An aqueous solution of ammonia is added to a solution containing hydrochloric acid - HSC - SSCE Chemistry - Question 34 - 2024 - Paper 1

The graph displays a decrease in conductivity as the volume of ammonia solution is added up to 4.5 mL, followed by a plateau. Initially, as ammonia reacts with hydrochloric acid, the reaction neutralizes the strong acid, which releases H⁺ ions (high conductivity) and produces NH₄⁺ ions (lower conductivity). Thus, the conductivity decreases during this phase due to the lower conductivity of NH₄⁺ compared to H⁺.

Before the equivalence point (before 4.5 mL), the following reaction occurs:

$H^+(aq) + NH_3(aq) \rightarrow NH_4^+(aq)$

As more NH₃ is added, the H⁺ concentration diminishes, resulting in lower overall conductivity. After reaching the equivalence point, more ammonia results in excess OH⁻ and NH₄⁺ being produced. While both contribute to conductivity, OH⁻ has a higher conductivity than NH₄⁺, leading to a slight increase in conductivity beyond the equivalence point. Thus, at higher volumes, conductivity rises as the effect of OH⁻ supersedes the low conductivity of NH₄⁺.

The reaction can be represented as follows:

$NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$

This reaction is notable because the NH₄⁺ ions are already present in solution at lower concentrations, and in accordance with Le Châtelier's Principle, the conductivity increases slightly after the equivalence point.