Scarica dispensa di elettrochimica e più Dispense in PDF di Chimica Inorganica solo su Docsity! Prof. Marilena Tolazzi docente di chimica e chimica ambientale Università degli Studi di Udine Facoltà di Ingegneria Centro Polifunzionale di Pordenone ELETTROCHIMICA 2Università degli Studi di Udine Terminology • Galvanic cells. – Produce electricity as a result of spontaneous reactions. • Electrolytic cells. – Non-spontaneous chemical change driven by electricity. • Couple, M|Mn+ – A pair of species related by a change in number of e-. 3Università degli Studi di Udine 1. Electrode Potentials and Their Measurement Cu(s) + 2Ag+(aq) Cu2+(aq) + 2 Ag(s) Cu(s) + Zn2+(aq) No reaction 4Università degli Studi di Udine An Electrochemical Cell 5Università degli Studi di Udine Terminology • Electromotive force, Ecell. – The cell voltage or cell potential. • Cell diagram. – Shows the components of the cell in a symbolic way. – Anode (where oxidation occurs) on the left. – Cathode (where reduction occurs) on the right. •Boundary between phases shown by |. •Boundary between half cells (usually a salt bridge) shown by ||. 6Università degli Studi di Udine Terminology Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s) Ecell = 1.103 V 7Università degli Studi di Udine 2. Standard Electrode Potentials • Cell voltages, the potential differences between electrodes, fem= E+- E-, are among the most precise scientific measurements. E+= E+° - log Q n 0.0592 The Nernst Equation: Es. Cu2+(1M) + 2 e- Cu(s) Q = [Cu(s)]/[Cu2+] Zn2+(1M) + 2 e- Zn(s) Q = [Zn(s)]/[Zn2+] 8Università degli Studi di Udine Standard Hydrogen Electrode (SHE) 2 H+(a = 1) + 2 e-H2(g, 1 bar) E° = 0 V Pt|H2(g, 1 bar)|H +(a = 1) •Arbitrary zero is chosen. 17Università degli Studi di Udine Concentration Cells Two half cells with identical electrodes but different ion concentrations: 2 H+(1 M) 2 H+(xM) Pt|H2 (1 atm)|H +(xM)||H+(1.0 M)|H2(1 atm)|Pt(s) 2 H+(1 M) + 2 e-H2(g, 1 atm) H2(g, 1 atm) 2 H+(xM) + 2 e- 18Università degli Studi di Udine Concentration Cells Ecell = 0 - log 2 0.0592 x2 1 Ecell = - 0.0592 log x Ecell = (0.0592) pH 2 H+(1 M) 2 H+(xM) 2 0 2 [ ]0.059 log 2 [ ] anodo catodo cell cell H E E H + + ∆ = ∆ − 2 0 0.059 log 2 1 cell cell x E E∆ = ∆ − 19Università degli Studi di Udine Measurement of Ksp Ag+(0.100 M) Ag+(sat’d M) Ag|Ag+(sat’d AgI)||Ag+(0.10 M)|Ag(s) Ag+(0.100 M) + e- Ag(s) Ag(s) Ag+(sat’d) + e- 20Università degli Studi di Udine Example Using a Voltaic Cell to Determine Ksp of a Slightly Soluble Solute. Calculate Ksp for AgI: AgI(s) Ag+(aq) + I-(aq) Let [Ag+] in a saturated Ag+ solution be x: Ag+(0.100 M) Ag+(sat’d M) 0 0 0.1 [ ]0.059 0.059 log log [ ] sat cell cell cell M Ag E E Q E n n Ag + + ∆ = ∆ − = ∆ − Ag|Ag+(sat’d AgI)||Ag+(0.10 M)|Ag(s) 21Università degli Studi di Udine Example Ksp = x 2 = 7.3 ⋅ 10-17 0 0.1 [ ]0.059 log [ ] sat cell cell M Ag E E n Ag + + ∆ = ∆ − 0 0.059 log 0.1 cell cell x E E n ∆ = ∆ − 0.417 0 0.059log 0.059log0.1x= − + 0.417 0.059log0.1 log 0.059 x − + = log 8.07x = − 8.07 910 8.55 10x − −= = ⋅ 22Università degli Studi di Udine Glass electrode f.e.m. = k + 0.059pH 23Università degli Studi di Udine 6. Batteries: Producing Electricity Through Chemical Reactions • Primary Cells (or batteries). – Cell reaction is not reversible. • Secondary Cells. – Cell reaction can be reversed by passing electricity through the cell (charging). • Flow Batteries and Fuel Cells. – Materials pass through the battery which converts chemical energy to electric energy. 