i, c ngha l in cc b phn cc. V vy, qu th c gy nn bi s phn cc ny gi l qu th khuch tn; v s phn cc trong trng hp ny cng gi l s phn cc nng .89

7.3.2. Qu th ho hcHu ht cc qu trnh in cc u c km theo s bin i ho hc thun tu. S bin i ho hc ny xy ra trc hoc sau qu trnh in ho v theo c ch ng th hoc d th. Chnh s bin i ho hc ny lm thay i nng cht phn ng b mt in cc v lm cho th in cc lch khi gi tr cn bng. Do , qu th xut hin trong trng hp ny gi l qu th ho hc. Qu th khuch tn v qu th ho hc thuc loi phn cc nng .

7.3.3. Qu th in hoNu trong qu trnh in ho giai on phn ng in ho thun tu (giai on chuyn electron b mt in cc) l giai on chm quyt nh tc qu trnh, th s phn cc in cc c gy nn bi giai on ny gi l s phn cc in ho. Qu th xut hin trong trng hp ny gi l qu th in ho. Nh vy, qu th in ho l s lch th in cc khi ga tr cn bng do cc phn ng in ho in cc gy nn. C hai loi phn cc in ho: phn cc ant v phn cc catt. S phn cc ant (qu th ant) c trng cho kh nng bt thun nghch ca qu trnh ant. S phn cc catt (qu th catt) c trng cho kh nng bt thun nghch ca qu trnh catt. (7.1) K = -c.b A = -c.b (7.2)

Qu th ph thuc vo mt dng, bn cht ca cc tiu phn tham gia phn ng in cc, vo vt liu v tnh cht ca b mt in cc, vo s c mt ca cht hot ng b mt, vo nhit ... S ph thuc ca vo cc yu t c biu th qua phng trnh kinh nghim Tafel: = a + blgi (7.3) i: mt dng; a,b l hng s. a: ph thuc ch yu vo vt liu in cc; b: c trng ch yu cho cc qu trnh in ho.

7.3.4. ngha ca qu thQu th c ngha ln trong in ho ng dng; do c qu th cao ca hiro m ngi ta c th gii phng kim loi t dung dch. Nhng gi tr ln ca qu th hiro v oxi cn l c s ca nhiu phn ng ho hc hoc oxi ho bng con ng in ho. 7.4. Th phn hu90

Trong qu trnh in phn cho cc qu trnh xy ra b mt in cc th ngi ta phi p t vo trong h mt hiu th : V = ( cbA -Kc.b) + (A -K) + IR. (7.4)

Vi i lng IR l st th ohm ca dung dch. Th V gi l th phn hu ca cht phn ng. 7.5. Tc qu trnh in cc

7.4.1. Mt dng ini v phn ng in ha tc phn c o bng s mol cht chuyn t tng ny sang tng khc trn 1cm2 b mt trong mt n v thi gian (mol/cm2.s). V cc tiu phn tham gia phn ng l cc phn t tch in, s chuyn ng ca chng to nn dng in; do trong h in ho thay cho vic biu th tc phn ng bng mol/cm2.s, ngi ta dng i lng in lng. Vi 1mol cht : nF Coulomb/mol Nh vy: i = n.F.V = ampe/cm2 gi l mt dng in v c xem l i lng o tc in ho. i=I S

(A/cm2)

(7.5)

I: cng dng in qua in cc; S: din tch in cc.

7.5.2. Tc qu trnh quyt nh bi tc phng in chmXt qu trnh in ho: oxh + ne kh

i : mt dng catt; i : mt d dng ant

Qu trnh in ho ny gm nhiu giai on ni tip nhau. Gi s giai on phng in l giai on quyt nh tc ca ton b qu trnh. T cc kt qu ca ng ho hc v ng hc in ho, ta c:i = nF.k0 .Cxy. e nF /RT

(7.7) (7.8)

i = nF. k0 . Ckh. e (1- ) n.F /RT

Vi n l s electron trao i, k0 v k0 l hng s ng vi hai qu trnh khi = 0; Cxy , Ckh l nng dng oxh v dng kh; l th in cc khng cn bng, l h s chuyn 0 < < 1. Nu i = i = 0 th h t trng thi cn bng ng vi th cn bng c.b v i0 c gi l dng trao di. Do : i = i = i0 = nF k0 . Coxy.e-.n.F/RT

= nF. k0 . Ckh. e (1-) n.F

/RT

(7.9)

91

V h trng thi cn bng nn: i = i - i = 0 Nu qu trnh in ho l khng cn bng; gi s qu trnh xy trn catt th i >i.

Tc phn ng s l: i k = i - i . ik = nF [ k0 . Coxy.e-.n.F V = - c.b , nn: ik = i0 { e-.n.F/RT /RT

- k0 . Ckh. e (1-) n.F

/RT

]

(7.10)

- e (1-) n.F

/RT

}

(7.11)

Phng trnh (7.11) l phng trnh Volmer-Butler v tc phn ng in cc.

