Learning focus
Explain electrochemical and energetic processes using ions, electrons, balanced equations, observations, energy pathways and quantitative evidence.
Electrode redox
Reduction is gain of electrons and occurs at the cathode. Oxidation is loss of electrons and occurs at the anode. The electrode signs identify where each process occurs in electrolysis.
Cathode half-equations
A positive ion is written on the left and electrons are added to balance charge. For copper deposition: Cu2+ + 2e- -> Cu. For hydrogen formation: 2H+ + 2e- -> H2.
Anode half-equations
A negative ion loses electrons. For bromide: 2Br- -> Br2 + 2e-. For hydroxide producing oxygen: 4OH- -> O2 + 2H2O + 4e-.
Checking equations
Count each atom on both sides, then add total charge on each side. A correct half-equation has equal atom numbers and equal overall charge, even though electrons appear on one side.
Worked example
For chloride oxidation, two chloride ions are needed because chlorine is diatomic: 2Cl- -> Cl2 + 2e-. Charge on each side is -2.
Practical or data skill
Complete half-equations with missing coefficients and electrons, then identify each as oxidation or reduction.
Examination tip
Electrons appear on the left for reduction and on the right for oxidation.
Review questions and suggested answers
Question 1
Write the half-equation for silver-ion reduction.
Suggested answer
Ag+ + e- -> Ag.
Question 2
Write the half-equation for iodide oxidation.
Suggested answer
2I- -> I2 + 2e-.
Question 3
Where does oxidation occur?
Suggested answer
At the anode.
