Question

A solution of $I_{2}$ in aqueous KI on reaction with an aqueous solution of $Na_{2}S_{2}O_{3}$ gets decolourised. The reaction taking place here is

• A. $Na_{2}S_{2}O_{3}+H_{2}O+I_{2} \rightarrow Na_{2}S_{2}O_{4}+2HI$
• B. $2Na_{2}S_{2}O_{3}+I_{2} \rightarrow Na_{2}S_{4}O_{6}+2NaI$
• C. $Na_{2}S_{2}O_{3}+2H_{2}O+2I_{2} \rightarrow Na_{2}S_{2}O_{5}+4HI$
• D. $Na_{2}S_{2}O_{3}+2H_{2}O+2I_{2} \rightarrow Na_{2}S_{4}O_{8}+4HI$

Answer 1

Answer: (B)

On running sodium thiosulphate solution into an iodine solution (in KI solution), the reddish-brown colour is gradually discharged through yellow to colourless.
$I_{2}+2e^{-} \rightarrow 2I^{-}$
$2Na_{2}S_{2}O_{3} \rightarrow Na_{2}S_{4}O_{6}+2Na^{+}+2e^{-}$
$2Na_{2}S_{2}O_{3}+I_{2} \rightarrow Na_{2}S_{4}O_{6}+2Na^{+}I^{-}$