kimia reaksi anorganik

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17/12/2011 [email protected] 1 Kimia Reaksi Anorganik Yuniar Ponco Prananto 2 nd Mid-Semester Topics Frontier Orbitals (2 sks) HSAB Application (5 sks) Heterogeneous Acid Base Reaction (2 sks) Reduction and Oxidation (12 sks): - Extraction of the Elements - Reduction Potential - Redox Stability in Water - Diagrammatic Presentation of Potential Data Huheey, J.E., Keiter, E.A., and Keiter, R.L., 1993, Inorganic Chemistry, Principles of Structure and Reactivity, 4 th ed., Harper Collins College Publisher, New York Miessler, D. L. and Tarr, D. A., 2004, Inorganic Chemistry, 3 rd ed., Prentice Hall International, USA Atkins, P., Overton, T., Rourke, J., Shriver, D. F., Weller, M., and Amstrong, F., 2009, Shriver and Atkins’ Inorganic Chemistry, 5 th ed., Oxford University Press, UK Gary Wulfsberg, 2000, Inorganic Chemistry, University Science Book, California, USA • Frontier orbitals, which are HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), has been focused nearly since the beginning of HSAB theory. Koopman’s theorem, the energy of HOMO → the ionization energy, while the energy of LUMO → the electron affinity for a closed-shell spesies, in which both orbitals are involved in the electronegativity and HSAB relationships. • Hard species → have a large HOMO – LUMO gap • Soft species → have a small HOMO – LUMO gap. Frontier Orbitals …..summary

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Page 1: Kimia Reaksi Anorganik

17/12/2011

[email protected] 1

Kimia Reaksi AnorganikYuniar Ponco Prananto

2nd Mid-Semester Topics

• Frontier Orbitals (2 sks)

• HSAB Application (5 sks)

• Heterogeneous Acid Base Reaction (2 sks)

• Reduction and Oxidation (12 sks):

- Extraction of the Elements

- Reduction Potential

- Redox Stability in Water

- Diagrammatic Presentation of Potential Data

Huheey, J.E., Keiter, E.A., and Keiter, R.L., 1993, Inorganic

Chemistry, Principles of Structure and Reactivity, 4th

ed., Harper Collins College Publisher, New York

Miessler, D. L. and Tarr, D. A., 2004, Inorganic Chemistry, 3rd ed., Prentice Hall International, USA

Atkins, P., Overton, T., Rourke, J., Shriver, D. F., Weller, M., and Amstrong, F., 2009, Shriver and Atkins’ Inorganic Chemistry, 5th ed., Oxford University Press, UK

Gary Wulfsberg, 2000, Inorganic Chemistry, University Science Book, California, USA

• Frontier orbitals, which are HOMO (highest occupied

molecular orbital) and LUMO (lowest unoccupied molecular

orbital), has been focused nearly since the beginning of

HSAB theory.

• Koopman’s theorem, the energy of HOMO → the

ionization energy, while the energy of LUMO → the

electron affinity for a closed-shell spesies, in which both

orbitals are involved in the electronegativity and HSAB

relationships.

• Hard species → have a large HOMO – LUMO gap

• Soft species → have a small HOMO – LUMO gap.

Frontier Orbitals …..summary

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Frontier orbitals is the HOMO (filled or partly filled) and

the LUMO (completely or partly vacant) of a

MOLECULAR ENTITY.

HOMO

is the highest-energy molecular orbital of an atom or

molecule containing an electron. Most likely the first

orbital, from which an atom will lose an electron.

LUMO

is the orbital that can act as the electron acceptor,

since it is the innermost (lowest energy) orbital that

has room to accept an electron.

HSAB Principle …..summary

• A structure-reactivity concept formulated in terms of

acid-base interaction.

• According to this principle soft acids react faster and

form stronger bonds with soft bases, whereas hard acids

react faster and form stronger bonds with hard bases,

everything else being assumed aproximately equal.

• The soft-soft preference is characteristic mostly of the

reactions controlled by the orbital interaction, and the

hard-hard preference relates to reactions in which

electrostatic factors prevail.