7.3.2. Tc phn ng in cc c quyt nh bi tc giai on chuyn cht ti in ccNu xem s chuyn cht ti in cc l giai on chm nht quyt nh tc ca qu trnh, th s chuyn cht c thc hin bi: a- Hoc bng s khuch tn cc cht t lng dung dch ti b mt in cc khi c mt graien nng . b- Hoc bng s in chuyn ion ti in cc di tc dng ca in trng. c- Hoc bng s khuch tn i lu do s chuyn ng ca cc cht lng. Di iu kin no ta c th b qua s in chuyn v ch xem s chuyn cht ti in cc l do qu trnh khuch tn v khuch tn i lu m nhim. - Xt phn ng in cc: Mn+ + ne M Ta c c.b = 0 +RT ln Ci0fi0 nF

(7.12)

Ci0: nng ion kim loi trong dung dch; fi0: h s hot Khi c dng lu thng: = 0 +RT ln Cisfi0 nF

(7.13)

Cis : nng ion kim loi ti b mt in cc. V Cis # Ci0 nn ta c s phn cc: k = - c.b =RT lnCis/Ci0 nF

(7.14)

Tc qu trnh catt c biu th bng phng trnh:

92

iK = nFD( Ci0 - Cis)/ D: h s khuch tn, : b dy lp khuch tn. Khi Cis = 0 th ik t n gi tr gii hn iL iL = nFDCi0/ Chia (7.15) cho (7.16) ta c:C iS i =1- K o iL Ci

(7.15)

(7.16)

(7.17)

Thay (7.17) vo (7.14) ta c: k = ik = iL (1- enFRT ln(1 - ik/iL) nF/RT

(7.18) (7.19)

)

-Tng t ta xt cho qu trnh ant: M - ne Mn+ Ta c ia = iL(enF. - 1) (7.20)

Biu thc (7.19) v (7.20) l biu thc biu th tc qu trnh catt v ant ca phn ng in ho b khng ch bi giai on chuyn cht ti in cc. 7.6. ng hc mt s qu trnh in ho

7.6.1. Qu trnh gii phng hiroPhn ng thot hiro trong dung dch axit v trong dung dch kim xy ra theo cc cch khc nhau. i vi dung dch axit, H2 c thot ra t phn ng: 2H3O+ + 2e = H2 + H2O OH- : (a)

Trong dung dch kim, cc phn t H2O trc tip nhn e to thnh H2 v ion 2H2O + 2e = H2 + 2OH(b)

Phn ng (a) v (b) l phn ng tng qut ca s thot H2. Phn ng ny xy ra qua nhiu giai on ni tip nhau, v xy ra theo cc con ng khc nhau ph thuc vo iu kin phn ng.

93

nhau:

Phn ng phng in ca ion H3O+ c th xy ra qua cc giai on ni tip 1- Chuyn ion H3O+ t dung dch n b mt in cc H3O+d.d H3O+bm Giai on ny do qu trnh kghuch tn v in di m nhim. 2- Cc ion H3O+ b mt tham gia vo mt ngoi ca lp in kp. H3O+bm H3O+lk

Trong giai on ny cc ion H3O+ b thay i cu trc, chuyn t trng thi t hot ng sang trng thi hot ng hn. iu lm cho qu trnh chuyn dch electron t in cc sang H3O+ xy ra d hn. 3- Giai on phng in chm. Cc ion H3O+ than gia phn ng in ho in cc: H3O+ + e Hhp + H2O 4- Giai on kh Hhp : Sau khi phng in cc nguyn t H s b cc trung tm xc tc b mt in cc hp ph to thnh cc Hhp. Sau cc Hhp ny c th t hp vi nhau to thnh nhng phn t H2 hp ph b mt H2 hp. Hhp + Hhp H2bm 5- Giai on khuch tn phn t H2 ho tan t b mt in cc vo dung dch H2bm H2d.d 6- Giai on to bt kh t phn t H2 ho tan v tch khi dung dch. H2d.d H2kh Qua vic nghin cu cc giai on phng in ca ion hyro ta thy, ch c hai giai on c th l chm v gy ra qu th hiro: l giai on phng in ion hiro (giai on 3) v giai on kh hiro hp ph (giai on 4). Tng ng vi hai giai on xut hin hai thuyt ch yu v qu th hiro l thuyt phng in chm ca Volmer-Frumkin v thuyt t hp chm ca Tafel. Thuyt phng in chm ch ng trn nhng kim loi t hp ph hiro hay ni cch khc trn cc kim loi c qu th hiro cao v trung bnh. Thuyt t hp chm ch ng trn nhng kim loi hp ph hyro mnh, ngha l nhng kim loi c qu th hiro thp. Qu th hiro ca mt s kim loi tng theo dy sau:

94

Pt Pd W Ni Fe Ag Cu Zn Sn Pb

Hg

Bng 7.1: Qu th hydro trn mt s in cc in cc Pt (ph mui Pt) Pt (trn) Fe Cu C(graphit) Ni Ag Pb Hg Qu th (V) Ti i = 10-1 A/cm2 Ti i = 10 -2A/cm2 -0,03 -0,05 -0,10 -0,40 -0,56 -0,64 -0,58 -0,82 -0,70 -0,99 -0,75 -0,76 -1,08 -1,12 -1,04 -1,21

7.6.2. Qu trnh gii phng oxiQu trnh gii phng O2 c th xy ra theo nhng con ng khc nhau tu thuc vo thnh phn ca cc dung dch in phn. Trong dung dch kim, O2 c gii phng t s phng in ca ion OH4OH- - 4e = O2 + 2H2O (a)

Trong dung dch axit, phn t H2O b phng in to O2 2H2O - 4e = O2 + 4H+ (b)

Trong dung dch mui trung tnh O2 c th c gii phng bi s phng in ca ion OH- hoc ca phn t H2O. ng hc ca qu trnh gii phng O2 l mt c ch phc tp bao gm nhiu giai on v ph thuc vo nhiu yu t. Giai on chm ca qu trnh gii phng oxi c th l mt trong cc giai on sau: 1- S phng in ca ion OH- hay cu phn t H2O. 2- S kt hp ca cc nguyn t oxi. 3- S kh hp ph in ho ca gc hyroxyl OH. 4- S hnh thnh v phn hu ca cc oxit trung gian km bn ca kim loi in cc. Mt s c ch ca phn ng thot oxi t dung dch kim nh sau: 1. 2OH- - 2e = 2OH. 2. 2OH. + 2OH+ = 2O- + 2H2O 3. 2O- - 2e = 2O 1. 2OH- - 2e = 2OH. 2. 2OH. + 2OH- = 2O- + 2H2O 3. 2O- + 2MOx = 2MOx+1 + 2e95

