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Chemistry · Ch 7 — Electrochemistry

Summary

Summary

  • Electrochemical cells: a galvanic cell converts a spontaneous redox reaction into electrical energy; an electrolytic cell uses electrical energy to force a non-spontaneous redox reaction. In both, oxidation occurs at the anode and reduction at the cathode — only the electrode polarity differs between the two cell types.
  • Daniell cell: Zn(s)+Cu2+(aq)→Zn2+(aq)+Cu(s)\text{Zn}(s) + \text{Cu}^{2+}(aq) \to \text{Zn}^{2+}(aq) + \text{Cu}(s); zinc anode (oxidation, negative terminal), copper cathode (reduction, positive terminal), joined by a salt bridge that maintains electrical neutrality in both half-cells.
  • Cell notation: anode | anode-ion(conc) || cathode-ion(conc) | cathode — e.g. Zn(s) ∣ Zn2+(1M) ∣∣ Cu2+(1M) ∣ Cu(s)\text{Zn}(s)\,|\,\text{Zn}^{2+}(1\text{M})\,||\,\text{Cu}^{2+}(1\text{M})\,|\,\text{Cu}(s).
  • Standard electrode potential, E∘E^{\circ}: reduction potential measured against the standard hydrogen electrode (defined as 0.00 V0.00\ \text{V}), under 1 M1\ \text{M}/1 atm1\ \text{atm}/298 K298\ \text{K} conditions. Ecell∘=Ecathode∘−Eanode∘E^{\circ}_{cell} = E^{\circ}_{cathode} - E^{\circ}_{anode}.
  • Nernst equation: Ecell=Ecell∘−(0.059/n)log⁡QE_{cell} = E^{\circ}_{cell} - (0.059/n)\log Q at 298 K298\ \text{K}, extending cell EMF to non-standard concentrations; at equilibrium (Ecell=0E_{cell}=0), it gives log⁡K=nEcell∘/0.059\log K = nE^{\circ}_{cell}/0.059.
  • Gibbs energy and EMF: ΔG∘=−nFEcell∘\Delta G^{\circ} = -nFE^{\circ}_{cell} — a positive Ecell∘E^{\circ}_{cell} always means a negative (spontaneous) ΔG∘\Delta G^{\circ}, and vice versa.
  • Conductance: specific conductivity κ=(cell constant)/R\kappa = (\text{cell constant})/R; molar conductivity Λm=1000κ/C\Lambda_m = 1000\kappa/C. κ\kappa falls on dilution (fewer ions per unit volume); Λm\Lambda_m rises on dilution (greater ionic mobility, and for weak electrolytes, greater dissociation).
  • Kohlrausch's law: Λm0=xλ+0+yλ−0\Lambda_m^{0} = x\lambda^{0}_{+} + y\lambda^{0}_{-} — lets a weak electrolyte's Λm0\Lambda_m^{0} (not obtainable by extrapolation) be built from strong-electrolyte data; then α=Λm(c)/Λm0\alpha = \Lambda_m(c)/\Lambda_m^{0} and Ka=Cα2/(1−α)K_a = C\alpha^2/(1-\alpha) (Ostwald dilution law). …
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