Electrochemistry · Lesson 9 of 13
Predicting the Products of Electrolysis
“Choose among competing electrode reactions by considering solvent, electrode material, composition and overpotential.”
• Distinguish molten-salt electrolysis from aqueous electrolysis. • Predict cathode and anode products for the chapter’s representative electrolytes. • Explain why inert and reactive electrodes can give different products. • Balance half-reactions and recognise kinetic and concentration limitations.
The electrolyte formula is not a complete product list
A salt solution contains solvent molecules as well as salt ions. Water can be reduced to hydrogen or oxidised to oxygen, so it competes with dissolved species. A reactive electrode can dissolve rather than merely conduct electrons. Product prediction must therefore identify all possible reactants at each electrode before comparing their tendencies. The anode is positive and the cathode negative in the externally driven cell.
| Question to ask | Reason |
|---|---|
| Is the electrolyte molten or aqueous? | Water supplies additional possible electrode reactions only in the aqueous case. |
| Is the electrode inert under these conditions? | A reactive metal can itself be oxidised at the anode. |
| What species, concentrations and pH are present? | Actual equilibrium potentials depend on activities and gas pressures. |
| What current and electrode surface are used? | Kinetic overpotentials alter practical discharge preferences. |
| Can products react or mix after formation? | The isolated products depend on separation and subsequent chemistry. |
Molten sodium chloride
In molten NaCl, the mobile ions are Na⁺ and Cl⁻ and there is no aqueous solvent competing for reduction. The cathode produces sodium by Na⁺ + e⁻ → Na, while the anode produces chlorine by 2Cl⁻ → Cl₂ + 2e⁻. Adding gives 2NaCl(l) → 2Na(l) + Cl₂(g) at the molten-salt operating temperature. Sodium is liquid at the temperature of molten NaCl, so a solid state symbol would not describe that operating condition.
The two electrode products must be kept apart to prevent recombination. External electrical energy drives the overall non-spontaneous decomposition. This is why a metal that reacts strongly with water is not obtained simply by electrolysing its ordinary aqueous salt solution.
Aqueous brine introduces water competition
Comparing only Na⁺/Na with the standard H⁺/H₂ number is incomplete at neutral or alkaline pH. Nevertheless, the actual water/hydrogen reduction still occurs much more readily than producing sodium metal in aqueous solution. The cathode region becomes alkaline as OH⁻ is generated.
The tabulated +1.36 V for Cl₂/Cl⁻ and +1.23 V for O₂/H₂O are reduction potentials, even when discussing reversed anode reactions. Reversing a half-reaction reverses its potential sign. Equilibrium potentials alone do not settle practical product selection: oxygen evolution can require substantial extra voltage at a surface. This kinetic contribution is called overpotential. Chlorine can therefore be generated preferentially under suitable concentrated-brine conditions even though water oxidation appears thermodynamically easier in a simple comparison.
Reactive copper and inert platinum
With copper electrodes in CuSO₄, copper dissolves at the anode and deposits at the cathode. With inert platinum electrodes in a typical aqueous CuSO₄ solution, Cu²⁺ can deposit at the cathode while water is oxidised at the anode, producing O₂ and H⁺. Sulphate is generally a spectator under these ordinary dilute conditions. The anode reaction is now 2H₂O → O₂ + 4H⁺ + 4e⁻; the copper-ion concentration falls as copper deposits.
Problem
Compare electrolysis of aqueous AgNO₃ with silver electrodes and with inert platinum electrodes.
- 1.At either cathode, Ag⁺ + e⁻ → Ag is the ordinary silver-deposition reaction while Ag⁺ remains available.
- 2.With a silver anode, Ag → Ag⁺ + e⁻ transfers silver into solution, so silver moves from anode to cathode and bulk Ag⁺ concentration is approximately maintained.
- 3.With a platinum anode, the electrode is not the source of Ag⁺. Water oxidation gives O₂ and H⁺: 2H₂O → O₂ + 4H⁺ + 4e⁻. Nitrate normally remains in solution.
- 4.The balanced net ionic reaction for inert electrodes is 4Ag⁺ + 2H₂O → 4Ag + O₂ + 4H⁺. Charge is +4 on both sides.
Dilute acid and concentrated sulphate
Problem
Predict products for dilute aqueous H₂SO₄ with inert platinum electrodes.
- 1.At the cathode: 4H⁺ + 4e⁻ → 2H₂. At the anode: 2H₂O → O₂ + 4H⁺ + 4e⁻.