4. 2O = O2 1. 4OH- - 4e + M = 4OH 2. 4MOH = 2MO + 2M + 2H2O 3. 2MO = 2M + O2

4. 2MOx+1 = 2MOx + O2 1. 2OH- - 2e = 2OH 2. 2OH + 2OH- = 2H2O23. 2H2O2- = O22- + H2O 4. O22- = O2 + 2e

7.6.3. Qu trnh gii phng kim loiS kh catt cc cation kim loi l c s cho nhiu qu trnh v c ng dng ln trong thc tin. in kt tinh kim loi c s dng trong luyn kim thu nhn kim loi sch t qung thin nhin. Qu trnh gii phng kim loi trn catt xy ra theo phn ng chung sau: Mn+. mH2O + ne = M + mH2O Qu trnh gii phng kim loi t dung dch ch xy ra khi th in cc: < 0 +RT lnCM nF

= c.b

Qa trnh ny c th xy ra qua nhiu giai on theo cc cch khc nhau ph thuc vo bn cht kim loi v dung mi, vo iu kin kt tinh ca kim loi. Sau khi chuyn cc ion kim loi ti b mt in cc bng s khuch tn, kim loi c th kt tinh ln b mt bng nhiu cch khc nhau: 1. Ion kim loi c th b hp ph ln b mt in cc Men+ Mehpn+ Sau ion kim loi hp ph s b phng in to nguyn t kim loi Mehpn+ + ne Me Cc nguyn t kim loi ny s lin hp vi nhau to thnh cc tinh th rt nh, hoc s di chuyn trn b mt kim loi cho n khi ri vo mng li tinh th ca kim loi. 2. Ion kim loi b hp ph ln b mt kim loi Mehpn+. Cc ion b hp ph ny di chuyn trn b mt in cc n mt v tr thun li nht v mt nng lng s b phng in v nm lun vo mng li tinh th. 3. Ion kim loi c th va b hp ph, va b thay i cu trc ( b lp v hirat, gim s phi tr trong phc...), sau li bin i theo mt trong hai cch trn. 4. Ion kim loi c th va b hp ph, va phng in ri nguyn t kim loi s di chuyn trn b mt in cc nh trn.

96

Hnh 7.1: S qu trnh sp xp cc nguyn t kim loi Cc nguyn t kim loi to thnh s kt hp vi nhau to thnh cc tinh th nh. Trn b mt in cc xut hin mt tng mi. Vic nghin cu ng hc qu trnh gii phng kim loi gp nhiu kh khn do bi s thay i th theo thi gian, s thay i b mt in cc v nhiu yu t khc.

Cu hi v bi tp 1. Hy nu nhng c trng chung ca qu trnh in cc. 2. Qu th l g? Hy cho bit th t phng in ca cc cation sau trn catt Fe: Al3+, Fe2+, Zn2+, H+? Cho bit O(Al3+/Al) = -1,67V; O(Fe2+/Fe) = -0,44V; O (Zn2+/Zn) = -0,76V ; O(H+/H2) = 0,00V v Zn/Fe = Al/Fe = Fe/Fe = 0,00V; H2/Fe = -0,80V (S dng khi nim qu th gii thch). 3. Cn phi in phn trong bao lu mt dung dch NiSO4 vi cng dng l 2A thu c hon ton Ni trn catt, bit rng hiu sut dng bng 90%, th tch dung dch NiSO4 l 0,5 lit v nng dung dch l 0,1N. 4. in phn mt dung dch mui km vi catt bng Fe c din tch 1.000cm2. Xc nh b dy ca lp km ph nu in phn trong 25pht vi mt dng trung bnh bng 2,5A/dm2. Khi lng ring ca km bng 7,15g/cm3. 5. Sc in ng ca nguyn t: (-) Pt, H2 / HClO4 / / HClO4 / O2, Pt (+) bng 1,23V. Phn ng in cc v phn ng chung ca pin ny xy ra nh th no? chuyn phn ng chung theo chiu nghch cn mt th bng 1,7. Hi qu th bng bao nhiu? 6. Bit qu th hidro trn in cc Ni bng 0,21V v qu th oxi trn in cc Pt bng 0,46V, Hy tm th phn hy NiSO4 khi in phn dung dch ny bng cc in cc Pt v cho bit trn catt c th xy ra qu trnh thot Ni hon ton m khng c s gii phng kh hidro khng?

97

Chng 8 Mt s ng dng ca lnh vc in ho

8.1. Mt s khi nim c s Cc phng php in ho c s dng rng ri trong cc lnh vc khc nhau ca k thut v sn xut hin i, chng to thnh c s khoa hc in ho ng dng. in ho ng dng s dng mt s khi nim c s sau:

8.1.1. nh lut Faradaya- nh lut Faraday th nht: Khi cho dng in mt chiu i qua dung dch

cht in li (hay cht in li nng chy) th khi lng cc cht thot ra ant hay catt t l vi in lng Q i qua dung dch (hay qua cht in li nng chy), tc l t l vi cng dng I v thi gian t.m = K. Q = K.I.t (8.1)

Trong : m l khi lng cht thot ra trn mt in cc, tnh theo gam. I l cng dng tnh theo ampe (A) t: thi gian in phn (h) K: ng lng in ho (g/A.h) b- nh lut Faraday th hai: Nhng lng in nh nhau khi in phn s lm

thot ra nhng lng tng ng cc cht khc nhau. lm thot ra mt ng lng gam mt cht bt k no cn tiu tn mt in lng l 96.500 Coulomb (1 C = 1A.s). V 1F = 96.500 C = 26,8 A.h. Ta c biu thc chung cho nh lut Faraday:m= A I .t n.F

(8.2)

Nu t tnh bng gi (h) th F = 26,8 v t tnh bng giy (s) th F = 96.500.

