AQA GCSE Chemistry Foundation (8462), Foundation tier · Chemical changes › Electrolysis
Practise Electrolysis of molten ionic compounds. 7 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 is produced when a binary ionic compound (a metal combined with a non-metal) is electrolysed in the molten state: the metal forms at the cathode and the non-metal at the anode. Expect prediction questions.
Key facts
Molten binary compound: metal at the cathode (−), non-metal at the anode (+)
Lead bromide → lead + bromine
Halogens are diatomic: Cl2, Br2, I2
Must be molten so the ions can move
Notes
Molten binary ionic compounds
A binary ionic compound is made of just two elements, a metal and a non-metal, e.g. lead bromide, PbBr2.
It must be heated until it melts, so the ions are free to move.
With inert electrodes (e.g. graphite), the metal is produced at the cathode (negative) and the non-metal is produced at the anode (positive).
This is simpler than a solution: there is no water, so only the two ions of the compound are present.
Example: lead bromide
diagram
Lead ions, Pb2+, move to the cathode and form lead, which appears as a silvery metal (molten at this temperature).
Bromide ions, Br−, move to the anode and form bromine, seen as brown vapour.
The solid must be melted first so that its ions can move. Do this in a fume cupboard.
Bromine is toxic, so this is done in a fume cupboard.
Split the compound into its metal ion and non-metal ion.
Metal at the cathode; non-metal at the anode.
Name the non-metal as an element: chlorine, not chloride.
Example. Molten calcium bromide is electrolysed using inert electrodes. Name the product at (a) the cathode and (b) the anode.
Show the model answerHide the model answer
(a) calcium (1) (b) bromine (1)
Don’t lose marks
Writing 'chloride' or 'bromide' as the product at the anode instead of chlorine or bromine.
Putting the metal at the anode.
More tips
Memory tricks
Molten: the Metal goes to the cathode (M for molten, M for metal).
Name change at the anode: chloride → chlorine, bromide → bromine, iodide → iodine, oxide → oxygen.
Exam technique
Say 'molten' or 'melted', not 'dissolved', when the question is about a molten compound.
If asked why a fume cupboard is used, name the toxic product (e.g. bromine or chlorine).
Describe observations precisely: 'brown vapour at the anode', 'silvery metal at the cathode'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State the products from molten lead bromideLead is produced at the cathode and bromine at the anode.
Grade 4
Predict products for any molten binary compoundThe metal forms at the cathode and the non-metal forms at the anode.
Grade 5
Describe what is seen at each electrodeFor lead bromide, silvery lead forms at the cathode and brown bromine vapour at 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.
Name the product formed at the negative electrode (cathode).
lead
When a molten metal halide is electrolysed using inert electrodes, one product always forms at each electrode. State what is always produced at the negative electrode.
the metal
Electrolysis experiments usually use inert electrodes. Name two materials commonly used as inert electrodes.
graphite (carbon) and platinum
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
A teacher electrolysed molten lead bromide using inert electrodes in a fume cupboard.
(a) Name the product formed at the negative electrode (cathode).[1]
(b) Name the product formed at the positive electrode (anode).[1]
(c) Describe what the teacher would see at the positive electrode.[1]
(d) Explain why the lead bromide must be molten.[1]
Show the answer and mark scheme
(a)Answer: lead
lead
(b)Answer: bromine
bromine
(c)
(red-)brown / orange-brown vapour / gas
(d)
so that the ions are free to move
Question 2Medium3 marks
Molten ionic compounds must be heated to very high temperatures (often several hundred °C) before they can be electrolysed.
(a) Explain why the compound must be molten, rather than solid, for electrolysis to work.[2]
(b) Suggest one practical difficulty of electrolysing a molten ionic compound on an industrial scale.[1]
Show the answer and mark scheme
(a)
ions must be free to move (to the electrodes) to conduct electricity and be discharged
in a solid, the ions are fixed in a lattice and cannot move
(b)
allow: a large amount of energy is needed to melt and maintain the compound at a high temperature (expensive) / special heat-resistant equipment is needed for safety