Electrolysis of Water (WJEC GCSE Chemistry)
Revision Note
Electrolysis of Water
What is the electrolysis of water?
Electrolysis of water is the process of using electricity to split water into hydrogen and oxygen
2H2O (l) → 2H2 (g) + O2 (g)
This process can be done using a Hoffman voltameter
When the current flows:
Hydrogen ions, H+, in the water are attracted to the cathode
They gain electrons (reduced) and hydrogen gas is formed
2H+ (aq) + 2e– (aq) → H2 (g)
Hydroxide ions, OH–, in the water are attracted to the anode
They lose electrons (oxidised) and oxygen gas is formed
2OH– (aq) → O2 (g) + 2H+ (aq) + 4e–
The Hoffman voltameter
The Hoffman voltameter is used to split water into two gases, hydrogen and oxygen
In the overall equation you will notice that there are 2 moles of hydrogen gas produced compared to 1 mole of oxygen
This is shown in the experiment as the volume of hydrogen gas formed is double the volume of oxygen gas
The gases formed at each electrode can be tested for
Examiner Tips and Tricks
You should be able to write the a balanced equation to show the reaction taking place at the cathode, but you are not expected to recall the equation taking place at the anode.
Cathode:
2H+ (aq) + 2e– (aq) → H2 (g)
Anode:
2OH– (aq) → O2 (g) + 2H+ (aq) + 4e–
Higher tier candidates should be able to balance the equation for the reaction at the anode.
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