Pure Appl. Chem., Vol. 71, No. 10, pp. 1919-1981, 1999

Hardness is associated with:

1. Small atomic/ionic radius 2. High oxidation state

3. low polarizability, 4. high electronegativity

and 5. energy low-lying HOMO

(bases) or energy high-lying LUMO (acids)

Hard vs Soft

Softness is related to:1. large atomic/ionic radius

2. low or zero oxidation state

3. high polarizability, 4. low electronegativity,

and 5. energy high-lying

HOMO (bases) or energy low lying LUMO

(acids)

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1. Equilibrium direction (reactants or products)

ZnI2 + HgCl2 ZnCl2 + HgI2

b/l A – SB SA – b/l B b/l A – b/l B SA – SB

• The I- is a soft base and Hg2+ is a soft acid,

although Zn2+ is a b/line acid and Cl- is a

b/line base (relative to Hg2+ and I-), both

Zn2+ and Cl- are the harder species, hence

the SA-SB is HgI2 and HA-HB is ZnCl2

→ products are favored

CuI2 + Cu2O 2Cul + CuOb/l A – SB SA – HB SA – SB b/l A – HB

• The I- is a soft base and O2- is a hard base,

since Cu+ is softer acid than Cu+2 therefore

the SA-SB is CuI and the HA-HB is CuO

→ products are favored

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CdCI2 + CaSO4 CdSO4 + CaCl2b/l A – b/l B HA – HB b/l A – HB HA – b/l B

• Both Cd+2 and Cl- are b/line acid and b/line

base, while both Ca+2 and O-2 are hard acid

and hard base, therefore those species in

the left are favorable, due to its stronger

interactions, than those species in the right

→ reactants are favored

Exercise 1

Predict the favorable species (reactants or products) inthe following equilibrium!

1. Hg(CN)2 + Ca(ClO4) 2 Ca(CN) 2 + Hg(ClO4) 2

2. 3FeO + Fe2S3 3FeS + Fe2O3

3. PbCO3 + 2NaBr Na2CO3 + PbBr2

4. MgCl2 + AgI MgI2 + AgCl

5. MnS + NiF2 NiS + MnF2

6. Ti(NO3)2 + ZnCl2 Zn(NO3) 2 + TiCl2

7. Nb2S5 + 5HgO Nb2O5 + 5HgS

2. Solubility of Halides and Chalcogenides

[Ag(H2O)n]+ + [Cl(H2O)m]- AgCl(s) + (m+n)H2OSA – HB b/l B – HA SA – b/l B HA – HB

Hence:

We can predict that “the chlorides, bromides, and iodides of the

soft acids are insoluble in water” → most of the predictions are

generally true for the +1 and +2 ions of the soft-acid metals, i.e:

CuCl, AgCl, AuCl, TlCl, Hg2Cl2, OsCl2, IrCl2, PdCl2, PtCl2, and

PbCl2, except CuCl2, AuCl3, HgCl2, PtCl4.

Similar predictions of the solubility of pseudohalide salts, i.e:

soft base → cyanide (CN), thiocyanate (SCN),

b/l base → azide (-N=N+=N-)

→ “the cyanides, thiocyanates, and azides of the soft acidsare insoluble in water”

• Special case: the sulfides, selenides and tellurides of the soft and borderline acids are insoluble in water due

to the combination of softness and some strength in bases (S2-, Se2- and Te2- ions are stronger bases than

Cl-, Br- and I- ions)

• Moreover…– for a given soft acid, the solubility of the iodide <

bromide < chloride,– similarly, for a given soft acid, the solubility of

telluride < selenide < sulfide

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• Due to the absence of soft base involvement in the precipitation equilibrium, the HSAB principle is

irrelevant to the water solubility’s prediction of the salts from hard bases F- (weak base), OH- and O2-(strong

bases) → “the fluorides, oxides, and hydroxides of acidic

cations are insoluble in water”

• Because there is no soft acid is involved in the solubility equilibrium, the solubility of the sulfides,

selenides and tellurides of the hard acids cannot be predicted by the HSAB principle.

