Chapter 2 - Electrochemistry

Your mobile battery converts chemical energy to electrical energy - that is galvanic cell. Charger converts electrical to chemical - electrolytic cell. Both are electrochemical cells.

Galvanic spontaneous chemical reaction produces electricity like Daniell cell, car battery. Electrolytic needs electricity to drive non-spontaneous reaction like electroplating gold on jewellery, charging battery, extracting aluminium.

Every cell has two electrodes: anode where oxidation loses electrons, cathode where reduction gains electrons. Electrons flow from anode to cathode through wire outside.

Daniell cell: Zn rod in ZnSO4 solution anode oxidation Zn -> Zn2+ + 2e-, Cu rod in CuSO4 cathode reduction Cu2+ +2e- -> Cu. Electrons go Zn to Cu through wire lights bulb. Salt bridge U-tube with KCl agar-agar connects solutions completes circuit allows ions flow maintains neutrality. Without salt bridge charge builds up stops reaction.

Cell representation Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s). Left anode oxidation right cathode reduction || salt bridge.

Daily: car battery lead acid 6 cells 12V similar but uses Pb PbO2 H2SO4.

Each electrode has tendency to lose or gain electrons called electrode potential. Cannot measure alone need reference. Standard Hydrogen Electrode SHE taken as 0 V at 25 C 1 atm H2 1 M H+ solution.

Reduction potentials table: more positive means stronger oxidizer easily gains electrons like Cu2+/Cu +0.34 V F2/F- +2.87 strong oxidizer, more negative stronger reducer loses electrons like Zn2+/Zn -0.76 V Li+ -3.05 strongest reducer. Cell EMF Ecell = Ecathode - Eanode reduction potentials. If Ecell positive spontaneous.

Daily: rusting iron -0.44 V lower than oxygen +1.23 V so iron oxidizes rusts.

Battery voltage not always same 1.1 V for Daniell changes with concentration. When Zn2+ increases or Cu2+ decreases voltage drops battery dies.

Nernst equation: Ecell = E0cell - (0.0591/n) log Q at 25 C where n electrons transferred Q = [products]^coeff / [reactants]^coeff reaction quotient. For Zn+Cu2+->Zn2++Cu Q=[Zn2+]/[Cu2+]. As reaction proceeds Q increases log Q increases Ecell decreases to zero at equilibrium battery dead.

For single electrode: E = E0 - 0.0591/n log (1/[Mn+]) or E = E0 +0.0591/n log [Mn+]. If concentration low E more negative oxidation easier.

Daily: mobile battery full high ion concentration voltage high, low battery concentration equal voltage drops to zero.

Solution conducts electricity via ions moving. Resistance R = rho l/A rho resistivity. Conductance G=1/R unit Siemens S = ohm^-1. Conductivity kappa =1/rho = G * cell constant l/A. l distance between electrodes A area cell constant G* = l/A measured via known KCl solution.

Molar conductivity Lambda m = kappa *1000 / Molarity unit S cm2 mol-1. Conductivity per mole. Tells how well one mole conducts.

Daily: salty water conducts tap water low conductivity pure distilled non-conducting, seawater high conductivity.

On dilution molar conductivity increases because ions farther apart less interaction more free. For strong electrolytes like NaCl fully ionized Lambda m decreases only slightly as concentration increases graph Lambda m vs sqrt(c) slight slope straight line.

For weak electrolytes like acetic acid partially ionized on dilution dissociation increases more ions Lambda m increases sharply large rise.

Kohlrausch law: at infinite dilution ions independent Lambda m0 = lambda0+ + lambda0- sum of ion contributions. So can find Lambda m0 for weak electrolyte from strong ones. Example Lambda m0 acetic acid = Lambda m0 HCl + Lambda m0 CH3COONa - Lambda m0 NaCl.

Also degree of dissociation alpha = Lambda m / Lambda m0, Ka = c alpha^2/(1-alpha).

Electrolysis uses electricity to split compounds. Faraday first law: mass deposited m = Z I t where Z= M/nF electrochemical equivalent, or m = M I t / nF. More current or time more mass.

Faraday second law: if same charge passed through different solutions masses deposited proportional to equivalent weights M/n.

Charge Q=I t, 1 Faraday 96485 C charge of 1 mole electrons n=1. So moles of electrons = Q/F. Moles of substance = Q / nF.

Daily: gold plating jewellery uses Au3+ +3e -> Au mass control by current time. Aluminium extraction, copper refining, water electrolysis H2 O2 production.

Primary battery non-rechargeable Leclanche dry cell Zn anode MnO2 cathode 1.5 V used torch remote.

Secondary rechargeable lead acid car battery Pb anode PbO2 cathode H2SO4 electrolyte 6 cells 12V reaction Pb+PbO2+2H2SO4->2PbSO4+2H2O discharge charge reverse needs charging. Lithium ion mobile laptop light high energy LiCoO2 cathode graphite anode 3.7V rechargeable hundreds times.

Fuel cell H2 + O2 -> H2O + electricity clean only water product used in rockets buses. Efficiency high 70% vs thermal 40%. H2 at anode 2H2->4H+4e- O2 at cathode O2+4H+4e- ->2H2O overall 2H2+O2->2H2O.

Corrosion rusting Fe + O2 + H2O -> Fe2O3 rust is electrochemical cell Fe anode O2 cathode. Prevention painting, galvanizing Zn coating Zn oxidizes protects Fe sacrificial, alloy stainless, dry storage.