Problem 12 - Chemistry: An Atoms-Focused Approach

Question

Why does a solution of a weak base and its conjugate acid control pH better than a solution of the weak base alone?

Step-by-Step Solution

Answer

Question: Explain why a solution of a weak base and its conjugate acid is better at controlling pH changes compared to a solution containing only the weak base. Answer: A solution of a weak base and its conjugate acid is better at controlling pH changes because it acts as a buffer, neutralizing added acids or bases and minimizing pH changes. In a solution containing only the weak base, there is no buffering action, and pH changes are more significant when acids or bases are added.

Step 1: Understanding how a buffer system works

A buffer is a mixture of a weak acid or a weak base and its conjugate partner that resists significant changes in pH when an acid or a base is added. It works by neutralizing the added acid or base in the solution to maintain a nearly constant pH. The buffering action depends on a presence of relatively equal amounts of a weak base and its conjugate acid.

Step 2: The role of a weak base in controlling pH

When a weak base (e.g., ammonia, NH3) is dissolved in water, it only partially ionizes by accepting a proton (H+) from water molecules, forming its conjugate acid (ammonium ion, NH4+) and a hydroxide ion (OH-). The equilibrium reaction is as follows: NH3 (aq) + H2O (l) <=> NH4+ (aq) + OH- (aq) This reaction does not proceed to a significant extent, leading to only a moderate increase in the concentration of OH- ions and, therefore, affecting the pH only slightly.

Step 3: The role of a weak base and its conjugate acid in controlling pH

A solution containing a weak base and its conjugate acid, in relatively equal amounts, acts as a buffer. The buffering action allows the solution to better control pH changes. When an acid (H+) is added, the weak base (e.g., NH3) will react with the added H+ to form its conjugate acid (NH4+): NH3 (aq) + H+ (aq) => NH4+ (aq) When a base (OH-) is added, the conjugate acid (e.g., NH4+) will react with the added OH- to form the weak base (NH3) and water: NH4+ (aq) + OH- (aq) => NH3 (aq) + H2O (l) In both cases, the added acid or base is neutralized, and the pH changes are minimized due to the buffering action.

Step 4: Comparing the solutions of weak base alone and weak base with conjugate acid in controlling pH

A solution of the weak base alone is less effective in controlling pH changes as it does not have the buffering action of a weak base/conjugate acid mixture. When acids or bases are added to a solution of weak base alone, the pH will change more significantly because there are no species present to neutralize the added H+ or OH- ions, unlike in the buffer system. Thus, a solution of a weak base and its conjugate acid is better at controlling pH compared to a solution containing only the weak base.