How electrolysis works, why ionic compounds must be molten or dissolved, and the extraction of aluminium including why cryolite is used and why the anodes wear away.
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1.What is electrolysis?
Using an electric current to break down an ionic compound into its elements.
2.What is an electrolyte?
The molten or dissolved ionic compound that conducts the electricity during electrolysis.
3.Why must an ionic compound be molten or dissolved?
The ions must be free to move to the electrodes. In a solid they are held in fixed positions.
4.What is the cathode?
The negative electrode.
5.What is the anode?
The positive electrode.
6.Which ions move to the cathode, and why?
Positive ions, because they are attracted to the negative electrode.
7.Which ions move to the anode, and why?
Negative ions, because they are attracted to the positive electrode.
8.What is produced at the cathode during electrolysis of a molten compound?
The metal, because metal ions are positive and gain electrons there.
9.What is produced at the anode during electrolysis of a molten compound?
The non-metal, because non-metal ions are negative and lose electrons there.
10.Is the reaction at the cathode oxidation or reduction?
Reduction, because ions gain electrons.
11.Is the reaction at the anode oxidation or reduction?
Oxidation, because ions lose electrons.
12.What are the products of electrolysing molten lead bromide?
Lead at the cathode and bromine at the anode.
13.Why are inert electrodes often used?
So the electrodes do not take part in the reaction and affect the products.
14.Which ore is aluminium extracted from?
Bauxite, which contains aluminium oxide.
15.Why must aluminium be extracted by electrolysis rather than with carbon?
Aluminium is above carbon in the reactivity series, so carbon cannot remove the oxygen from its oxide.
16.What is cryolite and why is it used?
A compound that the aluminium oxide is dissolved in. It lowers the melting point, so less energy is needed and the process is cheaper.
17.What is produced at the cathode in aluminium extraction?
Molten aluminium, which collects at the bottom and is tapped off.
18.What is produced at the anode in aluminium extraction?
Oxygen.
19.Why must the anodes be replaced regularly?
They are made of carbon, and the oxygen produced reacts with them at the high temperature to form carbon dioxide, so they burn away.
20.Why is aluminium extraction so expensive?
It uses large amounts of electricity, both to melt the mixture and to drive the electrolysis.
21.Write the half equation at the cathode for aluminium.
Al3+ + 3e- gives Al
22.Write the half equation at the anode for oxide ions.
2O2- gives O2 + 4e-
23.Why is recycling aluminium worthwhile?
Melting recycled aluminium uses far less energy than extracting it from its ore by electrolysis.
24.What happens to the mass of the cathode during electrolysis of a molten metal compound?
It increases, as metal is deposited on it.
25.Why does electrolysis require a direct current rather than alternating?
The electrodes must keep a fixed charge so that each type of ion always travels to the same electrode.
26.What state must the electrolyte be in, and why does that need heating?
Molten. Ionic compounds have very high melting points because of the strong electrostatic forces in the lattice.
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