Exercise 2

• Identify all insoluble compounds and

explain it according to HSAB principle:

a. PtAs2 e. TiO2

b. AgI f. CdTe

c. CaF2 g. AgF

d. KI h. CoSO4

3. The Qualitative Analysis Scheme for Metal Ions

• Group I → precipitated with 0.3 M HCl

• Group II

→ precipitated with H2S from acidic solution• Group III

→ precipitated with (NH4)2S from basic solution• Group IV

→ precipitated with (NH4)2CO3

• Group V→ remains in solution

.: the member of each group does not belong to

certain / similar period or group in the periodic

table of elements

?

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Group I and II

• The chloride ion is a borderline base that will combine strongly to soft acid, and if a large excess of Cl- is

present, only Ag+, Hg2+, Tl+ and Pb2+ will be precipitated while others like Cu, Pd, Os, Ir, Pt and Au

will form soluble chloro anions [MCln]x-.

• The sulfide ion is a very soft and a stronger base than Cl-. Therefore when it reacts with Cu, Pd, Os, Ir, Pt

and Au, it will produce more insoluble sulfides.

Note: the scheme should be done properly and in order!!

Group III

• The sulfide ion is present in much higher concentration and also competing with another strong base plus

much harder hydroxide ion.

• Only hard-acid metal ions and small group of borderline acids from the +2 ions of the 1st row transition metals

(Zn, Ni, Co, Fe, and Mn) are in the solution and will be precipitated as sulfides.

• The remains, which is strong - hard acid metal (also all

of the f-block elements), will react with the strong –hard base OH- and gives precipitation of metal

hydroxides or metal oxides.

Group IV and V

• Only the non-acidic and weak acidic hard acid of the 1st two groups of the periodic table are present, and

then moderately-basic carbonate ion is added → only the weak acidic cations will be precipitated.

• The rest will only be the non-acidic cations that prefer

to bind with the (non-basic) hard base water in the form of hydrated-ion (to get the precipitation of these

metals, sometimes the test is continued by adding the non-basic anions)

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Exercise 3

A new qualitative analysis scheme that separate

cations into five existing groups using novel precipitation anions is prepared. Choose the best substitute for:

1. HCl in Group I → HBr, HF, HClO, or HClO4

2. H2S in Group II → H2O, H2Te, H2SO3, or H2SO4

3. (NH4) 2CO3 in Group IV → NH4OH, NH4ClO4, (NH4) 2SO4, (NH4) 4C

4. If you want to precipitate Group V, which compound would do this → LiOH, LiClO4, Li2SO4, Li4C.

Geochemical classification of the elements divided into Primary and Secondary. The primary emphasizes

behavior under condition at the surface of the earth, which are:

• Lithophilesmetals and nonmetals that tend to occur as cations in

oxides, silicates, sulfates and carbonates.

the anions posses oxygen as the donor atom hence the metal ions that attached are hard acid metals,

the nonmetals are, or have been oxidized by atmospheric oxygen to oxo-anions, hard bases.

4. The Geochemical Classification and

Differentiation of the Elements

• Chalcophiles

occur in nature as cations in sulfides (less commonly w/ other soft bases such as telluride, arsenic, etc). Mostly

from the borderline and some soft acids and many of

soft bases.

• Atmophiles

chemically unreactive nonmetals that occur in the

atmosphere in elemental form, i.e: N2 and noble gases

• Siderophiles

soft acid metals that tend to occur native in elemental form (subdivision of chalcopiles)

Hard Acid MetalHard Acid Metal

Gypsum

CaSO4

NatroxalateNa2C2O4

Fluorite CaF2

NatroliteNa2[Al2Si3O10]·2H2O

Sylvite KCl

Magnesite

MgCO3

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(Meta)variscite AlPO4.2H2O

Corundum

Al2O3

Beryl Be3Al2Si6O18

Barite

BaSO4

GaylussiteNa2Ca(CO3) 2·5(H2O) Rhodochrosite

MnCO3

MolybdeniteMoS2

Uvarovite

Ca3Cr2 (SiO4)3 Rutile TiO2

Rutherfordine(UO2)(CO3)

Erythrite Co3(AsO4)2·8(H2O)

