The process of electrolysis — AQA GCSE Combined Science
Test yourself on The process of electrolysis with AQA GCSE practice questions.
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The process of electrolysis explained
In a solid ionic compound, the ions are held in a fixed lattice and cannot move, so the solid does not conduct electricity.
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When the compound is melted or dissolved in water, the lattice breaks down and the ions become free to move within the liquid or solution. These mobile ions carry charge through the liquid, allowing it to conduct electricity. Such liquids and solutions are called electrolytes. For example, solid sodium chloride does not conduct, but molten sodium chloride and sodium chloride solution both conduct because Na⁺ and Cl⁻ ions can move. During electrolysis, positive ions move towards the negative electrode and negative ions move towards the positive electrode. The ability to conduct depends on the presence of mobile ions, not on the compound being an ionic solid.
Passing an electric current through electrolytes causes the ions to move to the electrodes. Positively charged ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode (the anode). Ions are discharged at the electrodes producing elements. This process is called electrolysis.
Electrolysis needs a direct current through an electrolyte, which is an ionic compound that is molten or dissolved so its ions can move. The power supply pushes electrons through the wires, making the cathode negative and the anode positive. Positive ions, called cations, are attracted to the negative cathode; negative ions, called anions, are attracted to the positive anode. At each electrode the ion gains or loses electrons and is discharged, forming a neutral element. For example, in molten lead bromide, Pb²⁺ ions reach the cathode and form lead metal, while Br⁻ ions reach the anode and form bromine. The products are elements, so electrolysis decomposes the electrolyte.
(HT only) Throughout Section 4.4.3 Higher Tier students should be able to write half equations for the reactions occurring at the electrodes during electrolysis, and may be required to complete and balance supplied half equations.
(HT only) Half equations represent the reactions occurring at individual electrodes during electrolysis, showing the transfer of electrons (e⁻). At the cathode, positive ions gain electrons (reduction), so e⁻ are written on the left side of the equation. For example, Pb²⁺ + 2e⁻ → Pb. At the anode, negative ions lose electrons (oxidation), so e⁻ appear on the right. For example, 2Br⁻ → Br₂ + 2e⁻. To write a half equation, identify the ion and the element formed, balance the atoms, and then add electrons to balance the charge. When completing a supplied half equation, always balance atoms first, then charge. Combining half equations gives the full balanced equation.
Your focus
- Explain why solid ionic compounds do not conduct electricity but their melts and aqueous solutions do.
- Define an electrolyte as a liquid or solution containing mobile ions that can conduct electricity.
- Describe the movement of ions towards the electrodes during electrolysis.
Show all 9 objectives
- Describe the movement of positive and negative ions to the cathode and anode during electrolysis.
- Explain why an ionic compound must be molten or dissolved to undergo electrolysis.
- Identify the elements produced at each electrode when given a simple molten or aqueous electrolyte.
- (HT only) Write balanced half equations for reactions at the cathode and anode.
- (HT only) Complete and balance a supplied half equation by adding electrons correctly.
- (HT only) Combine two half equations to produce a balanced overall equation for electrolysis.
The process of electrolysis exam tips
Marking Points
- Solid ionic compounds do not conduct electricity because their ions are fixed in a lattice and cannot move.
- Melting an ionic compound breaks down the lattice and releases ions that are free to move.
- Dissolving an ionic compound in water separates the ions so they are free to move in the solution.
- Mobile ions carry charge through the liquid or solution, so the liquid or solution conducts electricity.
- A liquid or solution that conducts electricity because of mobile ions is called an electrolyte.
- During electrolysis, positive ions move to the negative electrode and negative ions move to the positive electrode.
- An electrolyte is an ionic compound that is molten or dissolved in water so that its ions are free to move and carry charge.
- A direct current from a power supply makes one electrode negative (the cathode) and the other positive (the anode).
- Positively charged ions (cations) move towards the negative electrode, the cathode.
- Negatively charged ions (anions) move towards the positive electrode, the anode.
- At the electrodes, ions gain or lose electrons and are discharged, producing neutral atoms or molecules of elements.
- The overall process of breaking down an ionic compound using electricity is called electrolysis.
- (HT only) A half equation represents the reaction at one electrode and includes electrons as e⁻.
- (HT only) At the cathode, positive ions gain electrons (reduction), so electrons are written on the left of the arrow.
- (HT only) At the anode, negative ions lose electrons (oxidation), so electrons are written on the right of the arrow.
- (HT only) Half equations must balance both atoms and charge; the number of electrons added must match the charge change.
- (HT only) To complete a supplied half equation, balance the atoms of each element first, then balance the overall charge using electrons.
Examiner Tips
- 💡When explaining conduction, always link the structure to the movement of ions, for example 'the ions are free to move and carry charge'.
- 💡Use the term 'electrolyte' correctly for a liquid or solution that conducts electricity because of mobile ions.
- 💡If asked why a solid does not conduct, refer to the fixed positions of ions in the lattice rather than saying there are no ions.
- 💡Name the cathode and anode and state their charges before describing ion movement, so your answer is precise.
- 💡Use the words cation and anion correctly, and link each to its electrode.
- 💡When asked to explain products, trace one positive ion to the cathode and one negative ion to the anode, then name the element formed.
- 💡(HT only) Balance atoms before adding electrons, then check the total charge on both sides last.
- 💡(HT only) For a supplied half equation, count the charge on each side and add the missing electrons to the side that needs them.
Common Mistakes
- Thinking that solid ionic compounds conduct electricity because they contain ions. Correction: the ions must be free to move, which they are not in the solid lattice.
- Believing that electrons carry charge through the electrolyte. Correction: in the liquid or solution, it is the mobile ions that carry charge; electrons flow in the external circuit.
- Assuming that all liquids conduct electricity. Correction: only liquids or solutions containing mobile ions, called electrolytes, conduct.
- Thinking that positive ions move to the positive electrode: correct this by linking opposite charges attract, so positive ions go to the negative cathode.
- Believing that solid ionic compounds conduct during electrolysis: correct this by stating that the ions must be molten or dissolved so they can move.
- Writing that ions are 'turned into electricity' or 'melted' at the electrodes: correct this by saying ions are discharged, gaining or losing electrons to form elements.
- Putting electrons on the wrong side. Correction: remember reduction at the cathode gains electrons (e⁻ on the left), and oxidation at the anode loses electrons (e⁻ on the right).
- Forgetting to balance charge as well as atoms. Correction: check that the total charge is exactly the same on both sides of the arrow.
- Writing e instead of e⁻ or omitting the charge. Correction: always show the electron as e⁻ with its negative charge.