AN MON VA CHONG AN MONCHNG III AN MON IEN HOA

ay la loai an mon pho bien nhat. ac biet no pha huy cac vat lieu khi e ngoai tri An mon kh quyen (an mon am) III.1. AI CNG@ ac trng

An mon ien hoa la qua trnh hoa tan cac kim loai do s trao oi cac electron khi tiep xuc vi dung dch ien ly.@ C che

An mon ien hoa gom ba qua trnh:

+ Qua trnh catod (xay ra s kh): oxyhoa + ne kh (vung catod)+ Qua trnh anod (xay ra s oxyhoa): kim loai chuyen vao dung dch di dang ion va giai phong electron M Mn+ + ne (vung anod)+ S di chuyen cac electron t vung anod sang vung catodTuy theo moi trng, chat oxyhoa la khac nhau:

Moi trng acid: 2H+ + 2e H2Moi trng acid co oxy hoa tan: O2 + 4H+ + 4e 2H2OMoi trng co oxy hoa tan: O2 + 2H2O + 4e 4OH

(A)(C)MMn+ne-ne-2H+H2ne-O2OH-Nh vay khi an mon se hnh thanh vung anod va vung catod, trong qua trnh an mon co s trao oi cac electron

Trong qua trnh an mon, vung anod cua kim loai b an monQua trnh an mon hoat ong nh mot pin pin an mon.@ ieu kien

+ Vat lieu tiep xuc vi moi trng am + Co s hnh thanh cac vung ien the khac nhau tren be mat kim loai(A)(C)(A)(C)

Enthalpy t do cua phan ng, G, giup anh gia chieu hng cua qua trnh. Khi gia tr G cang am, kha nang xay ra phan ng theo chieu khao sat cang ln.III.2 NHIET ONG HOC IEN HOAa. Enthalpy t doV du: @ Mg + H2O + O2 Mg(OH)2 ; Go = 142,6 kcal/mol@ Cu + H2O + O2 Cu(OH)2 ; Go = 28,6 kcal/mol @ Au + H2O + O2 Au(OH)3 ; Go = +15,7 kcal/mol

Co the anh gia la Mg phan ng de dang vi nc co oxy hoa tan, kha nang nay kem hn vi Cu va khong the xay ra vi AuTuy nhien can chu y, khuynh hng an mon nay khong phai la thc o toc o an mon cua phan ng. Gia tr G cang am, co the hoac khong dan en toc o an mon ln v con cac yeu to khac tham gia quyet nh.

Ngc lai, neu G > O, phan ng an mon khong the xay ra trong ieu kien khao sat.

Phan ng kh : oxyhoa + ne khPhng trnh Nernst, the ien cc:Mot ien cc, vi phan ng: Mn+ + ne M (phan ng kh)b. The ien ccMot kim loai nhung vao dung dch cha ion kim loai o Tai ranh gii kim loai dung dch se xuat hien mot hieu the,

c anh gia bang the ien cc: 25oCEo la the ien cc chuan

V du: Cu/Cu2+; Cu2+ + 2e Cu 25oCNh vay the ien cc phu thuoc vao nong o ion kim loaiMot kim loai tr trong moi trng khao satV du: moi trng co oxy hoa tan, phan ng ien hoa xay raO2 + 2 H2O + 4e 4 OH ay kim loai tr khong tham gia phan ng, ma ong vai tro truyen dan electronThe ien cc

EO2 = 0,40 + 0,015 log [PO2] 0,059 log [OH]EO2 = 1,23 + 0,015 log [PO2] 0,059 pH ( vi log [OH] = pH 14 )

ay the ien cc phu thuoc vao pH cua moi trng va ap suat kh oxy, neu la khong kh PO2 = 0,2 atm.Da vao the ien cc chuan, anh gia kha nang cho nhan electron cua kim loai. Kim loai co the ien cc cang ln, cang kho nhng electron, cang kho b oxyhoa (kho b an mon), cang ben