8.1.2. in lng k (Coulometer) xc nh in lng tiu tn cho mt qu trnh in ha ngi ta tin hnh qu trnh in phn trong mt in lng k v o khi lng cht thot ra trn b mt in cc. Thng thng ngi ta thng o khi lng kim loi kt tinh trn mt in cc catt tr nh Pt. T khi lng kim loi thu c, in lng Q c xc nh bng cng thc sau:Q= m M / zF

(8.3)

in lng k thng c s dng l in lng k bc, gm mt cc bng Pt cha dung dch AgNO3 30% v mt thanh Ag nh hnh 8.1.98

Thanh Ag Bc xp

Cc Pt

Hnh 8.1: in lng k bc Cc Pt c ni vi cc m v thanh Ag c ni vi cc dng ca ngun in. Khi cho dng in i qua, th trn cc in cc xy ra phn ng sau: - Ti cc dng (Ag): Ag - Ti cc m (Pt): Ag+ + e

Ag+ + e Ag

Nh vy, ti ant thanh Ag tan ra, ti catt (cc Pt) Ag c kt tinh v bm vo cc Pt. ng thi xung quanh thanh Ag c bc bi mt li xp nhm ngn cn cc cht rt rn ri ra t in cc bc (do s ph hu c hc in cc Ag) trong qu trnh in phn ln b mt cc Pt. Cn cc Pt trc v sau khi in phn xc nh lng bc to thnh (m) v tnh in lng:Q(Coulomb) = m(mg ) 1,118

(8.4)

8.1.3. Hiu sut dng inTrong qu trnh in phn, lng cht thot ra in cc thng nh hn lng cht tnh theo nh lut Faraday. Nguyn nhn l do ngoi cc phn ng chnh lun c km theo cc phn ng ph nh s thot H2 catt v s thot O2 ant. T s lng cht thu c v lng cht tnh theo l thuyt c gi l hiu sut dng (Y). Y =mlt .100% mtn

(8.5)

8.1.4. Pin in ha v bnh in phnTrong in ha c hai qu trnh c th c xem l tri ngc nhau l pin in ha v bnh in phn. Pin in ha l h in ha bin nng lng ca phn ng99

oxi ha - kh xy ra trn hai in cc thnh nng lng in. Ngc li, bnh in phn l h in ha s dng nng lng in bn ngoi tin hnh phn ng oxi ha - kh trn b mt in cc. Mt pin in ha v bnh in phn gm c hai in cc (vt dn loi I) v h in ly (vt dn loi II). Trong c hai h in ha trn, th in cc ant l ni xy ra phn ng oxi ha v in cc catt l ni xy ra phn ng kh. - Ant l in cc xy ra phn ng oxi ha: V d: Zn - 2e Zn2+

4OH - - 4e O2 + 2H2O Catt l in cc xy ra phn ng kh: V d: 2H+ + 2e H2 Cu2+ + 2e Cu Khi pin in ha v bnh in phn hot ng th lun lun c mt dng in chy qua hai in cc v h in ly. Theo quy c th dng in chy t in cc dng sang in cc m v dng electron chuyn t cc m sang cc dng. Do vy, chng ta cn lu du ca cc in cc trong pin in ha v bnh in phn. Xt pin in ha: (-) Zn / Zn2+ (1M) // Cu2+ (1M) / Cu (+)wire

elect rons

Zn

Cu salt mui

bridge

Cu

Zn2+ ions

Cu2+ ions

Hnh 8.2: S pin in ha Zn - Cu * in cc Zn c th m hn in cc Cu nn Zn ng vai tr in cc m v Cu l in cc dng. Khi pin hot ng, dng electron s chy t cc Zn sang cc Cu v lm cho th ca in cc Zn phn cc v pha dng hn th cn bng cb Zn / Zn , do cn bng ca in cc Zn dch chuyn v pha ho tan Zn:2+

100

Zn - 2e Zn2+ y l phn ng oxi ha, nn in cc Zn (cc m) gi l in cc ant. * in cc Cu nhn dng electron t Zn nn th ca in cc Cu b phn cc v pha dng hn th cn bng cb Cu / Cu , do cn bng ca in cc Cu dch chuyn v pha to Cu: Cu2+ + 2e Cu2+

y l phn ng kh, nn in cc Cu (cc dng) gi l in cc catt. Xt bnh in phn: (-) Zn / dung dch CuSO4 / Cu (+)

Hnh 8.3: S bnh in phn dung dch CuSO4 * in cc Zn c ni vi cc m ca ngun in nn b mt in cc Zn d electron v di tc dng ca dng in, ion Cu2+ trong dung dch di chuyn n b mt in cc Zn. xy ra phn ng kh ca Cu2+: Cu2+ + 2e Cu trn catt, nn y l phn ng xy ra trn catt, nn catt l cc m. * in cc Cu c ni vi cc dng ca ngun in nn b mt in cc Cu thiu electron. Do , cn bng ca in cc Cu dch chuyn v pha ho tan Cu (phn ng oxi ha Cu): Cu - 2e Cu2+ ant, nn y l phn ng xy ra trn ant, nn ant l cc dng. vy: Nh vy:101

