Question

(a) When a moist blue litmus paper is dipped in a solution of hypochlorous acid, it first turns red and then latter gets decolourised. Explain.
(b) Iodine is liberated when `KI` is added to a solution of `Cu^(2+)` ions but `Cl_(2)` is not liberated when `KCl` is added to a solution of `Cu^(2+)` ions. why?

Answer 1

(a) Hypochlorous acid, `HOCl` behave as an acid and hence blue litmus paper turns red.
`HOCl rarr H^(o+)+OCl^(Theta)`
Hypochlorous acid also acts as a bleaching agent and thus decolourises red litmus by the nascent oxygen.
`HOCl rarr HCl+[O]`
(b) The iodide ion `(I^(Theta))` being a strong reducing agent reduces `Cu^(2+)` to `Cu^(o+)` and itself gets oxidised to `I_(2)`.
`2Cu^(2+)+4KI rarr Cu_(2)I_(2) rarr I_(2)+4K^(o+)`
Since `Cl^(Theta)` ions does not act as a reducing agent, therefore, `Cl_(2)` is not liberated when `KCl` is added to a solution of `Cu^(2+)` ions.