Kim loaiMg/Mg2+Al/Al3+Ti/Ti2+Zn/Zn2+Cr/Cr3+Fe/Fe2+Eo , V 2,371,661,630,760,740,44 Kim loaiNi/Ni2+ H2/H+Cu/Cu2+Ag/Ag+Pt/Pt2+ Au/Au3+Eo , V0,250,000,340,801,201,50

The ien cc tnh t phng trnh Nernst la the ien cc can bang khong co dong ien lu thong trong mach.III.3 ONG HOC IEN HOAa. S phan ccIVEcbES dch chuyen the ien cc khoi gia tr can bang khi co dong ien i qua c goi la s phan ccnh ngha: Qua the = E EcbQua the hay the ien cc E thay oi theo cng o dong I

S ton tai qua the do nhieu nguyen nhanQua the phu thuoc vao kha nang trao oi, truyen dan electronY ngha:

Qua the caothe hien s cham tre, kho khan cua qua trnh Qua the ien tr: do ien tr cua he, nh do lp phim co tnh dan ien kem hnh thanh tren be matlp oxyd hay lp kh bam

Qua the nong o: do s chenh lech nong o gia be mat ien cc va trong long dung dch. Qua the nay phu thuoc vao toc o khuech tan cua cac chat.Qua the hoat hoa: qua the nay phu thuoc vao kha nang xay ra phan ng trao oi electron. Co lien quan en nang lng hoat hoa cua phan ng.Qua the ket tinh: khi co s ket tinh tai catod (Mn++ ne M)Qua trnh nay oi hoi nhng nang lng khac nhau tren be mat (v tr khuyet tat, goc canh de ket tinh hn cac v tr khac) va khac nhau oi vi nhng ion khac nhau.

Theo nh luat Tafel, moi quan he gia ien the va log i la ng thang. Dng ng E=f(logi) oi vi ien cc anod va catod cua pin an monb. Gian o Evansng catod: oxyhoa + ne = kh qua trnh nhan e nen E giam theo ing anod: M = Mn+ + nequa trnh nhng e nen E tang theo iVlogilogi, an monMEC > EAECEACAT giao iem cua 2 ng nay (M) xac nh c dong ien an mon

Cng o dong ien phu thuoc vao ban chat kim loai va moi trng an monKim loai co ien the thap b an mon manh hn Moi trng co oxy hoa tan lam lam tang toc o an mon thay oi s phan cc catodECEA1lgi1 lgi2lgi1 lgi2ECEAEA2khong O2co O2 hoa tan

Moi trng co nong o cac chat thay oi thay oi s phan cc anod va catodMoi trng co ien the cao gay an mon manh hnlgi1 lgi2lgi1 lgi2ECEAEC1EAEC2NaCl 3%NaCl 3%+ H2O2 0,1%O2+H2O+2e = 2OH-2H++2e = H2

III.4 TOC O AN MON VA CNG O DONG IEN nh luat Faraday(chu y n v)Lng chat chuyen hoa trong qua trnh ien hoa ty le vi ien lng i qua mach va ng lng gam cua kim loaiV du: sat b an mon trong moi trng acid Fe+2HCl = FeCl2+H2 Mot ien lng 1F (96500 coulomb) i qua mach se lam hoa tan (an mon) mot lng sat la: mFe = MFe/2 = 56/2 = 28g

Trong mot pin an mon xay ra vi cng o dong ien I trong mot khoang thi gian dt, lng kim loai b an mon la Nh vay toc o an monNeu ap dung moi quan he nay cho trng hp sat b an mon thanh Fe2+, ta co : v (mg/h) = 1,045 I (mA)Nh vay khi vat lieu bang sat b an mon vi toc o 1mm/nam se tng ng vi mat o dong ien i = I/S = 0,085 mA/cm2

