Ncert Exercises & Solutions

In the reactions given below, thiosulphate reacts differently with iodine than with bromine.

$2 S_{2} O_{3}^{2 -} + I_{2} \to S_{4} O_{6}^{2 -} + 2 I^{-}$

$S_{2} O_{3}^{2 -} + 2 {Br}_{2} + 5 H_{2} O \to 2 {SO}_{4}^{2 -} + 2 {Br}^{-} + 10 H^{+}$

Which of the following statements best describes the above dual behaviour of thiosulphate?

1. Bromine is a stronger oxidant than iodine

2. Bromine is a weaker oxidant than iodine

3. Thiosulphate undergoes oxidation by bromine and reduction by iodine in these reactions

4. Bromine undergoes oxidation and iodine undergoes reduction in these reactions

Hint: Gain of electron is reduction and loss of electron is oxidation

The two reactions are as follows:

2 {\overset{+ 2}{S}}_{2} \overset{- 2}{O_{3}^{2 -}} (aq) + {\overset{0}{I}}_{2} (s) \to {\overset{2.5}{S}}_{4} O_{6}^{2 -} (aq) + 2 I^{-} (aq)

\overset{+ 2}{S_{2}} \overset{- 2}{O_{3}^{2 -}} + 2 \overset{0}{B} r_{2} (l) + 5 H_{2} O (l) \to 2 \overset{+ 6}{S} \overset{2 -}{O_{4}^{2 -}} (aq) + 2 {Br}^{-} (aq) + 10 H^{+} (aq)

Element	Change in oxidation state	Reaction
S	+2 ---> 2. 5	oxidation
I	0----> -1	reduction
S	+2-----> +6	Oxidation
Br	0 ----> -1	reduction

In both the reactions, halogen acts as an oxidising agent and oxidised $S_{2} O_{3}^{2 -}$ and it self gets reduce. The change in oxidation state of sulphur in case of ${Br}_{2}$ is more than in case of $I_{2}$ . Hence, ${Br}_{2}$ being stronger oxidisng agent than I₂,

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