Electrochemistry and Redox
Electrochemistry and redox
This domain tests oxidation and reduction, oxidation numbers, electrolysis and electrode products. GCE O-Level questions often ask you to identify the substance oxidised or reduced and to predict products at each electrode.
Core rules
- Oxidation is loss of electrons and increase in oxidation number; reduction is gain of electrons and decrease in oxidation number. Use OIL RIG: oxidation is loss, reduction is gain.
- In electrolysis, the cathode is the negative electrode and attracts cations; reduction occurs there. The anode is the positive electrode and attracts anions; oxidation occurs there.
- In molten compounds, the ions present are discharged. In aqueous solutions, water competes, so selective discharge depends on reactivity and concentration.
- Reactive metals such as sodium and aluminium are extracted by electrolysis of molten compounds, not by carbon reduction.
- Oxidation numbers: elements are 0; simple ions equal their charge; oxygen is usually -2; hydrogen is usually +1.
Worked example
Predict the products at the electrodes during electrolysis of concentrated sodium chloride solution using inert electrodes.
Reasoning: the solution contains Na+, H+, Cl- and OH-. At the cathode, H+ is discharged in preference to Na+ because sodium is very reactive: 2H+ + 2e- -> H2. At the anode, concentrated Cl- is discharged in preference to OH-: 2Cl- -> Cl2 + 2e-. So hydrogen forms at the cathode and chlorine at the anode. The remaining solution becomes sodium hydroxide. In dilute sodium chloride, oxygen may form at the anode instead because OH- discharge becomes relatively more favoured.
Traps that cost marks
Do not say positive ions go to the anode; cations go to the cathode. Cathode is reduction, anode is oxidation. Electrolysis of aqueous solutions is not the same as molten compounds. Do not assume the most reactive metal is always discharged in aqueous solution. Balance electrode equations and include state symbols when asked.
Key points
- Oxidation is electron loss; reduction is electron gain.
- Cathode attracts cations and reduces them.
- Anode attracts anions and oxidises them.
- Aqueous electrolysis involves competition from water.
- Oxidation numbers help identify redox changes.