Electrolysis: electrolysis of molten ionic compounds and aqueous solutions, predicting the products at the electrodes, writing half-equations, and the electrolysis of copper sulfate core practical.

What this dot point is asking

Edexcel wants you to explain electrolysis, describe the electrolysis of molten ionic compounds, use rules to predict the products at the cathode and anode of aqueous solutions, write half-equations, identify the oxidation and reduction at each electrode, and describe the electrolysis of copper sulfate core practical. Half-equations and product prediction are reliable exam marks.

What electrolysis is

For ions to move and carry the current, the compound must be molten or in solution, so the ions are free. The electrodes are usually inert (carbon or platinum):

• The cathode is the negative electrode; positive ions (cations) move to it.
• The anode is the positive electrode; negative ions (anions) move to it.

At the cathode, ions gain electrons (reduction); at the anode, ions lose electrons (oxidation). (Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons.)

Electrolysis of molten compounds

For a molten ionic compound there are only two ions, so the products are straightforward:

• Cathode: the metal.
• Anode: the non-metal.

For molten lead bromide, lead forms at the cathode and bromine at the anode. The half-equations are $Pb^{2+} + 2e^- \rightarrow Pb$ (cathode) and $2Br^- \rightarrow Br_2 + 2e^-$ (anode).

Electrolysis of aqueous solutions

In solution there are also $H^+$ and $OH^-$ ions from the water, so the products are decided by rules:

So electrolysis of copper sulfate solution gives copper at the cathode (copper is below hydrogen in reactivity) and oxygen at the anode (sulfate, not a halide). Electrolysis of concentrated sodium chloride solution gives hydrogen at the cathode (sodium is more reactive than hydrogen) and chlorine at the anode (concentrated chloride).

Writing half-equations

A half-equation shows the electrons gained or lost so that the charges balance:

• Cathode (reduction, electrons on the left): $2H^+ + 2e^- \rightarrow H_2$ or $Cu^{2+} + 2e^- \rightarrow Cu$.
• Anode (oxidation, electrons on the right): $4OH^- \rightarrow O_2 + 2H_2O + 4e^-$ or $2Cl^- \rightarrow Cl_2 + 2e^-$.

The core practical: electrolysis of copper sulfate

The Edexcel core practical investigates the electrolysis of copper sulfate solution, often comparing inert (carbon) and copper electrodes.

• With inert electrodes, copper is deposited on the cathode and oxygen is released at the anode, so the blue colour fades as copper ions are removed.
• With copper electrodes, the anode dissolves and copper is deposited on the cathode, so the cathode gains the mass that the anode loses (this is used in purifying copper).

Try this

Q1. State what is produced at the cathode when molten zinc chloride is electrolysed. [1 mark]

• Cue. Zinc metal.

Q2. Write the half-equation for the formation of oxygen at the anode from hydroxide ions. [2 marks]

• Cue. $4OH^- \rightarrow O_2 + 2H_2O + 4e^-$.

Q3. Explain why hydrogen, not sodium, is produced at the cathode when sodium chloride solution is electrolysed. [2 marks]

• Cue. Sodium is more reactive than hydrogen, so the less reactive hydrogen from the water is discharged in preference.