Electrolysis
5 marks · EasyA student investigates the electrolysis of two different substances using graphite electrodes:
Molten lead(II) bromide (PbBr₂) is electrolysed. Predict which element forms at the cathode and which at the anode. Explain how you know.
The student now electrolyses concentrated aqueous magnesium chloride (MgCl₂(aq)). After testing the gases, they find the volume of chlorine gas collected at the anode is much less than the volume of hydrogen collected at the cathode.
Explain why hydrogen, and not magnesium, is formed at the cathode.
Give a reason why less chlorine gas is collected at the anode than expected.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Cathode: lead; Anode: bromine. Metal (lead) forms at cathode, non-metal (bromine) at anode because metals form cations and non-metals form anions. · must show Cathode: lead, Anode: bromine, Reason for each (metal/non-metal to respective electrode)
(b(i)) Hydrogen formed because it is preferentially discharged over magnesium from aqueous solution; hydrogen ions are reduced more easily than magnesium ions. · must show Hydrogen ions reduced more easily than magnesium ions, In aqueous solution, hydrogen forms at cathode
(b(ii)) Chlorine is more soluble in water, so some dissolves instead of being collected as a gas, leading to a lower volume observed. · must show Chlorine is soluble in water, Some chlorine dissolves rather than being collected, Therefore, observed gas volume is less than expected
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Marking points
B1 (a) · 1 mark · States lead forms at the cathode and bromine at the anode (both correct).
B2 (a) · 1 mark · Explains why metal forms at the cathode and non-metal at the anode in molten lead(II) bromide (ions attracted to opposite electrodes).
B3 (b(i)) · 1 mark · Explains hydrogen formed at cathode because it is discharged in preference to magnesium from aqueous solution (hydrogen ions more easily reduced).
B4 (b(ii)) · 1 mark · Recognises that chlorine is soluble in water.
B5 (b(ii)) · 1 mark · Links chlorine’s solubility to lower observed gas volume at the anode (explanation).
Worked solution
a) During electrolysis of molten lead(II) bromide (PbBr₂):
At the cathode (negative electrode): Pb²⁺ ions gain 2 electrons and are reduced to lead metal.
At the anode (positive electrode): Br⁻ ions lose electrons and are oxidised to bromine gas.
Metals form positive ions and move to the cathode, while non-metals form negative ions and move to the anode.
b) Electrolysis of concentrated aqueous MgCl₂:
i) Although magnesium ions are present, hydrogen ions (from water) are reduced more easily at the cathode. Hydrogen forms as it is less reactive, so H₂ gas is collected at the cathode.
ii) Less chlorine gas is collected at the anode than expected because chlorine is quite soluble in water. Some of the chlorine formed dissolves into the solution rather than collecting as a gas, leading to a smaller observed volume.