Siderite FeCO3 Magnetite Fe3O4Hematite Fe2O3

Pyrite FeS2

StrengiteFePO4.2H2O Fayalite Fe2SiO4

Greenockite CdCO3

CassiteriteSnO3

Stibnite Sb2S3

SmithsoniteZnCO3 Morenosite NiSO4·7H2O

Hard Hard –– b/line Acid Metalb/line Acid Metal

Salesite Cu(IO3)(OH)

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Iodargyrite AgI

GalenaPbS

CinnabarHgS Calaverite AuTe2

Soft Acid MetalSoft Acid Metal

Marshite CuI Sartorite PbAs2S4

Elements Mineral Names Hard / Soft – Acid / Base

1. Sodium Halite, NaCl Hard Acid Na + B/line Base Cl

Natron, Na2CO3.10H2O …

Villiaumite, NaF …

Caliche, NaIO3 …

2. Calcium Calcite, CaCO3 Hard Acid Ca + Hard Base O

Gypsum, CaSO4.2H2O …

Apatite, Ca5(PO4)3OH …

Fluorite, CaF2 …

3. Kalium Sylvite, KCl Hard Acid K + B/line Base Cl

4. Magnesium Magnesite, MgCO3 Hard Acid Mg + Hard Base O

5. Alumunium Bauxite, Al2O3 Hard Acid Al + Hard Base O

Cryolite, NaAlF4 …

Gibbsite, Al(OH)3 …

Gahnite, ZnAl2O4 …

6. Chromium Chromite, FeCr2O4 …

7. Titanium Rutile, TiO2 …

Ilmenite, FeTiO3 …

Elements Mineral Names Hard / Soft – Acid / Base

8. Iron Hematite, Fe2O3 Hard Acid Fe + Hard Base O

Magnetite, Fe3O4 …

Siderite, FeCO3 …

Pyrite, FeS2 …

Arsenopyrite, FeAsS …

9. Manganese Pyrolusite, MnO2 Hard Acid Mn + Hard Base O

Psilomelane, BaMn5O11 …

10. Zinc Sphalerite, ZnS B/line Acid Zn + Soft Base S

Smithsonite, ZnCO3 …

11. Nickel Pentlandite, (Ni,Fe)9S8 …

Heazlewoodite, Ni3S2 …

Melonite, NiTe2 …

Morenosite, NiSO4.7H2O Hard Acid Ni + Hard Base O

Pararammelsbergite, NiAs2 …

Vaesite, NiS2

12. Tin Cassiterite, SnO2 B/line Acid Sn + Hard Base O

13. Antimony Stibnite, Sb2S3 B/line Acid Sb + Soft Base S

14. Molybdenum Molybdenite, MoS2 B/line Acid Mo + Soft Base S

Exercise 4

• Choose and explain the most likely

mineral sources of this elements based

on HSAB principle!

a. LaPO4, LaAs, LaI3, or LaCl3

b. ZrSiO4, ZrPbO4, or ZrCl4

c. PtAs2, PtN2, PtSiO4, or PtF2

d. CoAs2, CoCO3, CoSO4, or CoBr2

e. TeF4, Na2Te, PbTe, or TeI4

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5. Biochemistry, Biological Functions, Toxicology, and Medicinal

• HSAB principle can organize the chemistry of the metal ions in ecology, biochemistry and medicine

• The donor atom of biochemical ligands to which the

essential metal ions prefer to bind indicates the general pattern of HSAB principle, as well as the

kind of biochemical sites at which toxic metal ions are likely to bind.

• Mimicking the behavior of complicated biochemical

complexes with simpler coordination complexes (bioinorganic chemistry) → catalysts, metal-activated

enzymes, template, etc.

Some of the most important reactions involving the

Lewis acidity of inorganic compounds occur at solid

surfaces.

For example, SURFACE ACIDS, which are solids with

a high surface area and Lewis acid sites, are used as

catalysts in the petrochemical industry for the

isomerization and alkylation of aromatic compounds.

The surfaces of many materials that are important in

the chemistry of soil and natural waters also have

Lewis acid sites.

Heterogeneous Acid-Base Reaction

Surface acidity

Alumina (Al2O3)• Termasuk salah satu asam permukaan karena Al dgn biloks Al yang

tinggi (+3).