24Università degli Studi di Udine The Leclanché (Dry) Cell 25Università degli Studi di Udine Dry Cell Zn(s) → Zn2+(aq) + 2 e-Oxidation: 2 MnO2(s) + H2O(l) + 2 e - → Mn2O3(s) + 2 OH -Reduction: NH4 + + OH- → NH3(g) + H2O(l) Acid-base reaction: NH3 + Zn 2+(aq) + Cl-→ [Zn(NH3)2]Cl2(s)Precipitation reaction: 26Università degli Studi di Udine Alkaline Dry Cell Zn2+(aq) + 2 OH-→ Zn (OH)2(s) Zn(s) → Zn2+(aq) + 2 e- Oxidation reaction can be thought of in two steps: 2 MnO2(s) + H2O(l) + 2 e - → Mn2O3(s) + 2 OH -Reduction: Zn (s) + 2 OH- → Zn (OH)2(s) + 2 e - 27Università degli Studi di Udine Lead-Acid (Storage) Battery • The most common secondary battery 28Università degli Studi di Udine Lead-Acid Battery PbO2(s) + 3 H +(aq) + HSO4 -(aq) + 2 e- → PbSO4(s) + 2 H2O(l) Oxidation: Reduction: Pb (s) + HSO4 -(aq) → PbSO4(s) + H +(aq) + 2 e- PbO2(s) + Pb(s) + 2 H +(aq) + HSO4 -(aq) → 2 PbSO4(s) + 2 H2O(l) E°cell = E°PbO2/PbSO4 - E°PbSO4/Pb = 1.74 V – (-0.28 V) = 2.02 V 37Università degli Studi di Udine Complications in Electrolytic Cells • Overpotential: v = Vd-Ed • Competing reactions. • Non-standard states. • Nature of electrodes. 38Università degli Studi di Udine Electrolysis of water In presence of Na2SO4, NaNO3, H2SO4, NaOH (in order to avoid charges build-up) (-) 2H2O + 2e - → H2(g) + 2OH - (+) 3H2O → ½O2(g) + 2e - +2H3O + Pt,H2(g, 1atm) | H3O + 10-7M || OH- 10-7M | O2(g,1atm) , Pt Due to H2 ed O2 overpotential, it’s necessary >1.23V f.c.e.m. = E(+) -E(-) = E°O2/OH-(pH=14) + 0.059pOH + 0.059pH= = 0.403 +0.059pKw= 1.23V 39Università degli Studi di Udine Faraday Laws 1 mol e- = 96485 C The quantities of chemicals which react at electrodes are directly proportional to the quantity of charge passed through the cell 1st Law: Q = charge (C) eqel = electrochemical equivalent w = mass of metal oxidised (g) I = current (C/s) or (A) w = eqel·Q = eqel· I·t 40Università degli Studi di Udine Faraday Laws The quantity of substance produced by electrolysis is directly proportional to its equivalent weight. 2nd Law: An equivalent corresponds to an amount in grams equal to the equivalent weight of the substance (PE). For ν electrons per mol of substance: 1F = 96485 C/eq PM PE ν = el PE eq /g C F = 41Università degli Studi di Udine Faraday Laws PE W= Q F W Q = PE F neq= n·ν n :number of moles ν : in eq/mol ut tot Q ×100 Q η = Qut :utilized electricity Qtot : total electricity 42Università degli Studi di Udine Chlor-Alkali Process 2H2O + 2e - → H2(g) + 2OH - E H2O/H2,OH-= -0.828 V a pH = 7 Na+ + e- → Na(s) E°Na+/Na= -2.71 V H2(g) 2Cl- → Cl2(g) + 2e - E°Cl2/Cl- = 1.36 V 3H2O → O2(g) + 2e - +2H3O + EO2/OH-= 0.816 V a pH = 7 Cl2(g) 43Università degli Studi di Udine Electrolitical purification of copper (-) Cu(grezzo) → Cu2+ + 2e- (+) Cu2+ + 2e- → Cu(puro) Cu(grezzo) → Cu(puro) sludge 0.3 V