Which reaction are used in Black and White photography ?
Decomposition by light
When silver chloride is kept in the sun, it decomposes to form chlorine gas and silver. As the reaction proceeds, the white coloured silver chloride turns grey because of the formation of silver. Chlorine produced in the reaction escapes into the environment as it is produced in the gaseous state.
$$2AgCl(s) + light \rightarrow 2\,Ag(s) + Cl_{2}(g)$$
Silver chloride Silver chlorine
Silver bromide also undergoes decomposition in a similar manner when exposed to sunlight.
$$2AgBr(s) + light \rightarrow 2\,Ag(s) + Br_{2}(g)$$
Silver bromide Silver Bromine
As the above reactions are sensitive to light, they are used in black and white photography.
It is seen that decomposition reactions require a source of energy in the form of heat, light, or electricity to decompose the compound involved. Hence, it can be concluded thatdecomposition reactions are endothermic in nature.