Tuy nhien thc te, thng xay ra s sai lech so vi tnh toan.@ Khi ong thi co xay ra s thoat kh oxy ( 4OH 4e = O2 + 2H2O) se t kim loai hoa tan hn so vi d kien@ Khi cac hat kim loai b tach ra do troc goc va b keo theo vao moi trng trong qua trnh an mon.se co nhieu kim loai mat i hn so vi d kien

III.5 PIN AN MONS an mon xay ra, bi hnh thanh cac vung co ien the khac nhau, la do s khong ong nhat, bi nhieu ly do+ S hien dien cua cac kim loai khac nhau trong vat lieu

+ S khong ong nhat ve vi cau truc cua kim loai

+ S khong ong eu cac vung cua moi trng

Hp kim hay kim loai lan tap chat co s khong ong nhat ve thanh phan hay cau truc. S khac nhau nay tao nen cac vung co ien the khac nhau.a. Do cac kim loai khac nhauVung co ien the am hn la anod Vung co ien the dng hn la catod. Vung anod b an mon ( kim loai hoa tan )Vung catod co s thoat kh H2 hay tieu thu oxy tuy theo moi trng ien ly ma kim loai tiep xuc.

V du : kim loai Zn co lan Cu Zn (A)Cu (C)ZnZn2+2e-2e-2H+H22e-O2OH-Ecu > Ezn

b. Do s thong kh khac nhau Vung tiep xuc vi dung dch co nong o oxy cao hn se ong vai tro la catod.

Vung tiep xuc vi moi trng t thong kh se ong vai tro anod va b an mon. Mot kim loai tiep xuc vi moi trng dung dch ien ly co s khac nhau ve nong o oxy hoa tan Trong trng hp nay oxy ong vai tro bao ve vung kim loai ma no tiep xuc (catod), con nh a biet th s hien hien cua oxy trong moi trng la yeu to gay an mon.

Nh vay oxy anh hng en hai tac ong ngc nhau.

Mot so trng hp an mon theo cach nayLam cho vung nay la anod va tao thuan li cho kim loai ay vet ntranh b an mon. Tai vung pha tren, oxy de hoa tan, nong o caovung catod + An mon tai cac lo ranhCo mot vet nt tren be mat kim loai O2O2(C)(C)(A)ZnZn2+2eOH-O2Zn(OH)2 Zn(OH)2 ket tua bam tai mieng vet nt-ranh cang lam cham s khuech tan oxy vao trong, cang lam tang qua trnh an monOxy rat kho xam nhap c vao tan ay cua ranh

+ An mon trong cac ng ong dan dauDau va cac san pham dau mo thng cha mot lng nho nc. Trong qua trnh van chuyen, nc se lang xuong ay ng ong. FedauO2O2O2O2Dau cha nhieu O2 hn nc. O2 t dau khuech tan vao lp nc. (A)(C)(C)Hnh thanh pin tai be mat ben trong cua ng ong ni co ncKhi oxy tiep tuc cung cap va lp nc khong the bay hi, qua trnh an mon tiep tuc xay ra va co the lam thung ng ong.

+ An mon cau kien kim loai nhung trong ncKhao sat mot coc kim loai ong vao at va ngam trong nc Qua trnh an mon se xay ra tai vung ranh gii gia phan nam trong at va phan ngam trong nc. An mon nay rat nguy hiem v khong quan sat c t pha tren, kho phat hien.atnccau kien kim loaioxy(C)(A)Vung pha tren thong kh de dang vung catod; vung pha di cho tiep vi at vung anod.

+ An mon a chatVung at set co tnh chong tham con vung at cat hay at a voi th nc tham de dang do o oxy co the khuech tan vao. Khi ng ong c lap trong vung a chat co nhng loai at khac nhau. at setat catng ongkhong kh(C)(A)V the vung at set la vung anod va tai ay ng ong se b an mon ngay sat vi vung at cat, at a voi vung catod.

An mon xay ra tai cho ng ong nam trong at do s thong kh oxy thap hn pha tren khong kh. + An mon a chatKhi mot ng ong nho ra khoi mat atat ng ongkhong kh(C)(C)(A)(A)

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