Question

When hydrogen sulphide gas is passed through a blue solution of copper sulphate, the color of the solution fades and a black precipitate is obtained.
a) Name the type of reaction mentioned above.
b) Why does the color of the solution fade away?
c) Write the chemical name of black precipitate formed.
d) Give the balanced chemical equation for the reaction involved.

Answer 1

A chemical reaction occurs when two compounds undergo some chemical change. This results in various types of reactions like, combination, decomposition, displacement etc. Copper sulphate solution is blue in color. A balanced chemical equation is written when the amount of the reactants is exactly equal to the amount of the products.

Complete answer:
a) When a gas of hydrogen sulphide ${{H}_{2}}S$ is passed through copper sulphate $CuS{{O}_{4}}$ then a reaction occurs where a black precipitate is obtained. A reaction where there is a formation of a precipitate is termed as a displacement reaction. This is because copper displaces the hydrogen from ${{H}_{2}}S$ and a precipitate is formed as a product.
b) The color of the blue copper sulphate solution fades away as there is a formation of a black precipitate due to the displacement reaction. So, the blue copper sulphate is completely utilized as the precipitate forms.
c) The black precipitate is formed as a result of the displacement reaction. The precipitate is copper sulphide $CuS$.
d) The balanced chemical equation for the displacement reaction involved is:
$CuS{{O}_{4}}+{{H}_{2}}S\to CuS+{{H}_{2}}S{{O}_{4}}$

Note:
The displacement reactions depend on the ability of metals to displace other metals. This is due to the activity series. In the activity series copper is placed above hydrogen. Hydrogen is in the middle as a reference. So, copper displaces hydrogen and forms copper sulphide. The activity series is based on the reduction potential of various metals.