• Ketika alumunium oksida hidrat yang baru mengendap dipanaskan di

atas 150°C, proses dehidrasi terjadi dan permukaan mengalami

reaksi:

• Reaksi tsb menghasilkan spesi Al3+ di permukaan, yg bertindak sbg

asam Lewis, begitu juga spesi O2-, yg bertindak sbg basa Lewis.

• Kedua asam-basa Lewis ini akan dihasilkan bersamaan, namun situs

asam Lewis lebih utama untuk katalisis permukaan.

Al

OH

Al

OH

Al

OH

Al Al Al

OHO

+ H2O

Aluminosilikat (Al2O3)• Berbeda dgn alumina, aluminosilikat memperlihatkan sifat keasaman

Brownsted dimana pembentukkannya diperkirakan melalui

kondensasi unit Si(OH)4 dgn unit H2OAl(OH)3:

• Reaksi tsb menghasilkan asam Brownsted yang kuat pada situs

dimana proton dipertahankan untuk menyeimbangkan muatan

positif yang lebih kecil dari Al3+.

• Karakter dan kekuatan asam permukaan dapat dipelajari melalui

metoda sederhana seperti titrasi dgn larutan basa. Selain itu,

spektrofotometri inframerah dari molekul yang teradsorb dapat juga

digunakan untuk mempelajari situs permukaan yang reaktif.

Si

OH

Al

OH2

Si Al

HO

+ H2O

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Silika (SiO2)• Permukaan silika tidak seketika membentuk situs asam Lewis

karena gugus OH terikat kuat pada permukaan turunan SiO2, dan

keasamaan Brownsted lebih dominan.

• Keasaman Brownsted permukaan silika relatif sedang dibandingkan

asam asetat, berbeda dgn aluminosilikat yang menunjukkan

keasamaan Brownsted yang kuat.

• Reaksi permukaan oleh situs asam Brownsted gel silika digunakan

untuk membuat lapisan tipis beberapa gugus organik, yang salah

satunya dipakai untuk memodifikasi permukaan fase diam kolom

kromatografiSi + H2OOH + HOSiR3

Si O SiR3

Si + HClOH + ClSiR3 Si O SiR3

REAKSI OKSIDASI – REDUKSI

• Ekstraksi Unsur

• Potensial Reduksi

• Kestabilan Redoks dalam Air

• Diagram Data Potensial

Ekstraksi Unsur

• Reduksi bijih logamreaksi redoks tidak selalu mencapaikesetimbangan sehingga hukum termodinamikadapat digunakan untuk memprediksi reaksimana yang lebih disukai, yaitu berdasarkan nilaiΔG yang negatif (pada T - P tetap), dimana nilaiΔG standar dirumuskan:

ΔG° = - RT ln K

Nilai ΔG yang negatif menandakan bahwa nilai K> 1 yang juga berarti reaksi bergeser ke kanan

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• Energi bebas (ΔG) reaksi reduksi oksida logam vssuhu → Diagram Ellingham

Energi bebas standar yang disajikan dalam diagram ini adalah

untuk pembentukan oksida dari logam:

2/x M(s or l) + O2(g) → 2/x MOx(s) ΔG°(M)

dan tiga jenis reaksi oksidasi karbon:

2C(s) + O2(g) → 2CO(g) ΔG°(C, CO)

2CO(g) + O2(g) → 2CO2(g) ΔG°(CO, CO2)

C(s) + O2(g) → 2CO2(g) ΔG°(C, CO2)

Latihan:

Pada 1000°C, tuliskan reaksi reduksi dan nilai energi Gibbs dari

logam Ag, Zn, Al, dan Ca!

Jika ΔG° suatu logam bernilai positif pada suhu

tertentu , maka dekomposisi termal suatu oksida

logam dapat terjadi tanpa adanya reduktor.

