Electrochemistry (A Level)

• Predict electrolysis products from state, E^⦵ and concentration.
• F = Le; Q = It; masses and volumes liberated; finding L by electrolysis.
• Standard electrode and cell potentials, the standard hydrogen electrode, and how E^⦵ is measured.
• E^⦵_cell for polarity, electron flow and feasibility; oxidising and reducing strength; redox equations.
• Concentration effects and the Nernst equation; ΔG^⦵ = −nE^⦵_cellF.

Predicting the products

Molten electrolyte: only the compound’s own ions are present. The metal forms at the cathode (−) and the non-metal at the anode (+). Molten PbBr₂ gives Pb and Br₂.

Aqueous electrolyte: water also provides H⁺ and OH⁻, so the ions compete.

• Cathode: the species with the more positive E^⦵ is reduced. Metals below hydrogen (Cu²⁺/Cu +0.34 V, Ag⁺/Ag +0.80 V) are deposited. For reactive metals (Na, K, Mg, Al…) hydrogen is given off instead.
• Anode (inert electrode): oxygen is given off from water/OH⁻ unless a halide is present. Br⁻ and I⁻ give Br₂ and I₂. Cl⁻ gives Cl₂ when concentrated, but mainly O₂ when dilute. SO₄²⁻ and NO₃⁻ are not discharged.
• Concentration matters because it shifts electrode potentials: a high [Cl⁻] makes oxidation of Cl⁻ easier than that of water.

Quantitative electrolysis

The Faraday constant, F, is the charge on one mole of electrons: F = Le = 6.02 × 10²³ × 1.60 × 10⁻¹⁹ = 9.65 × 10⁴ C mol⁻¹.

Finding the Avogadro constant by electrolysis

1. Weigh a clean, dry copper cathode. Set up two copper electrodes in CuSO₄(aq) in series with an ammeter, a variable resistor and a d.c. supply.
2. Pass a constant, measured current for a measured time (keep I steady with the variable resistor).
3. Remove the cathode, wash with water then propanone, dry and reweigh. The mass gained is copper deposited.
4. Q = It; n(e⁻) = 2 × mass/63.5; F = Q/n(e⁻); L = F/e.

Definitions

Standard electrode (reduction) potential, E^{⦵ −3}

Standard cell potential, E^⦵_cell

The standard hydrogen electrode

^{−3 + −3 + −} ₂ ^⦵

^⦵

^⦵

• ^⦵