How does electrolysis split compounds, and what forms at each electrode?
Electrolysis of molten ionic compounds and of aqueous solutions including brine, predicting the products at each electrode, and writing electrode half-equations.
A CCEA GCSE Chemistry answer on electrolysis, covering how molten ionic compounds and aqueous solutions are split by an electric current, the rules for predicting the products at the cathode and anode including the electrolysis of brine, and writing electrode half-equations.
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What this dot point is asking
CCEA wants you to explain how electrolysis splits ionic compounds, predict the products at each electrode for molten compounds and for aqueous solutions including brine, and write the half-equations for the electrode reactions.
How electrolysis works
Ions are attracted to the oppositely charged electrode: positive ions (cations) to the cathode, negative ions (anions) to the anode. At the electrodes the ions gain or lose electrons, forming neutral elements. This is a redox process: reduction at the cathode, oxidation at the anode.
Electrolysis of molten compounds
This is how reactive metals such as aluminium are extracted: the molten oxide is electrolysed, with the metal forming at the cathode.
Electrolysis of solutions
In an aqueous solution, water also provides and ions, so there is competition at each electrode:
For brine (concentrated sodium chloride solution): sodium is too reactive, so hydrogen forms at the cathode; chloride is present, so chlorine forms at the anode; and sodium hydroxide is left in the solution. All three products are valuable industrially.
Worked example
Examples in context
Example 1. Extracting aluminium. Aluminium is made by electrolysing molten aluminium oxide, with aluminium forming at the cathode and oxygen at the anode. Because aluminium is too reactive for carbon to reduce, electrolysis is the only practical route, a direct link to the reactivity series.
Example 2. The chlor-alkali industry. Electrolysing brine produces chlorine, hydrogen and sodium hydroxide on an industrial scale, feeding the manufacture of bleach, plastics and soap. This single process supplies several major chemical industries from cheap salt water.
Try this
Q1. State what forms at the cathode when molten zinc chloride is electrolysed. [1 mark]
- Cue. Zinc (the metal).
Q2. Name the gas formed at the anode when brine is electrolysed. [1 mark]
- Cue. Chlorine.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20194 marksMolten lead(II) bromide is electrolysed. Name the products at each electrode and write the half-equation for the reaction at the cathode.Show worked answer →
Markers want both products and a correct cathode half-equation.
Molten lead(II) bromide contains and ions. Positive ions move to the negative cathode; negative ions move to the positive anode.
At the cathode, lead ions gain electrons and lead metal forms. At the anode, bromide ions lose electrons and bromine forms.
The cathode half-equation (reduction) is:
Markers reward lead at the cathode, bromine at the anode, and the balanced cathode half-equation.
CCEA 20214 marksConcentrated sodium chloride solution (brine) is electrolysed. State the products at each electrode and name one use of each product.Show worked answer →
The marks are for the three products and their uses.
At the cathode, hydrogen ions are discharged in preference to sodium (sodium is too reactive), so hydrogen gas forms. Hydrogen is used to make ammonia or margarine.
At the anode, chlorine gas forms from the chloride ions. Chlorine is used to make bleach or to disinfect water.
The solution left behind contains sodium hydroxide, used to make soap or paper.
Markers reward hydrogen at the cathode, chlorine at the anode, sodium hydroxide remaining, and a sensible use of each.
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)