Which of the following compounds listed below enter into a reaction with $N a_{2} S_{2} O_{3}$ ?
(a) $H C l$
(b) $N a C l$
(c) $N a I$
(d) $I_{2}$
(e) $K M n O_{4}$
If the initial substance becomes decolorised :

H C l

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N a C l

N a I

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I_{2}

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K M n O_{4}

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Solution

The correct options are
C $I_{2}$
D $K M n O_{4}$
Thiosulphate salts characteristically decompose upon treatment with acids. Initial protonation occurs at sulfur.Under normal conditions, acidification of solutions of this salt excess with even dilute acids results in complete decomposition to sulfur, sulphur dioxide, and water.
$N a_{2} S_{2} O_{3} + 2 H C l 2 N a C l + S + S O_{2} + H_{2} O$ .

This reaction is known as "clock reaction", because when the sulfur reaches a certain concentration, the solution turns from colourless to a pale yellow.The sulphur thus produced is colloidal.
The thiosulfate anion reacts stoichiometrically with iodine in aqueous solution, reducing it to iodide as it is oxidized to tetrathionate,

$2 S_{2} O_{3}^{2} + I_{2} S_{4} O_{6}^{2} + 2 I$
Here, the colour of the solution changes from violet to colourless. Iodine is violet in color.
Thiosulphate ion decolourises permanganate ion (pink) .The reaction takes place in an acidic medium.

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I_{2} + N a_{2} S_{2} O_{3} \to N a_{2} S_{4} O_{6} + N a I

I_{2}

(N a_{2} S_{2} O_{3})

(N a_{2} S_{2} O_{3}) + I_{2} ⟶ N a I + \dots \dots

I_{2} + N a O H \to I O_{3} + N a I + 3 H_{2} O

I_{2} + N a O H \to N a I O_{3} + N a I + H_{2} O

N a_{2} S_{2} O_{3}

I_{2}

N a_{2} S_{4} O_{6}

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