1- i vi pin in ha: catt l cc dng (+) v ant l cc m (-). (2- i vi bnh in phn: catt l cc m (-) v ant l cc dng (+). (3- Catt l ni xy ra qu trnh kh v ant l ni xy ra qu trnh oxi ha (d ha l pin hay bnh in phn).8.2. in kt tinh kim loi S kh catt cc cation kim loi l c s cho nhiu qu trnh in kt tinh kim loi v c ng dng trong thc tin. in kt tinh kim loi c s dng trong luyn kim thu nhn kim loi sch t qung thin nhin. Mt s kim loi c sn xut v tinh ch bng phng php in kt tinh nh Cu, Zn, Ni, Cd, Sn, Pb, Ag, Au, Mn, ... Theo nhit ng hc th qu trnh phng in cu cc cation kim loi t dung dch c th xy ra khi iu kin sau y c tho mn: < 0 +RT ln [Men+ ] = c.b nF

(8.6)

Phng php in kt tinh kim loi c s dng to mt mng mng kim loi ln mt kim loi, cn gi l phng php m in. Ngy nay, k thut m kim loi tr thnh mt k thut pht trin mnh m hu ht cc nc trn th gii, phc v cho mi ngnh khoa hc k thut, sn xut v i sng ca con ngi. in kt tinh kim loi c s dng to lp m trn b mt cc chi tit my, dng c sinh hot, phng tin sn xut, giao thng vn ti, khai thc m i cht, thng tin lin lc, k thut in, c kh chnh xc, thit b y t, dng c phng th nghim... M kim loi c kh nng to ra lp m bn chc, chu mi mn, chu p lc... i vi cc chi tit s dng trong ngnh k thut. K thut m cn gip phc hi cc chi tit my mc qu gi, cc mch in trong k thut in t. M kim loi khng ch nhm mc ch bo v kim loi nn khi b n mn, m cn c tc dng bo v, trang tr cc tc phm ngh thut. 8.3. Mt s ng dng trong phn tch in ho Cc phng php in ho c ng dng rng ri khi tin hnh phn tch trong khi kim tra cc qa trnh sn xut khc nhau v trong nghin cu khoa hc. Phng php chun o dn in da trn cc qui lut dn in ca dung dch, y l php o dn in ca dung dch nghin cu. Phng php ny cho php tm hm lng ca cc cht ring r trong dung dch, nu ta xy dng c ng chun ca s ph thuc dn in vo nng cht nghin cu. Nh phng php ny c chnh xc cao, nn ngi ta xc nh tan ca hp cht kh tan v hng s cn bng ion. Trong phng php chun dn, ngi ta dng cc php o dn xc nh im cui ca php chun . Php chun dn dng ngay trong cc phn ng to thnh kt ta.

102

Php o th in cc l c s ca phng php in th. Tu theo loi phn ng c dng khi chun , ngi ta chia php chun in th theo phng php kt ta, to phc, trung ho v chun oxi ho -kh. Trn c s nh lut Faraday ngi ta a ra cc phng php phn tch in trng lng v php o Coulomb. Phn tch in trng lng gm s tch trn in cc cu t xc nh dng sn phm bit (kt ta kim loi, oxit, mui...) v ghi s thay i khi lng sau ca in cc. Trong php o Coulomb, hm lng ca cht c xc nh theo lng in tiu tn bin i in ho n thnh sn phm khc. Tu thuc vo thnh phn ca dung dch nghin cu, php phn tch in lng v Coulomb c tin hnh trong ch dng tnh (galvanosatic) hoc th tnh (potentiostatic). Trong s cc phng php phn tch in ho, phng php cc ph v nhiu bin th ca phng php cc ph chim v tr c bit v tnh ph bin v c ng dng rng ri, bi v phng php cc ph c nhy cao 10-7 - 10-8 mol/l.

a) Nguyn tc chung ca phng php cc phPhng php phn tch cc ph do Jaroslav Heyrovsky (Tip Khc) pht minh nm 1922 v da trn qu trnh phn cc catt Hg. Ni dung ca phng php l theo di s bin i gia cng dng v th trong qu trnh in phn trn in cc git Hg khi cht phn ng chuyn n catot ch bng cong ng khuch tn. Cu to bnh in phn gm: - Catt lm bng mao qun 0.03 0.05 mm cha Hg. o th catt thng s dng in cc Calomen bo ho. - Ant l lp Hg c din tch ln gp 100 ln din tch catt.

Hnh 8.4: S bnh o cc ph c in Khi in phn, in p E t vo 2 cc l: E = A- K + IR R l in tr thun cht in phn, do nng cht in phn nn rt ln nn in tr R rt b v gim th IR rt nh c th b qua.

103

Cng dng in dng in phn rt b, khong 10-6A nn mt dng ant rt nh. Khi thay i in p E trong gii hn nht nh, mt dng ant coi nh khng ., do th A coi nh khng i. Tri li in tch catot rt b, mt dng catot rt ln v vy coi nh in p E tc dng ch yu cho qu trnh phn cc catot: E = - K Bng cch chuyn dch con chy trn R vi tc 0.1V/s v ghi chp cng dng in trn in k A, ta xy dng c cc ph c dng nh sau:

i

1 /2

K

Hnh 8.5: Dng ng cong th - dng trong phn tch cc ph Do s c mt ca mt lng ln cht in li nn, v th ion Mn+ chuyn n b mt in cc bng con ng khuch tn, tc khuch tn t l vi gradien nng , Mn+ trong dung dch v nng ca n trn b mt git Hg. Khi t n th kh ion phn tch: Mn+ + ne + Hgn+

M(Hg)

