How can passing electricity through a molten ionic compound break it down into its elements?
Describe electrolysis of a molten binary ionic compound, name the electrodes and products, and explain why the compound must be molten
A focused answer to the N(A) Chemistry outcome on electrolysis of molten compounds. The electrodes, why the compound must be melted, and how to predict the metal at the cathode and the non-metal at the anode.
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What this dot point is asking
The syllabus wants you to describe the electrolysis of a molten (melted) ionic compound made of two elements, to name the electrodes and the products at each, and to explain why the compound must be molten. The key idea is that electrolysis uses electricity to break a compound down into its elements, and it only works when the ions are free to move.
The answer
What electrolysis is
Electrolysis is breaking down an ionic compound into its elements using electricity. The melted (or dissolved) compound is called the electrolyte. Two rods called electrodes dip into it and are connected to a power supply.
The two electrodes
The electrodes are named by their charge:
- the cathode is the negative electrode,
- the anode is the positive electrode.
Opposite charges attract, so positive ions move to the negative cathode and negative ions move to the positive anode.
Why the compound must be molten
In a solid, the ions are locked in a fixed lattice and cannot move, so the solid does not conduct and cannot be electrolysed. When the compound is melted, the ions become free to move. They can then travel to the electrodes and carry the charge, so the melted compound conducts and can be broken down.
The products
For a molten compound of two elements (a metal and a non-metal):
- the metal forms at the cathode. The positive metal ions move to the negative cathode and gain electrons to become metal atoms.
- the non-metal forms at the anode. The negative non-metal ions move to the positive anode and lose electrons to become non-metal atoms.
So electrolysing molten lead(II) bromide gives lead at the cathode and bromine at the anode.
Examples in context
Example 1. Extracting aluminium. Aluminium is too reactive to extract by heating with carbon, so it is obtained by electrolysing molten aluminium oxide. Aluminium metal forms at the cathode and oxygen at the anode, a direct industrial use of molten electrolysis to win a reactive metal.
Example 2. Showing a compound is ionic. If a melted substance conducts electricity and is broken down into elements, it must be made of ions. So electrolysis is also a test that helps confirm a compound is ionic rather than covalent.
Try this
Q1. State why solid lead(II) bromide does not conduct electricity but molten lead(II) bromide does. [2 marks]
- Cue. In the solid the ions are fixed and cannot move; when molten the ions are free to move and carry the charge.
Q2. Molten potassium iodide is electrolysed. State the product at the cathode and the product at the anode. [2 marks]
- Cue. Potassium metal forms at the cathode; iodine forms at the anode.
Q3. Explain why the metal forms at the cathode in the electrolysis of a molten compound. [2 marks]
- Cue. The metal ions are positive, so they are attracted to the negative cathode, where they gain electrons to become metal atoms.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksMolten lead(II) bromide () is electrolysed using carbon electrodes. (a) Name the product formed at the cathode. (b) Name the product formed at the anode. (c) Explain why the lead(II) bromide must be melted before it will conduct.Show worked answer →
(a) Lead metal forms at the cathode (the negative electrode).
(b) Bromine forms at the anode (the positive electrode).
(c) When solid, the ions are fixed in place and cannot move. When melted, the ions are free to move to the electrodes and carry the charge, so the compound conducts and can be broken down.
What markers reward: lead at the cathode, bromine at the anode, and that melting frees the ions to move and carry charge.
Original4 marksMolten sodium chloride is electrolysed. (a) Name the two elements produced. (b) State at which electrode each is formed, and explain why the metal forms there.Show worked answer →
(a) Sodium and chlorine are produced.
(b) Sodium forms at the cathode (negative electrode) and chlorine at the anode (positive electrode). The sodium ions are positive (), so they are attracted to the negative cathode, where they gain electrons to become sodium metal.
What markers reward: sodium and chlorine named, the correct electrode for each, and positive sodium ions attracted to the negative cathode.
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