Namun kebanyakan oksida logam membutuhkan

reduktor, misalnya karbon:

C(s) + 2/x MOx(s) → 2/x M(l) + CO2(g)

ΔG° = ΔG° (C) - ΔG° (M)

Contoh Soal

• Berapakah suhu minimum yang diperlukan untuk

mereduksi (a) ZnO dan (b) MgO menjadi logamnya

dengan reduktor karbon ? (lihat diagram Ellingham)

Jawab:

C(s) + ZnO(s) → Zn(l) + CO(g) T > 950°C

C(s) + MgO(s) → Mg(l) + CO(g) T > 1850°C

C(s) + 2MgO(s) → 2Mg(l) + CO2(g) T > 2250°C

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• Diagram Ellingham juga dapat dipakai untuk

menentukan apakah suatu logam (M’) dapat dipakai

sebagai reduktor untuk oksida logam lain (M),

sebagai pengganti karbon:

M’ + MOx → M + M’Ox

Maka: ΔG° = ΔG° (M’) - ΔG° (M)

• Contoh:

Mg → Al2O3 pada suhu di bawah 1300°C

Mg → TiO2 pada suhu di bawah 1800°C

Mg → SiO2 pada suhu di bawah 2200°C

• Proses industri untuk mendapatkan logam melalui

reaksi reduksi memberikan banyak kemungkinan

dibandingkan analisa termodinamika yang ada, mulai

dari reduksi logam yang mudah, sedang, dan sulit.

Mudah � ekstraksi hidrometalurgi tembaga

Sedang � ekstraksi besi dalam tanur suhu tinggi

Sulit � ekstraksi silikon dari pasir silika atau quarts

Note: see Atkin's for detail!!

Reduksi Elektrolitik

• Reduksi secara langsung Al2O3 dgn C dapat dilakukan pada

suhu di atas 2000°C (sesuai diagram Ellingham) sehingga

relatif mahal dan sulit � pada 1886 digunakan proses Hall

- Heroult

• Proses ini merupakan suatu teknik untuk mengatur suatu

reduksi dgn merangkaikannya (melalui elektrode dan

sirkuit eksternal) ke sebuah reaksi dgn nilai ΔG yang lebih

negatif.

• Energi bebas yang diperoleh dari sumber luar dapat

diamati dari perbedaan potensial E yang dihasilkan

sepanjang elektrode tsb menggunakan hubungan

termodinamika

ΔG = - n.F.E

• Energi bebas yang diperoleh dari sumber luar dapat

diamati dari perbedaan potensial E yang dihasilkan

sepanjang elektrode tsb. menggunakan rumus

ΔG = - n.F.E

dimana:

n = mol elektron yang ditransfer

F = konstanta Faraday = 96500 C/mol

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• Sementara itu, perubahan energi bebas Gibbs total dari

pasangan proses dalam dan luar sistem adalah

ΔG + ΔG(luar) = ΔG - n.F.E(luar)

• Apabila beda potensial dari luar berlebih maka:

E(luar) = ΔG : nF

ket:

proses reduksi dapat berlangsung spontan dan

selanjutnya keseluruhan proses berjalan dgn

berkurangnya energi bebas

CONTOH

Tentukan beda potensial minimal yang diperlukan untuk

mereduksi alumina pada 500°C!

2/3 Al2O3 →4/3 Al + O2

Jawab:

Diagram Ellingham→ ΔG = +960kJ (setiap mol O2)

Jumlah mol elektron = 4/3 x Al3+ = 4 mol

maka

E(luar) = 960 kJ : (4 mol x 96,5 kC/mol) = 2,49V

.:. Setidaknya beda potensial 2,5V harus digunakan untuk

mereduksi alumina pada 500°C.

• Ekstraksi unsur dgn oksidasi yang paling penting

adalah golongan halogen.

• Energi bebas standar dari oksidasi Cl- dalam air

bernilai sangat positif, yang mengindikasikan bhw

diperlukan elektrolisis untuk reaksi berjalan ke kanan:

2Cl-(aq) + 2H2O(l) → 2OH-(aq) + H2(g) + Cl2(g) ΔG° = +422 kJ

• Perbedaan potensial minimum yang diperlukan

sekitar 2,2V (dimana n = 2 mol untuk reaksi di atas) ---

tunjukkan buktinya!

