Lesson 9 of 12 · 6 min
Products of electrolysis
NCERT §2.5.1
Pass a current through molten salt and shiny sodium appears at the cathode. Pass it through brine and the cathode fizzes with hydrogen instead. Why does the water win?
The lesson in notes
In short
The products depend on what is electrolysed and on the electrodes. An inert electrode (Pt, Au) only supplies or accepts electrons; a reactive one takes part in the reaction.
Among competing reactions, standard potentials decide, but some reactions are so slow that they need extra voltage (overpotential) and lose out.
Molten NaCl: only Na⁺ and Cl⁻ are present, so sodium metal forms at the cathode and Cl₂ at the anode.
Aqueous NaCl, cathode: H⁺ + e⁻ → ½H₂ (E° = 0.00 V) beats Na⁺ + e⁻ → Na (−2.71 V). With water supplying the H⁺, the net cathode reaction is H₂O + e⁻ → ½H₂ + OH⁻.
Aqueous NaCl, anode: water oxidation (E° = 1.23 V) should win over Cl⁻ → ½Cl₂ + e⁻ (1.36 V), but the overpotential of oxygen makes chlorine form instead.
Net: NaCl(aq) + H₂O(l) → Na⁺(aq) + OH⁻(aq) + ½H₂(g) + ½Cl₂(g). The products are NaOH, H₂ and Cl₂.
Concentrations shift the choice, since the Nernst potentials, not the standard ones, apply. H₂SO₄ at the anode: dilute acid gives O₂ from water (1.23 V); concentrated acid gives 2SO₄²⁻ → S₂O₈²⁻ + 2e⁻ (1.96 V).