Practise Electrolysis of molten ionic compounds. 12 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 and, at Higher tier, half equations.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State the products from molten lead bromideLead is produced at the cathode and bromine at the anode.
4
Predict products for any molten binary compoundThe metal forms at the cathode and the non-metal forms at the anode.
5
Describe what is seen at each electrodeFor lead bromide, silvery lead forms at the cathode and brown bromine vapour at the anode.
6
Explain how each product formsMetal ions are attracted to the cathode and gain electrons; non-metal ions are attracted to the anode and lose electrons.
7
Write half equations for molten electrolysisFor example, Pb2+ + 2e− → Pb and 2Br− → Br2 + 2e−.
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
Lead ions, Pb2+, move to the cathode, gain electrons and form lead, which appears as a silvery metal (molten at this temperature).
Bromide ions, Br−, move to the anode, lose electrons and form bromine, seen as brown vapour.
Bromine is toxic, so this is done in a fume cupboard.
Half equations for lead bromide: Pb2+ + 2e− → Pb (cathode) and 2Br− → Br2 + 2e− (anode). grade 7+
Halogens form diatomic molecules, so two halide ions are needed for each molecule: 2Cl− → Cl2 + 2e−. grade 7+
Cheatsheet
Molten binary compound: metal at the cathode (−), non-metal at the anode (+)
Lead bromide → lead + bromine
Cathode: Pb2+ + 2e− → Pb
Anode: 2Br− → Br2 + 2e−
Halogens are diatomic: Cl2, Br2, I2
Must be molten so the ions can move
How to answer each type of question
Predict the products at each electrode
2 marks4
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 answer
(a) calcium (1) (b) bromine (1)
Explain how the products form
4 marks6
For each ion, say its charge and which electrode it moves to.
Say whether it gains or loses electrons there.
Name the element formed.
Example. Molten potassium iodide is electrolysed. Explain how potassium and iodine are produced.
Show the model answer
Potassium ions are positive, so they move to the cathode / negative electrode (1) where they gain electrons to form potassium atoms (1) Iodide ions are negative, so they move to the anode / positive electrode (1) where they lose electrons to form iodine (molecules) (1)
Write the half equations
2 marks7
Cathode: metal ion + electrons (equal to its charge) → metal atom.
Anode: two halide ions → halogen molecule + 2 electrons.
Check that atoms and charges balance.
Example. Molten zinc chloride is electrolysed. Write half equations for the reactions at the cathode and at the anode.
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.
The charge on the metal ion is the number of electrons in its half equation: Pb2+ needs 2e−.
Where marks are lost
Writing 'chloride' or 'bromide' as the product at the anode instead of chlorine or bromine.
Writing Br instead of Br2 in a half equation.
Putting the metal at the anode.
Not balancing electrons: 2Br− → Br2 + 2e−, not + e−.
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'.
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
Molten sodium chloride is electrolysed. Write the half equation for the reaction at the negative electrode.
Na+ + e− → Na
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
Question 3Hard6 marks
During electrolysis, reactions at the electrodes can be represented by half equations.
(a) Molten sodium chloride is electrolysed. Write the half equation for the reaction at the negative electrode.[1]
(b) Write the half equation for the reaction at the positive electrode during the electrolysis of molten sodium chloride.[1]
(c) Molten lead bromide is electrolysed. Complete the half equations. Pb2+ + ........ → Pb ....Br− → Br2 + ........[2]
(d) Explain which electrode reaction in the electrolysis of molten lead bromide is oxidation and which is reduction.[2]
Show the answer and mark scheme
(a)Answer: Na+ + e− → Na
Na+ + e− → Na
(b)Answer: 2Cl− → Cl2 + 2e−
2Cl− → Cl2 + 2e−
(c)Answer: Pb2+ + 2e− → Pb; 2Br− → Br2 + 2e−
Pb2+ + 2e− → Pb
2Br− → Br2 + 2e−
(d)
at the negative electrode lead ions gain electrons, so this is reduction
at the positive electrode bromide ions lose electrons, so this is oxidation