Ekstraksi Unsur dgn Oksidasi

• Namun, ketika beda potensial yang digunakan hanya 1,2V

(atau setara dgn n = 4), maka akan terjadi kompetisi dgn

reaksi berikut:

2H2O(l) → 2H2(aq) + O2(g) ΔG° = +414 kJ

� Hal ini mengisyaratkan pentingnya faktor kinetika

• Kecepatan oksidasi air sangatlah rendah pada potensial

dimana reaksi tsb mulai spontan secara termodinamika �

reduksi memiliki overpotential yang tinggi.

• Overpotential (ή) merupakan beda potensial sebagai

tambahan terhadap nilai kesetimbangan yang dibutuhkan

untuk mencapai kecepatan reaksi yang signifikan.

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• Golongan halogen lain:

F2 � elektrolisis campuran HF/KF anhidrat dilakukan dan

meleleh pada 72°C

Br2 dan I2 � oksidasi larutan halida dgn gas klor (Cl2)

• Logam lain:

khususnya yang secara alami berada dalam bentuk

unsurnya seperti Emas (Au), yaitu dgn melarutkan dengan

CN- dan mereduksinya dgn logam reaktif seperti Zn:

2[Au(CN)2]-(aq) + Zn(s) → [Zn(CN)4]-2

(aq) + 2Au(s)

• Reaksi redoks dapat berjalan apabila ΔG negatif,

selain itu energi bebas Gibbs berhubungan dan

dapat juga dituliskan dalam bentuk lain yaitu

beda potensial (E).

• Kedua hal tsb (ΔG dan E) dapat digabungkan dan

menghasilkan cara lain yang bermanfaat dalam

membahas reaksi redoks.

Potensial Reduksi

Reaksi setengah redoks

• Spesies oksidasi dan reduksi pada reaksi setengah

membentuk pasangan redoks, dimana spesies yang

teroksidasi ditulis di depan spesies yang tereduksi yaitu

OKS/RED, misalnya:

Zn(s) → Zn2+(aq) + 2e- …..1)

2H+(aq) + 2e- → H2(g)

…..2)

maka pasangan redoks ditulis dgn:

rx 1 ==> Zn2+ / Zn

rx 2 ==> H+ / H2

Kesepakatan umum: ditulis dalam bentuk reaksi reduksinya!!

Potensial elektoda standar

• Potensial reduksi standar(E0) adalah voltase yang

berkaitan dengan reaksi reduksi pada elektroda jika

konsentrasi semua zat terlarut 1 M dan semua gas pada 1

atm.

• Energ Gibbs dari reduksi ion H+ ditetapkan bernilai nol dan

potensial reduksi zat lain dihitung berdasarkan acuan ini.

Zn2+(aq) + H2(g) → 2H+

(aq) + Zn(s) ΔG° = +147kJ/mol

sehingga diperoleh nilai energi Gibbs untuk reduksi Zn

Zn2+(aq) + 2e-→ Zn(s) ΔG° = +147kJ/mol

Page 16: Kimia Reaksi Anorganik

17/12/2011

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Nilai potensial elektroda yang setara dgn nilai ΔG° dari reaksi

setengah ditulis dgn E° dan disebut potensial reduksi standar.

Karena ΔG° dari reaksi reduksi H+ bernilai nol maka potensial

reduksi standar H+ / H2 juga nol pada semua suhu.

Dgn rumus ΔG = - n.F.E maka diperoleh hasil

2H+(aq) + 2e- → H2(g) E° = 0 V

Zn2+(aq) + 2e- → Zn(s) E° = -0,76 V

Sehingga reaksi akan berjalan spontan bila:

2H+(aq) + Zn(s) → Zn2+

(aq) + H2(g) E° = +0,76 V

Reaksi spontan bila ΔG < 0 atau E > 0

ΔG = - n.F.ESerial

elektrokimia

∆G0 = -RT ln K

∆G = -nFEsel

ΔG = - n.F.Esel

ΔG0 = - n.F.E°sel

n = jumlah mol elektron dalam reaksi

F = 96.500J

V • mol = 96.500 C/mol

ΔG0 = -RT ln K = = -n.F.Esel

E°sel =RT

nFln K

(8,314 J/K•mol)(298 K)

n (96.500 J/V•mol)ln K=

=0,0257 V

nln KE°sel =

0,0592 Vn

log KE°sel

Kespontanan Reaksi Redoks

atau