Electrolysis concepts

1. (a) An aqueous solution of copper (II) sulphate was electrolysed between graphite electrodes.
   (i) State what was observed at the cathode.
   (ii) Write equation for the reaction that took place at the anode.
   

   (b) The solution that remained after electrolysis in (a) was tested with litmus solution.
   (i) State what was observed.
   (ii) Give a reason for your answer in (b) (i)
   (c) the electrolysis in (a) was repeated using copper electrodes that had been weighed before the experiment. State the change in mass of the electrode that took place after the electrolysis.

   Suggested answers

   (a) When an aqueous solution of copper (II) sulfate is electrolyzed between graphite electrodes:
   (i) At the cathode, copper ions (Cu²⁺) from the solution are reduced to form copper metal (Cu).
   Cu²⁺ + 2e^- → Cu (reduction)
   

   Therefore, the observation would be the formation of a brown coating on the cathode as the copper metal is deposited.

   (ii) At the anode, both sulphate ions (SO₄^2- and hydoxide ions (OH^- will migrate to it, but hydroxide will be selectively discharged.
   4OH^- → 2H₂O + O₂ + 4e-
   (b) After electrolysis, the solution is tested with litmus solution:
   (i) The litmus solution would turn red. This indicates that the solution has become acidic.
   (ii) The reason for the solution turning acidic is due to the presence of hydrogen ions (H⁺) which is remained in the solution during the electrolysis process. These hydrogen ions increase the acidity of the solution, turning the litmus solution red.
   (c) When the electrolysis is repeated using copper electrodes that had been weighed before the experiment, the change in mass of the electrode that took place after the electrolysis would be as follows:
   - The cathode would gain mass due to the deposition of copper metal from the solution.
   - The anode would lose mass as copper from the electrode is oxidized and enters the solution as copper ions (Cu²⁺).
2. Give two properties of graphite that make it a suitable material for use as an electrode. Explain your answer.

Suggested answers

i. Graphite is a good conductor of electricity due to the presence of delocalized electrons.
ii. It is inert electrode; therefore, it does not react with most electrolytes and most products of the electrolysis.
3. If 0.2 faradays are used in the production of hydrogen gas by using electrolytic process then the gas formed allowed to pass through a hot tube which contains sample of copper(II) oxide and copper (II) sulphate.
   1. In which chemical change and physical change occurred in samples?
   2. If the product for the chemical change involved one of its products was 10g of the solid and other product was liquid. Calculate the volume of the hydrogen gas that should be supplied and the volume of the hydrogen gas remained respectively to 0.2 Faradays at S.T.P.