chem ce 1997 paper 1 #8a(iv) 個題ge filter paper(KI)有D咩野用??

Question

chem ce 1997 paper 1 #8a(iv) 個題ge filter paper(KI)有D咩野用??
點解佢會變色??(我知係 2I- -->I2 + 2e- 才變做brown,但係冇啦啦filter paper 會有redox occur??關唔關chlorine事,定係filter有咩野功用?)

Answer 1

In the experiment, chlorine bleach reacts with dilute hydrochloric acid to give chlorine gas. The experiment also shows that chlorine gas can react with KI solution (actually the I- ions in the solution).
Cl2(g) + 2I-(aq) → 2Cl-(aq) + I2(aq)
or Cl2(g) + 2KI(aq) → 2KCl(aq) + I2(aq)

But what container is used to hold the KI solution ?
If a common container (e.g. small test tube or spatula) is used, it is difficult for the chlorine gas to make contact with KI solution. A simple method is used a piece of filter paper to be moistened by KI solution. In this case, the filter paper acts as a "container" to hold KI solution. The colourless I-(aq) ion soaked in the filter paper changes to brown I2(aq) solution, but the filter paper does not change.

Similarly, when a drop of dilute acid is added to a piece of blue litmus paper, the litmus paper changes to red. The paper also acts as a "container" of litmus. Actually, the blue litmus changes to red, but the paper does not change at all.