Question

A salt of strong acid and a weak base is dissolved in water. Its hydrolysis in solution is (1) not affected by heating (2) increased by adding a strong acid (3) suppressed by adding a strong acid (4) suppressed by dilution

Short answer

Suppressed by adding a strong acid.

Step-by-step solution

Understand Hydrolysis

Hydrolysis occurs when the ion from the salt reacts with water to form a weak acid or weak base. In this case, the hydrolysis reaction is involving the salt of a strong acid and a weak base.

Effect of Temperature

Hydrolysis reactions can be affected by temperature. However, for salts of strong acids and weak bases, the hydrolysis is typically not significantly affected by heating.

Effect of Adding a Strong Acid

Adding a strong acid to the solution provides more H⁺ ions, which will shift the equilibrium of the hydrolysis reaction according to Le Chatelier's principle. This addition of H⁺ ions will suppress the hydrolysis reaction.

Effect of Dilution

Diluting the solution decreases the concentration of both the salt and water molecules. However, the effect of dilution typically does not suppress hydrolysis; it might actually increase it by reducing ion pairing.

Select the Correct Option

Given the analysis, the most appropriate answer is 'suppressed by adding a strong acid,' which corresponds to option (3).

Key concepts

strong acid and weak base

When a salt is formed from the combination of a strong acid and a weak base, it behaves differently in water compared to salts formed from other combinations. The reason lies in their dissociation properties. Strong acids, like HCl, dissociate completely in water to give H⁺ ions. On the other hand, weak bases, like NH₃, do not fully dissociate in water. This results in unique hydrolysis reactions where the salt, such as ammonium chloride (NH₄Cl), when dissolved in water, leads to the formation of H⁺ ions, making the solution slightly acidic due to the incomplete dissociation of the weak base component.

Le Chatelier's principle

Le Chatelier’s principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system tends to adjust in a manner that counteracts the change. In the context of salt hydrolysis, when you add a strong acid to the solution, you introduce more H⁺ ions. The hydrolysis reaction of the salt will shift to reduce the concentration of H⁺ ions to re-establish equilibrium.

For example, if you add HCl to an NH₄Cl solution, the extra H⁺ ions will combine with OH⁻ ions, forming water and shifting the hydrolysis balance to the left, thereby suppressing the reaction. This aligns with option (3) from the exercise, indicating that the addition of a strong acid suppresses the hydrolysis process.

effect of temperature on hydrolysis

Temperature can influence many chemical reactions, including hydrolysis. Generally, increasing the temperature speeds up reaction rates. However, the impact on hydrolysis for salts of strong acids and weak bases is minimal. This is because the hydrolysis equilibrium in such salts is already established mainly by the nature of the acid and base involved.

In our example, heating a solution of NH₄Cl does not significantly affect the hydrolysis process. The equilibrium constants for these reactions do not change considerably with temperature. Thus, the hydrolysis reaction is relatively unaffected by heating, aligning with option (1) of the exercise.

effect of strong acid on hydrolysis

Adding a strong acid to a solution containing a salt of a strong acid and a weak base introduces more H⁺ ions into the system.
This causes the hydrolysis equilibrium to shift in response to the added H⁺ ions. According to Le Chatelier's principle, the system will resist this change by decreasing hydrolysis.

For instance, if HCl is added to an NH₄Cl solution, it increases the H⁺ concentration. The system reacts by reducing the dissociation of NH₄⁺, therefore suppressing hydrolysis and shifting the equilibrium to the left.
This was summarized in step 3 of the original step-by-step solution and it supports option (3) in the exercise.

effect of dilution on hydrolysis

The effect of dilution on hydrolysis pertains to the changes in reactant concentrations. When the solution is diluted, the concentration of both the salt and the water molecules decreases. This reduces ion pairing and might actually increase the extent of hydrolysis.
In essence, dilution increases the degree of ion separation in the solution, making hydrolysis more probable.

This is contrary to option (4) in the exercise, which suggests hydrolysis is suppressed by dilution. In reality, dilution tends to favor hydrolysis by increasing the dissociation tendencies of the ions.