Note: In the following questions a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the choices given below each question.
Assertion (A): The ionisation of hydrogen sulphide in water is low in the presence of hydrochloric acid.
Reason (R) : Hydrogen sulphide is a weak acid.
H2S ⇋ H+ + S2-
The ionisation of hydrogen sulphide in water is low in the presence of hydrochloric acid. This can be explained on the basis of common ion effect and Le Chatilier principle.
On addition of HCl, concentration of H+ will increase as it is common to both HCl and H2S. So the system accommodates the concentration of H+ by combining the H+ and S2- back toH2S and to do so the Equilibrium will shift in backward direction. Therefore, ionisation of H2S is low in presence of HCl due to common ion effect.