Practise The process of electrolysis. 13 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name the electrodes and their chargesThe cathode is the negative electrode and the anode is the positive electrode.
4
Define an electrolyteA molten or dissolved ionic compound, which conducts electricity because its ions are free to move.
5
Predict which electrode each ion moves toPositive ions move to the cathode and negative ions move to the anode.
6
Explain why solid ionic compounds do not conductIn a solid the ions are held in fixed positions in a lattice, so they cannot move to carry charge.
7
Explain what happens when ions are dischargedAt the electrodes, ions gain or lose electrons and become atoms or molecules of elements.
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
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.
Discharge at the electrodes
At the electrodes the ions are discharged: they gain or lose electrons and form elements.
At the cathode, positive ions gain electrons, so a metal (or hydrogen) is produced.
At the anode, negative ions lose electrons, so 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.
Cheatsheet
Electrolyte = molten or dissolved ionic compound; its ions are free to move
Discharged = an ion gains or loses electrons to become an element
Inert electrodes (graphite, platinum) do not react
How to answer each type of question
Define key terms
1 mark each4
Learn short, exact definitions of electrolyte, electrolysis, cathode and anode.
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 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 marks5
Positive ions go to the cathode (negative); negative ions go to the anode (positive).
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 answer
(a) Zinc ions / Zn2+ (1) (b) They are positive, so they are attracted to the negative electrode (1)
Explain why an ionic compound conducts only when molten or dissolved
2 to 3 marks6
Say what the ions do in the solid: fixed in position in a lattice, cannot move.
Say what changes when it melts or dissolves: the ions are free to move.
Link moving ions to carrying charge through the liquid.
Example. Solid sodium chloride does not conduct electricity, but molten sodium chloride does. Explain why.
Show the model answer
In the solid, the ions are held in fixed positions (in a lattice), so they cannot move (1) When molten, the ions are free to move (1) so they can carry charge / current through the liquid (1)
Shortcuts and 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.
Where marks are lost
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.
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.
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)
Question 3Hard5 marks
A student placed a crystal of potassium manganate(VII) in the centre of a strip of filter paper soaked in sodium sulfate solution. Each end of the strip was connected to a d.c. power supply. After 20 minutes, a purple colour had spread from the crystal towards the positive electrode. Potassium manganate(VII) contains K+ ions and MnO4− ions.
(a) Which ion causes the purple colour? Explain your answer.[2]
(b) Suggest why the filter paper was soaked in sodium sulfate solution.[1]
(c) Potassium ions are colourless. Describe the movement of the potassium ions during the experiment.[1]
(d) Predict what would happen to the purple colour if the connections to the power supply were reversed.[1]
Show the answer and mark scheme
(a)
the manganate(VII) ion / MnO4−
it moved towards the positive electrode, so it must be negatively charged
(b)
so the paper contains ions that can move / so the paper conducts electricity (acts as an electrolyte)
(c)
they move towards the negative electrode / cathode
(d)
it would move in the opposite direction / back towards the new positive electrode