Q2 of 7 Page 84

Compare the electrolysis of molten potassium chloride and solution of potassium chloride. What are the processes taking place at the cathode and the anode?

On electrolysis of molten potassium chloride, potassium chloride dissociates into K+ and Cl-. The anode is connected to positive terminal and cathode to the negative terminal of the battery. So, the oxidation occurs at anode and reduction occurs at the cathode. At the cathode, there are only K+ ions so they reduced and potassium will deposit on the cathode. And at the anode, there are only Cl- ions so they will oxidized and chlorine gas will evolve at the anode.

At anode- 2Cl-(aq) Cl2(g) + 2e-


At cathode- K+(aq) + e- K(s)


On electrolysis of a solution of potassium chloride, potassium chloride dissociates into K+ and Cl-. The anode is connected to positive terminal and anode to the negative terminal of the battery. So, the oxidation occurs at anode and reduction occurs at the cathode. At the cathode, there are K+ ions and water. So, while comparing K+ ions and water, water has greater tendency to get reduced. Hence hydrogen gas is liberated at the cathode. Similarly, at anode, there are Cl- ions and water. So, on comparing Cl- ions, and water, oxidation occurs to Cl-. Hence chlorine gas is liberated at the anode.


At anode- 2Cl- Cl2 + 2e-


At cathode- 2H2O + 2e- H2 + 2OH-


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• In which test tube the iron nail undergoes a colour change?


• What is the reaction taking place here?


• Justify your answer. (Refer reactivity series of metals).

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 1

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(i) At which electrode does a colour change occur-anode or cathode?


(ii) Is there any change in the blue colour of the copper sulphate solution?


(iii) Write down the chemical equations for the changes occurring here.

 2

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