Revision notes for AQA GCSE Chemistry Electrolysis of molten ionic compounds. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Electrolysis of molten ionic compounds
What you'll learn
- Why ionic compounds must be molten or dissolved before they conduct electricity.
- What happens at the cathode and anode during electrolysis.
- How to predict the products when a binary ionic compound is electrolysed in the molten state.
- How to avoid the common exam trap of mixing up molten and aqueous electrolysis.
The big idea: electricity can split ionic compounds
Ionic compounds are made from ions: charged particles formed when atoms lose or gain electrons. Metals usually form positive ions. Non-metals usually form negative ions.
For example, lead bromide, PbBr2, contains:
- lead ions, Pb2+
- bromide ions, Br-
In solid lead bromide, the ions are held in a fixed giant ionic lattice. They can vibrate, but they cannot move from place to place. This means solid ionic compounds do not conduct electricity.
Definition
Electrolysis
Electrolysis is the decomposition, or breaking down, of an ionic compound using electricity.
Definition
Electrolyte
An electrolyte is a liquid or solution that contains mobile ions and can conduct electricity.
Why the compound must be molten
Definition
Molten
Molten means melted. A molten ionic compound is an ionic compound that has been heated until it becomes a liquid.
When an ionic compound melts, the ions are no longer fixed in position. They can move through the liquid.
That movement matters because an electric current in an electrolyte is carried by moving ions. If the ions cannot move, electrolysis cannot happen.
Key Idea
Why melting is needed
A molten ionic compound conducts electricity because its ions are free to move to the electrodes.
Example
Explaining why solid lead bromide does not electrolyse
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In solid PbBr2(s), the Pb2+ and Br- ions are held in fixed positions in a giant ionic lattice.
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Electrolysis needs charged particles to move through the electrolyte, so the ions must be able to travel towards electrodes.
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Because the ions in solid PbBr2(s) cannot move, it does not conduct electricity and cannot be electrolysed until it is melted to PbBr2(l).
The electrolysis cell
In molten electrolysis, the molten ionic compound is placed between two electrodes connected to a d.c. power supply.
Definition
Electrode
An electrode is a conductor, usually made of graphite or a metal, that allows electricity to enter or leave the electrolyte.
The two electrodes have different names:
- the cathode is the negative electrode
- the anode is the positive electrode
For this topic, the electrodes are usually inert.
Definition
Inert electrode
An inert electrode conducts electricity but does not react with the electrolyte or the products during electrolysis.
Graphite is commonly used because it conducts electricity and usually does not react in this type of school-level setup.
Here is a labelled setup for molten lead bromide, PbBr2(l).
Tip
Remember the electrode charges
In electrolysis, the cathode is negative and the anode is positive. A useful memory hook is: PANIC — Positive Anode, Negative Is Cathode.
Ion movement during electrolysis
Opposite charges attract.
So in electrolysis:
- positive metal ions move to the negative cathode
- negative non-metal ions move to the positive anode
For molten lead bromide:
- Pb2+ ions move to the cathode
- Br- ions move to the anode
Key Idea
Direction of ion movement
Positive ions go to the cathode. Negative ions go to the anode.
What forms at the cathode?
The cathode is negative, so it attracts positive metal ions.
At the cathode, positive metal ions gain electrons. This turns them into neutral metal atoms.
For molten lead bromide:
Pb2+(l) + 2e- → Pb(l)
Lead metal is produced at the cathode.
Definition
Reduction
Reduction is gain of electrons.
So the cathode is where reduction happens.
Example
Finding the cathode product from molten zinc chloride
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Zinc chloride, ZnCl2(l), contains Zn2+ ions and Cl- ions.
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The positive ion, Zn2+, is attracted to the negative cathode.
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At the cathode, Zn2+ ions gain electrons to become zinc atoms, so the product is zinc metal: Zn(l).
What forms at the anode?
The anode is positive, so it attracts negative non-metal ions.
At the anode, negative ions lose electrons. This turns them into neutral non-metal atoms or molecules.
For molten lead bromide:
2Br-(l) → Br2(g) + 2e-
Bromine is produced at the anode.
Definition
Oxidation
Oxidation is loss of electrons.
So the anode is where oxidation happens.
Tip
OIL RIG
Oxidation Is Loss, Reduction Is Gain of electrons.
The overall reaction
In molten lead bromide, lead bromide is broken down into lead and bromine:
PbBr2(l) → Pb(l) + Br2(g)
This is a decomposition reaction caused by electricity.
