Question

When acid is added to a buffer solution composed of a weak base (B) and its salt with strong acid, then the reaction which occur to maintain the $\mathrm{pH}$ is (1) $\mathrm{B}+{\mathrm{H}}_3{\mathrm{O}}^{+}⟶{\mathrm{BH}}^{+}+{\mathrm{H}}_2\mathrm{O}$ (2) ${\mathrm{OH}}^{-}+{\mathrm{BH}}^{+}⟶\mathrm{B}+{\mathrm{H}}_2\mathrm{O}$ (3) $\mathrm{B}+{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{BH}}^{-}+{\mathrm{OH}}^{-}$ (4) ${\mathrm{BH}}^{+}+{\mathrm{H}}_2\mathrm{O}⟶\mathrm{B}+{\mathrm{H}}_3{\mathrm{O}}^{+}$

Short answer

Reaction (1) $\text{B}+{\text{H}}_3{\text{O}}^{+}\to {\text{BH}}^{+}+{\text{H}}_2\text{O}$ maintains the pH when acid is added to the buffer.

Step-by-step solution

- Understand the Buffer System

A buffer solution composed of a weak base (B) and its salt with a strong acid is designed to resist changes in pH when an acid (or a base) is added. In this scenario, the weak base (B) can react with any added acid to neutralize it and maintain the pH.

- Identify the Added Acid

When an acid is added to the buffer solution, it contributes hydronium ions ${\text{H}}_3{\text{O}}^{+}$ to the solution. We need to find the reaction where these hydronium ions are neutralized to understand how the buffer maintains the pH.

- Analyze the Given Reactions

Let's examine each of the provided reactions to see which one describes the neutralization of hydronium ions by the weak base: (1) $\text{B}+{\text{H}}_3{\text{O}}^{+}\to {\text{BH}}^{+}+{\text{H}}_2\text{O}$(2) ${\text{OH}}^{-}+{\text{BH}}^{+}\to \text{B}+{\text{H}}_2\text{O}$(3) $\text{B}+{\text{H}}_2\text{O}\to {\text{BH}}^{-}+{\text{OH}}^{-}$(4) ${\text{BH}}^{+}+{\text{H}}_2\text{O}\to \text{B}+{\text{H}}_3{\text{O}}^{+}$

- Determine the Correct Reaction

The weak base $\text{B}$ reacts with the added ${\text{H}}_3{\text{O}}^{+}$ ions to form the conjugate acid ${\text{BH}}^{+}$ and water. Thus, the reaction that maintains the pH of the buffer solution is: $1)\text{B}+{\text{H}}_3{\text{O}}^{+}\to {\text{BH}}^{+}+{\text{H}}_2\text{O}$

- Verify the Equilibrium Shift

Adding the acid shifts the equilibrium to the right, producing more ${\text{BH}}^{+}$ and ${\text{H}}_2\text{O}$, and thus stabilizing the pH of the buffer solution by neutralizing the added hydronium ions.

Key concepts

Acid-Base Chemistry

Acid-base chemistry revolves around the behavior of acids and bases in various solutions. In an aqueous environment, acids donate protons $H^{+}$ to the solution, while bases accept these protons. For instance, when hydrochloric acid $HCl$ dissolves in water, it releases $H^{+}$ ions and chloride ions $C{l}^{-}$. Conversely, a base like ammonia $N{H}_3$ can accept $H^{+}$ ions to form ammonium ions $N{H}_4^{+}$. The interaction between acids and bases is crucial for pH regulation in different contexts, including biological systems and buffer solutions.
Understanding this chemistry is important because it helps explain how and why certain reactions occur in these systems.

Weak Base and Salt

In the context of buffer solutions, a weak base and its corresponding salt play vital roles. A weak base only partially ionizes in water, establishing an equilibrium between the base and its conjugate acid. For example, the weak base ammonia $N{H}_3$ only partially ionizes to form $N{H}_4^{+}$ and $O{H}^{-}$. Meanwhile, its salt with a strong acid, such as ammonium chloride $N{H}_{4}Cl$, fully dissociates into $N{H}_4^{+}$ and $C{l}^{-}$ ions in water.
This combination helps form a buffer solution that resists pH changes. When an acid is added to the buffer, the weak base neutralizes the added $H^{+}$ ions, while if a base is added, the salt's conjugate acid neutralizes the added $O{H}^{-}$ ions. This delicate balance allows the buffer solution to maintain a relatively constant pH.

pH Maintenance

Buffers are crucial for maintaining stable pH levels in various systems. The pH refers to the measure of acidity or basicity in a solution, calculated as the negative logarithm of the hydrogen ion concentration. Buffer solutions work by neutralizing added acids or bases to prevent drastic changes in pH. For instance, in the given exercise, a buffer composed of a weak base and its salt with a strong acid can neutralize added hydronium ions $H_3{O}^{+}$ through the reaction:
\mathrm{B} + \mathrm{H}_3\mathrm{O}^+ \rightarrow \mathrm{BH}^+ + \mathrm{H}_2\mathrm{O}\
This reaction shifts the equilibrium to stabilize the pH, protecting the system from significant pH fluctuations.