Question

Sodium bromide is added to an aqueous solution of a metal nitrate $\mathrm{A}$. A yellow precipitate $\mathbf{B}$ is obtained which is used in photography. Compound $\mathbf{B}$ is photosensitive and decomposes to its constituents on exposure to sunlight, accompanied by the evolution of a reddish- brown gas. Write down the balanced equation and identify $\mathrm{A}$ and $\mathbf{B}$.

Answer 1

Reaction:

$\mathrm{NaBr}\left(\mathrm{aq}\right)+{\mathrm{AgNO}}_3\left(\mathrm{aq}\right)\to \mathrm{AgBr}\left(\mathrm{s}\right)+{\mathrm{NaNO}}_3\left(\mathrm{aq}\right)$

Sodium bromide Silver nitrate Silver bromide Sodium nitrate

Here yellow precipitate of $\mathrm{AgBr}$ is formed which is used in black and white photography.

$\mathrm{AgBr}$ is photosensitive and it decomposes to $\mathrm{Ag}$and ${\mathrm{Br}}_2$.

Decomposition reaction: Reaction in which reactant is broken down into two or more products in the presence of heat, light or electricity. $\mathrm{A}\to \mathrm{B}+\mathrm{C}$

2. Reaction:

$2\mathrm{AgBr}\left(\mathrm{s}\right)\stackrel{\mathrm{sunlight}}{\to }2\mathrm{Ag}\left(\mathrm{s}\right)+{\mathrm{Br}}_2\left(\mathrm{g}\right)$

Silver bromide Silver bromine

(yellow) (grey) (reddish-brown)

In presence of sunlight $\mathrm{AgBr}$ which is yellow colour solid undergoes photochemical decomposition that is it breaks down into $\mathrm{Ag}$ which is grey in colour and ${\mathrm{Br}}_2$ gas is released which is reddish-brown in colour.

So compound $\mathrm{A}$ is ${\mathrm{AgNO}}_3$ and compound $\mathbf{B}$ is $\mathrm{AgBr}$.