Do nng ion Mn+ trn b mt git Hg gim xung tc khuch tn ca M tng ln, cng dng tng ln. Khi nng ion Mn+ trn b mt Hg bng khng, tc khuch tn t n tc cc i v t l vi Mnn+ trong dung dch. Cng dng in t n gi tr gii hn iL, thng gi l dng khuch tn gii hn. Khi d tip tc tng th th cng dng cng khng tng na anot c qu trnh: Hg - 2e + Cl- Hg2Cl2 Calomen to ra trn chnh b mt in cc anot, hnh thnh in cc calomen. Khi gim nng phn tch ion Mn+, th phn tch chuyn dch v pha m ta thu c mt h cc ng cong, nhng cc ng cong ny u i xng vi th bn sng 1/2.

b) Phng trnh dng khuch tn id

104

Git Hg chy t ng mao qun chuyn ng theo chiu thng ng trong lng cht lng, bn knh git Hg b thay i thng xuyn v b mt git Hg ln dn t 0 n gi tr cc i S. Tc khuch tn ion phn ng n b mt git Hg phi thay i theo thi gian ngha l dC / dt 0, do dC / dt const. tm gradien nng dC / dt trong iu kin khuch tn khng n nh, ta s dng nh lut Fiek II . Trong to cu:dC 2 C 2 x C = D. 2 + . . dt 3 t x x

(8.7)

Gii phng trnh (8.7) vi cc iu kin: Cx,t = Co t = 0, x = 0 Cx,t = C t > 0 , x 0 =>

(C o C ) dC = 1/ 2 dx 3 . .D.t 7

(8.8)

Gradien nng dC/dx thay i theo thi gian nn dng khuch tn id cng thay i theo thi gian.it = nFD Co C dC = nFD 1/ 2 dx 3 Dt 7

(8.9)

Gi S l din tch in cc git Hg, th cng dng:I t = S .it = n.F .D.S Co C 3 Dt 7 1/ 2

(8.10)

Din tch b mt git Hg (S) c xc inh nh sau: Gi m l tc ri ca git Hg tnh ra mg/s th khi lng 1 git Hg l: Q = m.t = 4/3. .r3. Q: khi lng 1 git Hg t: Thi gian to ra 1 git Hg : khi lng ring Hg = 13,534 g/cm3 T (8.11) ta c: (8.11)

105

3.m.t r = 4. .

2/3

(8.12)

Thay S theo r vo (8.10) ta c: 3.m.t I t = n.F .D.4 4. . 2/3

.

Co C 3 . .D.t 7 1/ 2

(8.13)

Thay cc gi tr , vo (8.13) v n gin ta c: It = 0.732 n.F.m2/3. t1/6. D1/2. (Co- C) t(s); Co, C (mmol/l); D(cm2/s); m(mg/s); It (A = 10-6A) Khi th tch tng ln th C = 0, ta c dng gii hn: IL = 0.732.n.F.m3/2.t1/6.D1/2.Co (8.15) (8.14)

Dng IL theo (8.15) l dng t bin. Trong thc t ngi ta khng o dng t bin m o tr s trung bnh ca dng ny nh dao ng k in t:IL = 1

I L dt = a.0

t 0

1

1/ 6

dt

IL =

6 1/ 6 a 7

Vi a = 0,732.n.F.Co.m2/3.D1/2 V phng trnh Inkovit vi dng trung bnh l:I L = 0,627.nF .m 2 / .D1 / 2 1 / 6 .C 0

(8.16)

l chu k ri ca git thu ngn.

C) Phng trnh sng cc ph thun nghchTa ch xt cc qu trnh in cc ng vi trng thi cn bng. Cng dng ch ph thuc vo tc khuch tn cht t lng dung dch ti b mt in cc v t b mt in cc i ra. Mn+ + ne + Hg M(Hg)

106

Qu trnh c cc giai on sau: =o +Cs RT ln M nF C S

CSM: Nng kim loi trn b mt catt CS : Nng ion kim loi trn b mt git Hg Qu trnh khuch tn ion Mn+ n b mt catt c m t bng phng trnh Inkovit: I = 0.732.n.F.m2/3.t1/6.D1/2.(Co- CS) = a1(Co - CS) = IL - a1CS Vi a1 = 0.732.n.F.m3/2.t1/6.D1/2 => CS =IL I a1

(8.17)

S khuch tn nguyn t kim loi t b mt catot vo trong lng Hg c nng nguyn t kim loi CoM cng tun theo phng trnh Inkovit: I = 0.732.n.F.m3/2.t1/6.D1/2.(CSM - CoM) I = a2 (CSM - CoM) (8.18)

Thc t th nng kim loi trong lng git Hg CoM = 0. Do , phng trnh (8.18) tr thnh: I = a2 CSM => CSM =I a2

(8.19)

Thay CS (8.17) v CSM (8.19) vo phng trnh th:(I L I ) a1 RT = o + ln I nF a2

=o +

Cs RT ln M nF C S

hay = o +

RT a 2 RT I L I ln + ln nF a1 nF I

(8.20)

107

v

a 2 DM RT DM RT I L I , nn (14) => = o + = ln + ln nF D nF I a1 D

= 1 / 2 +

RT I L I ln nF I

1/2 khng ph thuc nng ion trong dung dch m ch ph thuc vo bn cht ion v v nht ca Hg v nc gn ng nht nn D ~ DM, nn: 1/2 ~ 0 Mi cation Mn+ c mt th bn sng 1/2 xc nh, v vy da vo gi tr th bn sng ta xc nh c cation Mn+ v o dng gii hn xc nh nng Mn+ trong dung dch.

