Question

An aqucous solution of hydrogen sulphide shows the cquilibrium ${\mathrm{II}}_2\mathrm{S}\rightleftharpoons {\mathrm{II}}^{-}\mathrm{I}{\mathrm{IIS}}^{-}$ If dilute hydrochloric acid is added to an aqucous solution of hydrogen sulphide without any change in temperature, then (1) the equilibrium constant will change (2) the concentration of HS will increase (3) the concentration of nondissociated hydrogen sulphide will decrease (4) the concentration of HS will decrease

Short answer

The concentration of HS⁻ will decrease.

Step-by-step solution

Understand the Reaction and Its Equilibrium

The given equilibrium in the aqueous solution of hydrogen sulfide is ${\text{H}}_2\text{S}\rightleftharpoons {\text{H}}^{+}+{\text{HS}}^{-}$. This indicates that hydrogen sulfide dissociates into hydrogen ions (${\text{H}}^{+}$) and bisulfide ions (${\text{HS}}^{-}$).

Analyze the Effect of Adding HCl

Adding dilute hydrochloric acid (HCl) to the solution introduces more hydrogen ions (H⁺) into the solution. Since $\text{HCl}\to {\text{H}}^{+}+{\text{Cl}}^{-}$, the concentration of H⁺ ions will increase.

Apply Le Chatelier's Principle

According to Le Chatelier's Principle, increasing the concentration of H⁺ ions will shift the equilibrium to the left in order to reduce the stress. This means more hydrogen sulfide (H₂S) will form from HS⁻ and H⁺.

Determine the Changes in Concentrations

As the equilibrium shifts left, the concentration of ${\text{HS}}^{-}$ will decrease, and the concentration of nondissociated hydrogen sulfide (${\text{H}}_2\text{S}$) will increase.

Validate the Effect on Equilibrium Constant

The equilibrium constant (K) is only affected by changes in temperature and is not influenced by the addition of reactants or products. Here, the temperature remains unchanged, so the equilibrium constant does not change.

Key concepts

Chemical Equilibrium

Chemical equilibrium occurs when a chemical reaction's forward and reverse reactions proceed at the same rate. At this point, the concentrations of reactants and products remain constant. For the reaction ${\text{H}}_2\text{S}\rightleftharpoons {\text{H}}^{+}+{\text{HS}}^{-}$, this means the rates at which hydrogen sulfide dissociates into hydrogen ions and bisulfide ions are equal to the rates at which they recombine. Equilibrium doesn't mean the reactants and products are equal in concentration, but their amounts stay constant over time.

Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system adjusts itself to counteract the change and restore a new equilibrium. In our exercise, adding $\text{HCl}\to {\text{H}}^{+}+{\text{Cl}}^{-}$ increases the concentration of ${\text{H}}^{+}$ ions. To counteract this change, the equilibrium shifts left, forming more ${\text{H}}_2\text{S}$ and reducing ${\text{HS}}^{-}$. This principle helps predict the direction of the shift when changes occur in concentration, pressure, or temperature. However, it does not affect the equilibrium constant if the temperature is constant.

Acid-Base Reactions

Acid-base reactions involve the transfer of hydrogen ions between species. In the equilibrium ${\text{H}}_2\text{S}\rightleftharpoons {\text{H}}^{+}+{\text{HS}}^{-}$, ${\text{H}}_2\text{S}$ acts as an acid (proton donor), and ${\text{HS}}^{-}$ acts as its conjugate base (proton acceptor). When $\text{HCl}$ is added, it dissociates into ${\text{H}}^{+}$ ions, increasing the acidity. The system responds by shifting the equilibrium towards the formation of more ${\text{H}}_2\text{S}$, a less dissociated form, thus acting to 'absorb' the excess protons.

Equilibrium Constant

The equilibrium constant (K) expresses the ratio of the concentrations of products to reactants, each raised to the power of their coefficients in the balanced equation. For the reaction ${\text{H}}_2\text{S}\rightleftharpoons {\text{H}}^{+}+{\text{HS}}^{-}$, the equilibrium constant is $$K=\frac{[{\text{H}}^{+}][{\text{HS}}^{-}]}{[{\text{H}}_2\text{S}]}$$. It only changes with temperature and remains unaffected by changes in concentrations of reactants or products at constant temperature. Here, as $\text{HCl}$ adds more ${\text{H}}^{+}$ ions, the equilibrium shifts to maintain the constant K value by altering the concentrations of reactants and products.