Electric Chemistry
How electrolysis splits ionic compounds with electricity: why they conduct only when molten or dissolved, what forms at the negative cathode and positive anode, how reactive metals like aluminium are extracted, and the rules for aqueous solutions.
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Splitting compounds with electricity
Some compounds are so stable that only electricity can break them apart. Electrolysis passes a current through a molten or dissolved ionic compound and splits it into its elements, and it is how we get reactive metals like aluminium out of their ores. This module works through why it conducts, what forms at each electrode, and the rules for solutions.
Electrolysis words to know
Learn these before you switch on the current.
Cathode against anode
The two electrodes pull opposite ions and make opposite products.
Say why it conducts, then use the rules
In the exam, first say why it conducts: an ionic compound conducts only once it is melted down or dissolved in water, because that sets its ions free to move. Then apply the electrode rules. At the negative cathode you collect the metal when it is molten, but in a solution you collect hydrogen unless the metal is less reactive than hydrogen. At the positive anode you collect the non-metal: oxygen, unless a halide is present, when you collect the halogen. Work cathode first, then anode.
Match the term
- electrolysis
- cathode
- anode
- electrode
- splitting an ionic compound with electricity
- the negative electrode where metal forms
- the positive electrode where non-metals form
- a rod that carries the current in
Match the product
- molten lead bromide at the cathode
- molten lead bromide at the anode
- a reactive-metal solution at the cathode
- extracting aluminium
- lead metal forms
- bromine forms
- hydrogen forms
- uses electrolysis of molten aluminium oxide
Why does it conduct?
Why does an ionic compound conduct electricity only when it is molten or dissolved?
- Its ions are then free to move
- It turns into a metal
- It stops being ionic
- It gains extra electrons from the air
True about electrolysis
Select the TWO true statements about electrolysis.
- At the cathode you can get the metal or hydrogen
- Reactive metals like aluminium are extracted using electrolysis
- The anode is the negative electrode
- A solid ionic compound conducts electricity well
Count the electrons
Each aluminium ion has a charge of 3 plus, so it needs 3 electrons to become an aluminium atom. Multiply 3 by 4 to find how many electrons are needed to make 4 aluminium atoms. What is the answer?
Order the process
Put the steps of electrolysis in order, earliest first.
- Melt or dissolve the compound so the ions can move
- Dip two electrodes in and switch on the current
- Positive ions move to the negative cathode
- Negative ions move to the positive anode
Complete the rules
Splitting an ionic compound using electricity is called _____. It only works when the compound is molten or dissolved, so the ions are free to _____. The negative electrode is the _____. The positive electrode is the _____.
Spot the true electrolysis facts
Tap the TWO true statements about electrolysis.
- an ionic compound conducts when molten or dissolved
- the metal forms at the negative cathode
- a solid ionic compound conducts electricity
- the anode is the negative electrode
Predict the product
Read each case and choose what forms or why.
- Molten lead bromide is electrolysed. What forms at the negative cathode?
- A solution of copper chloride is electrolysed, and a halide is present at the anode. What forms there?
- Why is aluminium extracted by electrolysis rather than by heating its ore with carbon?
Explain electrolysis
Explain what electrolysis is and how to predict the products at each electrode.
- Explain why an ionic compound must be molten or dissolved to conduct
- Explain what forms at the cathode and the anode for a molten compound
- Explain the cathode and anode rules for an aqueous solution