IGCSE Chemistry · 0620

IGCSE Chemistry: Electrochemistry and Electrolysis Questions

Electrolysis questions reward you for tracking ions to the right electrode and writing correct half equations. These worked examples cover molten lead bromide, why ions must be free to move, and cathode half equations.

What you need to know

In electrolysis, positive ions (cations) move to the negative cathode; negative ions (anions) move to the positive anode.
The compound must be molten or in solution so the ions are free to move and carry the charge.
At the cathode, positive ions gain electrons (reduction); at the anode, negative ions lose electrons (oxidation).
Molten binary compounds produce the metal at the cathode and the non-metal at the anode.

A simple electrolysis cell: cations move to the cathode, anions to the anode.

Practice questions with answers

Question 12 marks

Molten lead(II) bromide is electrolysed. Name the product formed at each electrode.

At the cathode, lead (Pb) is formed. At the anode, bromine (Br2) is formed.

Mark schemeLead at the cathode (1). Bromine at the anode (1).

Common mistakeSwapping the electrodes. The metal always forms at the cathode.

Exam tipRemember the metal goes to the cathode. Positive metal ions are attracted to the negative electrode.

Question 22 marks

Explain why lead(II) bromide must be molten before it can be electrolysed.

The ions must be free to move so they can carry the charge to the electrodes. In the solid, the ions are held in fixed positions in the lattice and cannot move, so no electrolysis occurs. Melting the compound frees the ions.

Mark schemeIons must be free to move to carry charge (1). In the solid ions are fixed in the lattice (1).

Common mistakeSaying the solid does not conduct without explaining that the ions cannot move.

Exam tipAlways link conduction to ions being free to move. That is the reasoning examiners want.

Question 32 marks

Write the half equation for the reaction at the cathode when lead ions, Pb2+, are discharged.

Pb2+ plus 2e- gives Pb. This is a reduction because electrons are gained.

Mark schemeCorrect species and balance: Pb2+ + 2e- gives Pb (1). Correct number of electrons, 2e- (1).

Common mistakeAdding electrons to the wrong side or using the wrong charge on the ion.

Exam tipAt the cathode electrons are always added on the left. Reduction is gain of electrons.

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Frequently asked questions

What is the difference between the anode and cathode?

The anode is the positive electrode where anions lose electrons. The cathode is the negative electrode where cations gain electrons.

Why does electrolysis need a molten or aqueous compound?

The ions must be free to move to carry the charge. In a solid they are fixed in the lattice, so it cannot be electrolysed.

What forms at the cathode?

Positive ions are attracted to the cathode. In a molten binary compound the metal forms there. In solution, hydrogen forms unless the metal is below hydrogen in reactivity.

Is oxidation at the anode or cathode?

Oxidation, the loss of electrons, happens at the anode. Reduction, the gain of electrons, happens at the cathode.

More IGCSE Chemistry practice

Stoichiometry Acids, bases and salts Atoms, elements and compounds