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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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  • 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.

    Koopmans 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 aregenerally 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 solubilitys 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. TiO2b. 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 HClO42. H2S in Group II H2O, H2Te, H2SO3, or H2SO43. (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) 25(H2O) Rhodochrosite

    MnCO3

    MolybdeniteMoS2

    Uvarovite

    Ca3Cr2 (SiO4)3 Rutile TiO2

    Rutherfordine(UO2)(CO3)

    Erythrite Co3(AsO4)28(H2O)

    Siderite FeCO3 Magnetite Fe3O4Hematite Fe2O3

    Pyrite FeS2

    StrengiteFePO4.2H2O Fayalite Fe2SiO4 Greenockite CdCO3

    CassiteriteSnO3

    Stibnite Sb2S3

    SmithsoniteZnCO3 Morenosite NiSO47H2O

    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 150C, 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 nilaiG yang negatif (pada T - P tetap), dimana nilaiG standar dirumuskan:

    G = - RT ln K

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

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    Energi bebas (G) 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) G(M)

    dan tiga jenis reaksi oksidasi karbon:

    2C(s) + O2(g) 2CO(g) G(C, CO)

    2CO(g) + O2(g) 2CO2(g) G(CO, CO2)

    C(s) + O2(g) 2CO2(g) G(C, CO2)

    Latihan:

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

    logam Ag, Zn, Al, dan Ca!

    Jika G 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)

    G = G (C) - G (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 > 950C

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

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

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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 + MOx

    Maka: G = G (M) - G (M)

    Contoh:

    Mg Al2O3 pada suhu di bawah 1300C

    Mg TiO2 pada suhu di bawah 1800C

    Mg SiO2 pada suhu di bawah 2200C

    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 2000C (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 G yang lebih

    negatif.

    Energi bebas yang diperoleh dari sumber luar dapat

    diamati dari perbedaan potensial E yang dihasilkan

    sepanjang elektrode tsb menggunakan hubungan

    termodinamika

    G = - n.F.E

    Energi bebas yang diperoleh dari sumber luar dapat

    diamati dari perbedaan potensial E yang dihasilkan

    sepanjang elektrode tsb. menggunakan rumus

    G = - 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

    G + G(luar) = G - n.F.E(luar)

    Apabila beda potensial dari luar berlebih maka:

    E(luar) = G : 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 500C!

    2/3 Al2O3 4/3 Al + O2

    Jawab:

    Diagram Ellingham G = +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 500C.

    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) G = +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) G = +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 () 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 72C

    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 G negatif,

    selain itu energi bebas Gibbs berhubungan dan

    dapat juga dituliskan dalam bentuk lain yaitu

    beda potensial (E).

    Kedua hal tsb (G 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) G = +147kJ/mol

    sehingga diperoleh nilai energi Gibbs untuk reduksi Zn

    Zn2+(aq) + 2e- Zn(s) G = +147kJ/mol

  • 17/12/2011

    [email protected] 16

    Nilai potensial elektroda yang setara dgn nilai G dari reaksi

    setengah ditulis dgn E dan disebut potensial reduksi standar.

    Karena G dari reaksi reduksi H+ bernilai nol maka potensial

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

    Dgn rumus G = - 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 G < 0 atau E > 0

    G = - n.F.ESerial

    elektrokimia

    G0 = -RT ln K

    G = -nFEsel

    G = - n.F.Esel

    G0 = - n.F.Esel

    n = jumlah mol elektron dalam reaksi

    F = 96.500J

    V mol = 96.500 C/mol

    G0 = -RT ln K = = -n.F.Esel

    Esel =RT

    nFln K

    (8,314 J/Kmol)(298 K)

    n (96.500 J/Vmol)ln K=

    =0,0257 V

    nln KEsel =

    0,0592 Vn

    log KEsel

    Kespontanan Reaksi Redoks

    atau