Electrolysis
5 marks · EasyA student compares the electrolysis of molten lead(II) bromide with the electrolysis of aqueous sodium chloride solution.
State the product formed at the cathode during the electrolysis of molten lead(II) bromide. Explain your answer briefly.
Describe how to set up the apparatus to investigate the electrolysis of an aqueous sodium chloride solution. State one safety precaution.
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) Lead (Pb) forms at the cathode because Pb2+ ions gain electrons (are reduced) to become Pb atoms (reduction).
(b) Set up an electrolytic cell with inert electrodes (carbon/platinum) in a beaker containing concentrated NaCl solution; connect to a power supply; collect gases at the electrodes using test tubes or gas collection tubes. Safety: wear eye protection and use a fume cupboard or ventilated area because chlorine gas is poisonous.
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Marking points
B1 (a) · 1 mark · Product at cathode correctly stated as lead (Pb); accepts "lead metal".
B2 (a) · 1 mark · Reason correctly given: Pb2+ ions gain electrons (are reduced) at cathode.
B3 (b) · 1 mark · Describes correct arrangement of apparatus for electrolysis: beaker (or glass tube), NaCl(aq), inert electrodes, power supply.
B4 (b) · 1 mark · Mentions collection of gases at electrodes using test tubes or similar.
B5 (b) · 1 mark · States a valid safety precaution (eye protection, fume cupboard, avoid inhaling chlorine, etc).
Worked solution
a) In molten lead(II) bromide, Pb2+ ions move to the cathode.
At the cathode, Pb2+ ions gain 2 electrons and are reduced to Pb atoms:
So, lead forms at the cathode because the metal ion is reduced there.
b) To investigate the electrolysis of aqueous sodium chloride:
- Place concentrated sodium chloride solution in a beaker or glass tube.
- Insert two inert electrodes (carbon or platinum) into the solution so they do not touch.
- Connect electrodes to a power supply.
- Place an upside-down test tube or a gas collection tube over each electrode to collect any gases formed.
- Safety: Wear eye protection and use a fume cupboard or well-ventilated room because chlorine gas is poisonous.