Question

Why is there a black coating in silver?

Answer 1

Hint: We have to know that the hydrogen sulfide is a gas having a pungent or rotten egg like smell. Hydrogen Sulfide (\[{H_2}S\]) is a gas commonly found during the drilling and production of crude oil and natural gas, plus in wastewater treatment and utility facilities and sewers. It is colorless, flammable, poisonous and corrosive.

Complete answer:
We need to remember that the black coating in silver is due to the formation of silver sulfide i.e. \[A{g_2}S\]. The chemical equation can be represented as:
\[2Ag + {H_2}S \to A{g_2}S + {H_2}\]
Silver sulfide is a dense black solid and is insoluble in all solvents but can be degraded by using strong acids. Since it is made of silver and sulfur, it has covalent bonds. The molecular weight of silver sulfide is $247.88g/mol$. The crystal structure for this compound is orthogonal. Silver belongs to group $11$ while sulfur belongs to group $16$. Silver sulfide naturally occurs as the tarnish on silverware. When combined with silver, hydrogen sulfide gas creates a layer of black silver sulfide patina on the silver, protecting the inner silver from further conversion to silver sulfide. Silver whiskers can form when silver sulfide forms on the surface of silver electrical contacts operating in an atmosphere rich in hydrogen sulfide and high humidity. Such atmospheres can exist in sewage treatment and paper mills.

Note:
We have to know that the reaction that is carried out between silver and hydrogen sulfide is an exothermic reaction as hydrogen gas is being released. Silver sulfide naturally occurs as the tarnish on silverware. Hydrogen sulfide gas is colorless, flammable, poisonous and corrosive.