Hnh 8.6: Dng ng cong phn tch cc ph ca dung dch cha nhiu ion Mn+ Ngoi ra, ngi ta cn dng phng php in ho nghin cu cu to lp in kp; qu trnh hp ph hiro, oxi, cc cht v c v hu c trn cc in cc; nghin cu ng hc v c ch cc qu trnh in cc. 8.4. Ngun in ho hc in Ngun in ho hc l thit b cho php bin i trc tip nng lng ho hc thnh nng lng in. Trn c s cc qu trnh oxi ho-kh khc nhau c th to nn rt nhiu ngun in ho hc. Song ch c mt s h in ho p ng yu cu thc t t ra. Sau y l nhng yu cu ca cc ngun in ho hc hin i. 1- C kh nng cho i lng sc in ng ln: i lng sc in ng ca ngun in c th tnh theo cc qui lut nhit ng in ho. Song cn phi tnh n kh nng khi t c hiu th xc nh, trn cc in cc ca ngun in xut hin kh nng xy ra phn ng c s tham gia ca dung mi.108

V d trong cc dung dch nc, khi hiu th ca cc in cc vt 1,23V th trn cc in cc xy ra phn ng thot hiro v oxi. V vy vic to nn ngun in vi dung dch nc ca cc cht in li c E ~ 1,23 V l khng c kh nng.Tuy nhin, trong thc t c th to c cc ngun in vi dung dch nc ca cc cht in li c E > 1,23V. iu ny c gii thch rng dng trao i ca cc phn ng tch hiro v oxi trn hng lot in cc l b, tc l cc qu trnh ny xy ra vi qu th ln. Do , sc in ng ca cc ngun in dng dung dch in li nc t ti 2,0 - 2.2V. 2- C s sai lch hiu th gia cc in cc so vi sc in ng khi ngun in lm vic rt b. S sai lch ny gn lin vi s phn cc cc in cc ca ngun v s h th trn in tr ni Rn ca ngun. Hiu th U hai u in cc chu ti l : U = E - K - A vi K, A l phn cc catt v ant, I l cng dng in. Vic tng U c th t c bng hai cch. Mt mt lm gim s phn cc ant v catt nh to nhng iu kin ti qu trnh in ho xy ra cc nhanh. Mt khc lm gim ti a in tr ni R ca ngun in bng cch lm gim khong cch gia cc in cc hay dng cc cht c dn in cao... 3- C in dung ca ngun ln, tc l d tr in nng ca ngun tnh cho mt n v khi lng hay n v th tch ln. Ngi ta xc nh i lng ny nh phng in, chnh l s ph thuc gia in p ca ngun vo thi gian phng in khi I = const. 4- C cng sut ring ln, tc l c lng nng lng ln cc i m ngun cung cp trong mt n v thi gian ca mt n v khi lng hay th tch ca ngun in. B mt tip xc ca vt liu in cc vi dung dch l mt trong nhng yu t quan trng quyt nh i lng ny. V vy ngi ta thng dng in cc xp c b mt ln. Ngoi ra, vic dng cc in cc nh vy s lm gim s phn cc in cc, do ti I cho mt dng b gim v S ln. 5- C s t phng in thp. T phng in l s mt mt in dung ca ngun khi mch h. Nguyn nhn t phng in l do s hnh thnh cc vi nguyn t cc b trn in cc, dn ti s tiu tn cc cht hot ng in ho mt cch v ch v lm hng in cc. Cc ngun in ho hc c chia thnh 3 nhm: cc ngun s cp, cc ngun th cp v my pht in ho. a- Ngun s cp: - Pin Leclanch: (-) Zn / NH4Cl (20%), ZnCl2 / MnO2, C (+) c E = 1,5 - 1.6 V.

109

Ant

Catt

Lp cch in Np thp Keo dn Lp bt Catt C

NH4Cl. ZnCl2, MnO2, bt nhoLp ngn xp ng km ant V bc ngoi

Hnh 8.7: Cu to pin Leclanch Phn ng xy ra ti cc in cc nh sau: - Ant: - Catt: Zn Zn2+ + 2e 2MnO2 + 2NH4+ + 2e 2MnOOH + 2NH3

V phn ng cht in ly: Zn2+ + 2NH3 [Zn(NH3)2]2+ Phn ng tng cng xy ra trong pin: Zn + 2MnO2 + 2NH4Cl 2MnOOH + [Zn(NH3)2]Cl2 - Pin km-khng kh: Zn / NaOH / O2 (C) c E =1,4V Zn + NaOH + 1/ 2O2 NaHZnO2 - Pin oxit thu ngn: Zn / KOH / HgO / C c E = 1,34V Pin ny c cng sut ring ln, t phng in thp, phn cc b v c th sn xut di dng vin nh. Zn + HgO + 2KOH K2ZnO2 + H2O + Hg Cc ngun s cp s hng sau khi tiu ph ht cc cht tham gia phn ng to dng.