- 2.Adding cancels H⁺ and electrons: 2H₂O(l) → 2H₂(g) + O₂(g). The expected ideal gas amount ratio H₂:O₂ is 2:1.
- 3.The acid provides conducting ions but is not consumed by this net reaction. Gas volumes can differ from the ideal ratio if dissolution, leakage or side reactions occur.
At sufficiently concentrated sulphate or bisulphate, high anode potential and suitable electrodes, peroxodisulphate formation can become important: 2SO₄²⁻ → S₂O₈²⁻ + 2e⁻. At ordinary dilute-acid conditions, oxygen evolution is the main comparison. High acid concentration alone is not a universal guarantee of peroxodisulphate: current density, electrode surface, temperature and competing oxygen kinetics also matter.
Aqueous copper chloride
For aqueous CuCl₂ with inert electrodes, the usual cathode product while Cu²⁺ is available is copper. Chlorine is commonly produced at the anode when chloride concentration and electrode conditions favour chloride oxidation. Oxygen evolution competes, especially for dilute chloride or different anode surfaces. Thus “Cu and Cl₂” is a conditional product prediction, not an unconditional statement for every aqueous CuCl₂ sample.
Problem
Write the ideal half-reactions when copper deposition and chlorine evolution dominate.
- 1.Cathode: Cu²⁺ + 2e⁻ → Cu. Anode: 2Cl⁻ → Cl₂ + 2e⁻.
- 2.Add to obtain Cu²⁺ + 2Cl⁻ → Cu + Cl₂, or CuCl₂(aq) → Cu(s) + Cl₂(g).
- 3.This overall form assumes those products dominate and remain available. If water oxidation becomes important, the anode chemistry and net reaction change.
A single reduction-potential table gives a thermodynamic starting point, not a complete electrolysis recipe. Include concentration, solvent, electrode material and kinetic competition in any defensible product prediction.
Quick Quiz
Quiz
What is the normal cathode product during electrolysis of molten NaCl?
No water solvent is present; Na⁺ can be reduced to sodium at the cathode.
What is usually produced at the cathode of aqueous brine?
Water is reduced to hydrogen and hydroxide rather than sodium metal being deposited.
Why can concentrated brine give chlorine rather than only oxygen at its anode?
Actual composition and the kinetic difficulty of oxygen evolution can favour chloride oxidation.
Which change is expected when a silver anode replaces platinum in AgNO₃ electrolysis?
Reactive silver can undergo Ag → Ag⁺ + e⁻ instead of the ordinary inert-anode water oxidation.
What is the ideal net gas amount ratio for electrolysis of dilute sulphuric acid at inert electrodes?
The net reaction 2H₂O → 2H₂ + O₂ gives twice as many moles of hydrogen as oxygen.
Practice Problems
- Compare molten and aqueous sodium chloride electrolysis, including balanced half-reactions.
- Why can copper sulphate electrolysis preserve Cu²⁺ concentration with copper electrodes but not with platinum electrodes?
- Write the net reaction for aqueous AgNO₃ electrolysis with platinum electrodes and identify spectator ions.
- Under what conditions is “copper and chlorine” a reasonable product prediction for CuCl₂(aq) at platinum electrodes?
- Explain overpotential without changing the definition of standard electrode potential.
- Balance sulphate oxidation to peroxodisulphate and check its charge.
1. Molten: cathode Na⁺ + e⁻ → Na; anode 2Cl⁻ → Cl₂ + 2e⁻. Net 2NaCl(l) → 2Na(l) + Cl₂(g). 2. Aqueous concentrated brine: cathode 2H₂O + 2e⁻ → H₂ + 2OH⁻; anode 2Cl⁻ → Cl₂ + 2e⁻ under appropriate conditions. 3. The net separated-cell process gives 2NaCl + 2H₂O → 2NaOH + H₂ + Cl₂. Water competition explains why sodium metal is not the aqueous cathode product.
Key Takeaways
• Aqueous electrolysis includes water as a competing electrode reactant. • Molten sodium chloride produces sodium and chlorine. • Concentrated aqueous brine commonly produces hydrogen, chlorine and alkaline solution in separated compartments. • Reactive electrodes can dissolve and replenish dissolved metal ions. • Inert electrodes provide an electronic surface without serving as the main reactant. • Actual potentials depend on composition and pH. • Overpotential introduces a kinetic influence on product selection. • Balance both atoms and charge before adding electrode reactions.