For many simple molten ionic compounds, the pattern is very reliable:
- the metal forms at the cathode
- the non-metal forms at the anode
Key Idea
Main rule for molten binary ionic compounds
When a molten binary ionic compound is electrolysed using inert electrodes, the metal is produced at the cathode and the non-metal is produced at the anode.
Predicting products for binary ionic compounds
Definition
Binary ionic compound
A binary ionic compound contains ions from two elements: usually one metal and one non-metal.
Examples include:
| Molten ionic compound | Ions present | Product at cathode | Product at anode |
|---|---|---|---|
| NaCl(l) | Na+, Cl- | sodium, Na(l) | chlorine, Cl2(g) |
| PbBr2(l) | Pb2+, Br- | lead, Pb(l) | bromine, Br2(g) |
| ZnCl2(l) | Zn2+, Cl- | zinc, Zn(l) | chlorine, Cl2(g) |
| MgO(l) | Mg2+, O2- | magnesium, Mg(l) | oxygen, O2(g) |
Notice that some non-metals form diatomic molecules, meaning molecules made of two atoms. Chlorine is Cl2, bromine is Br2, and oxygen is O2.
Example
Predicting products from molten sodium chloride
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Sodium chloride, NaCl(l), contains Na+ ions and Cl- ions.
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Na+ ions are positive, so they move to the negative cathode and gain electrons. Sodium forms: Na(l).
-
Cl- ions are negative, so they move to the positive anode and lose electrons. Chlorine forms as Cl2(g).
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The overall balanced equation is: 2NaCl(l) → 2Na(l) + Cl2(g).
Why inert electrodes matter
If the electrodes are inert, they do not take part in the chemical reactions. This makes prediction simpler: the products come only from the ions in the molten compound.
For example, in molten lead bromide using inert electrodes:
- Pb2+ ions become Pb(l)
- Br- ions become Br2(g)
The graphite electrodes just provide a surface for electron transfer.
Common Mistake
Forgetting the electrodes are inert
Do not write graphite or carbon as a product just because the electrodes are made from graphite. In this topic, inert electrodes conduct electricity but are not used up in the reaction.
Molten is not the same as aqueous
This topic is about molten ionic compounds, not solutions.
Definition
Aqueous
Aqueous means dissolved in water, shown by the state symbol (aq).
Aqueous electrolysis is more complicated because water also provides ions, so extra products like hydrogen or oxygen may form. You do not use those aqueous rules here.
For molten ionic compounds, there are only the ions from the compound itself.
Common Mistake
Using aqueous rules for molten compounds
If the question says molten, ignore water completely. There is no water present, so you predict products only from the ions in the ionic compound.
A note on safety in practical work
Lead bromide is a common teaching example because the product pattern is clear: lead forms at the cathode and bromine forms at the anode.
However, lead compounds and bromine are hazardous. In practical demonstrations, a safer alternative may be anhydrous zinc chloride.
Definition
Anhydrous
Anhydrous means containing no water. Anhydrous zinc chloride is used because this topic is about molten compounds, not aqueous solutions.
Molten zinc chloride produces zinc at the cathode and chlorine at the anode:
ZnCl2(l) → Zn(l) + Cl2(g)
The prediction method
Use this simple method whenever the question gives you a molten binary ionic compound.
- Split the formula into its ions.
- Identify the positive metal ion.
- The positive metal ion goes to the cathode and becomes the metal.
- Identify the negative non-metal ion.
- The negative non-metal ion goes to the anode and becomes the non-metal.
Example
Predicting products from molten magnesium oxide
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Magnesium oxide, MgO(l), contains Mg2+ ions and O2- ions.
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Mg2+ is the positive metal ion, so it moves to the cathode and gains electrons. The cathode product is magnesium, Mg(l).
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O2- is the negative non-metal ion, so it moves to the anode and loses electrons. The anode product is oxygen, O2(g).
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The balanced overall equation is: 2MgO(l) → 2Mg(l) + O2(g).
Exam technique
In the exam
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Check whether the compound is molten or aqueous before predicting products.
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For molten binary ionic compounds, send the metal ion to the cathode and the non-metal ion to the anode.
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Include state symbols in equations when asked: molten compound is (l), metals produced are often (l), and halogens such as chlorine or bromine are usually (g).
Self review
Check yourself
- Why can molten lead bromide conduct electricity but solid lead bromide cannot?
- What forms at the cathode and anode when molten zinc chloride is electrolysed?
- Why must you not use aqueous electrolysis rules for a molten ionic compound?
Electrolysis
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