110

b- Ngun th cp: Ngun th cp hay cn gi c qui, l nhng nguyn t galvani cho php s dng nhiu ln. Cc cht to dng ban u s c ti to khi dn qua c qui ngun mt chiu t bn ngoi. - c qui ch: c quy ch c pht minh nm 1859 bi nh vt l Php Gaston Plant. S cu to ca c quy ch: (-) Pb, PbSO4(r) / H2SO4 (32 - 34%) / PbO2 / Pb (+) E = 1,95 - 2,15V Cc phn ng xy ra trn in cc: - Ti in cc m:phongdien Pb -2e + SO42- PbSO4 (r) napdien

-

Ti in cc dng:phongdien PbO2 (r) +2e + 4H+ + SO42- PbSO4(r) + 2H2O napdien

Phn ng tng cng:phongdien Pb + PbO2 + 2H2SO4 2PbSO4 + 2H2O napdien

-

c qui kim- Fe hay Cd- Ni: (Cd) Fe / KOH / NiOOH / Ni c E = 1,3 - 1,4V Fe + 2NiOOH + 2H2O 2Ni(OH)2 + Fe(OH)2 Cd + 2NiOOH + 2H2O 2Ni(OH)2 + Cd (OH)2

- c qui bc-km: Zn / KOH + K2ZnO2 / Ag2O / Ag c E = 1,85V 2Zn + 2Ag2O + KOH 2K2ZnO2 + 4Ag + 2H2O c qui bc - km l ngun in c cng sut ring v in dung ring cao. - c qui Ni- Zn: Zn / KOH + K2ZnO2 / NiOOH / Ni E = 1,7V 2NiOOH + Zn + 2KOH 2Ni(OH)2 + K2ZnO2 c- My pht in ho:111

My pht in ho hay pin nhin liu l thit b bin trc tip ho nng thnh in nng b qua giai an tch t nng lng nh i vi cc ngun in ho hc khc (pin, c qui). Trong cc pin nhin liu, cht oxi ho hu nh dng oxi nguyn cht hay oxi khng kh. Cht kh (hay gi l nhin liu) l hiro, metanol, hyrazin, axit focmic, oxitcacbon, hyrocacbon, than... Cho n nay mi c ba loi pin nhin liu vi cc cht nhin liu H2, hirazin, metanol c ng dng thc t; v t c thnh tu ln nht l ch to pin nhin liu H2 - O2. Trong nhng nm gn y, nhng nhm ngun in ho mi dc ch to. l nhng ngun dng cc kim loi kim v nhng ngun cc cht in li rn c dn in ion cao.

Hnh 8.8: S pin nhin liu H2 - O2 hp 8.5. Tng hp cc hp cht hu c - v c bng phng php in ho T th k 19 Reynol v Irman bit s dng in ho vo vic tng hp cc cht hu c nhm nng cao hiu sut phn ng hu c. Grothus quan st c mt hin tng l th: Khi in phn indigo trng trong mi trng kim, trn ant c cn mu xanh. Cn ny mt i khi i chiu dng in. Nhng nguyn tc in kh v in oxi ho ch c thit lp t sau pht hin ca Kolbe nm 1845 khi ng nghin cu vic th clo bng hidro trong axit clometylsunfonic. Trong nhng nm cui th k 19 Gatterman, Haber, Tafel c cng ng gp rt nhiu vo vic nghin cu cc quan h nh lng khi nghin cu in kh hu c. Ngy nay phng php in kh in ho c s dng kh ph bin nhiu nc sn xut cc hp cht hu c c gi tr cao, p ng nhu cu cho cc ngnh dc phm, m phm v cng nghip ho cht. Do cc phn ng in ho dng tc nhn oxi ho kh l dng in mt chiu, nn trong mt s trng hp phng php tng hp in ho cc hp cht hu c c tnh u vit hn so vi cc phng php ho hc v tit kim ho cht, d iu khin,

112

phn ng xy ra trong iu kin nhit v p sut bnh thng. Sn phm to ra c tinh khit cao, chn lc cao, d tch; do cho php h gi thnh v nng cao hiu qu kinh t, k thut ca phn ng tng hp cc hp cht hu c. Tng hp in ho hu c thng l s kt hp ca 2 qu trnh: Qu trnh in ho (E) v qu trnh ho hc (C). Th t ny c th c lp li hoc qu trnh (C) v (E) c th c kt hp trong nhng cch khc nhau nh E.E.C, E.C.E, C.E.C.E; cng c th l C.E. C s ca qu trnh (E) l s trao i electron gia in cc v cc cht phn ng, vo loi qu trnh oxi ho kh. Cc qu trnh (E) c th xy ra nh sau:A2+ A+ A AA A2A+

Nu tt c cc yu t ca iu kin th nghim trong tng hp in ho c gi khng i, th yu t c bn nh hng n qu trnh in ho (E) l th in cc. C s ca qu trnh ho hc (C) trong qu trnh in ho l s tham gia phn ng ca cc cht trung gian c hnh thnh t qu trnh (E). Cc cht trung gian ny c th tham gia phn ng cng, tch, phn ng th... to thnh cc sn phm c cu trc mong mun. Sn phm cui cng to thnh ph thuc vo dung mi, bn cht ca cht in li, vt liu in cc, mt dng hay th in cc, nhit , pH... S chung cho qu trnh in ho c th c m t nh sau:Anode Cathode e Process E e Reactive intermediate Prcess C Substrate Product

Sn xut cc hp cht ho hc bng cc phng php in ho l mt ngnh ln ca cng nghip ho hc hin i. Vic iu ch xt, clo bng phng php in ho trn c s in phn dung dch NaCl chim v tr hng u trong nn in ho qui m ln. in phn nc thu hiro v oxi cng nh nc nng bit t lu v da trn qui lut tch hiddro trn catt v oxi trn ant. Sn xut in ho hipoclorit, axit cloric, peclorat, H2O2, MnO2 v cc cht v c khc c ngha quan trng trong cng nghip. Cc phng php in ho m ra kh nng to ln trong tng hp cc hp cht hu c khc nhau.

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Bng phng php in ho ngi ta c th thc hin mt s qu trnh chuyn ho cc hp cht h c m bng phng php ho hc thun tu khng thc hin c hoc kh tin hnh. Qu trnh tng hp cc hp cht hu c bng phng php kh in ho c ng dng rng ri nh qu trnh kh axetylen thnh etilen, kh axit phtalic thn