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4.4.3.1The process of electrolysis

AQA GCSE Chemistry Foundation (8462), Foundation tier · Chemical changes › Electrolysis

Practise The process of electrolysis. 9 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.

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Revision notes

What electrolysis is, why an ionic compound must be molten or dissolved to be electrolysed, and which way the ions move. This is the basis of every other electrolysis subtopic and often opens a longer question.

Key facts

  • Electrolyte = molten or dissolved ionic compound; its ions are free to move
  • Cathode = negative electrode; anode = positive electrode
  • Positive ions → cathode; negative ions → anode
  • PANIC: Positive is Anode, Negative Is Cathode
  • Discharged = an ion becomes an element at an electrode
  • Inert electrodes (graphite, platinum) do not react

Notes

Electrolytes

  • Ionic compounds conduct electricity when melted (molten) or dissolved in water, because the ions are then free to move.
  • Solid ionic compounds do not conduct: the ions are held in fixed positions in the lattice and cannot move.
  • A molten or dissolved ionic compound that conducts electricity is called an electrolyte.
  • Electrolysis is breaking down an electrolyte into elements using electricity (a direct current).

Movement of ions

diagram
  • Two electrodes dip into the electrolyte and are connected to a d.c. power supply.
  • The cathode is the negative electrode. Positive ions, such as metal ions and H+, are attracted to it.
  • The anode is the positive electrode. Negative ions, such as Cl−, Br− and O2−, are attracted to it.
  • Opposite charges attract, so each ion moves towards the electrode with the opposite charge.
  • d.c. supplycathode (−)anode (+)+−electrolytepositive ions → cathode; negative ions → anode
    Opposite charges attract. The short thick line of the cell symbol is the negative terminal.

Discharge at the electrodes

  • At the electrodes the ions are discharged and form elements.
  • At the cathode, a metal (or hydrogen) is produced.
  • At the anode, a non-metal such as a halogen or oxygen is produced.
  • Inert electrodes, e.g. graphite or platinum, are used so that the electrodes do not react with the electrolyte or the products.

How to answer each type of question

Define key terms

1 mark eachGrade 4
  1. Learn short, exact definitions of electrolyte, electrolysis, cathode and anode.
  2. Include the charge when you name an electrode.

Example. (a) What is an electrolyte?
(b) What is the name of the positive electrode?

Show the model answerHide the model answer
(a) A molten or dissolved ionic compound / a liquid or solution that conducts electricity (1)
(b) Anode (1)

Predict which electrode ions move to, and why

2 marksGrade 5
  1. Positive ions go to the cathode (negative); negative ions go to the anode (positive).
  2. Explain using attraction between opposite charges.

Example. Molten zinc chloride, ZnCl2, is electrolysed.
(a) Which ions move to the cathode?
(b) Explain why.

Show the model answerHide the model answer
(a) Zinc ions / Zn2+ (1)
(b) They are positive, so they are attracted to the negative electrode (1)

Don’t lose marks

  • Saying electrons move through the electrolyte: in the electrolyte it is the ions that move.
  • Mixing up which electrode is positive and which is negative.
  • Saying solid ionic compounds do not conduct because they have no ions: they have ions, but the ions cannot move.
  • Writing 'molecules' or 'atoms' when you mean ions.

More tips

Memory tricks

  • PANIC: Positive Anode, Negative Is Cathode.
  • Cations go to the cathode and anions go to the anode (match the first letters: cat–cat, an–an).
  • Opposites attract: positive ions to the negative electrode, negative ions to the positive electrode.

Exam technique

  • In conduction questions, the phrase 'ions are free to move' earns the key mark.
  • Name an electrode and give its charge: 'the cathode (negative electrode)'.
  • If you draw the apparatus, label the d.c. supply, both electrodes (cathode and anode) and the electrolyte.

What each grade needs

What you need to be able to do, from the first marks up to the top grade.

  1. Grade 3
    Name the electrodes and their chargesThe cathode is the negative electrode and the anode is the positive electrode.
  2. Grade 4
    Define an electrolyteA molten or dissolved ionic compound, which conducts electricity because its ions are free to move.
  3. Grade 5
    Predict which electrode each ion moves toPositive ions move to the cathode and negative ions move to the anode.

Quick recall

Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.

What name is given to a liquid or solution that conducts electricity and is broken down by electrolysis?
electrolyte
A student electrolysed molten zinc chloride, ZnCl2, using inert electrodes. Give the name of the positive electrode.
anode
Name the electrode connected to the positive terminal of the power supply.
the anode
State which electrode, the cathode or the anode, is connected to the negative terminal of the power supply.
the cathode

Sample questions

Written for this site in the style of AQA exam questions. They are not taken from real past papers.

Question 1Easy4 marks
Electrolysis uses an electric current to break down ionic compounds.
(a) What name is given to a liquid or solution that conducts electricity and is broken down by electrolysis?[1]
(b) Solid sodium chloride does not conduct electricity. Molten sodium chloride does conduct electricity.
Explain why.[2]
(c) Which electrode do positive ions move towards during electrolysis?
Tick (✓) one box.[1]
  • the anode
  • the cathode
  • both electrodes
  • neither electrode
Show the answer and mark scheme
(a) Answer: electrolyte
  • electrolyte
(b)
  • in the solid the ions are held in fixed positions / cannot move
  • when molten the ions are free to move (and carry charge)
(c) Answer: the cathode
Question 2Medium5 marks
A student electrolysed molten zinc chloride, ZnCl2, using inert electrodes.
(a) Give the name of the positive electrode.[1]
(b) Which ions move towards the positive electrode? Explain why.[2]
(c) What is meant by an inert electrode?[1]
(d) Describe what happens to the ions when they reach the electrodes.[1]
Show the answer and mark scheme
(a) Answer: anode
  • anode
(b)
  • chloride ions / Cl−
  • they are negatively charged so are attracted to the positive electrode
(c)
  • an electrode that does not react (with the electrolyte or the products)
(d)
  • they are discharged: they gain or lose electrons to form atoms / molecules (elements)

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