Problem 37 - Chemistry The Molecular Nature of Matter

Question

The hydrogen phosphate ion is able to act as a buffer all by itself. Write chemical equations that show how this ion reacts with (a) \(\mathrm{H}^{+}\) and \((\mathbf{b}) \mathrm{OH}^{-} .\)

Step-by-Step Solution

Answer

The hydrogen phosphate ion (\text{HPO}_4^{2-}) reacts with \text{H}^+ to form \text{H}_2\text{PO}_4^-, and with \text{OH}^- to form \text{PO}_4^{3-} and H_2O.

Step 1: Reaction of Hydrogen Phosphate Ion with H+

The hydrogen phosphate ion (\text{HPO}_4^{2-}) can react with a hydrogen ion (\text{H}^+) by accepting the H+ to become dihydrogen phosphate (\text{H}_2\text{PO}_4^-). The equation for this reaction is: \[ \text{HPO}_4^{2-} + \text{H}^+ \rightarrow \text{H}_2\text{PO}_4^- \]

Step 2: Reaction of Hydrogen Phosphate Ion with OH-

Alternatively, the hydrogen phosphate ion can react with a hydroxide ion (\text{OH}^-) by donating a hydrogen ion to form phosphate ion (\text{PO}_4^{3-}) and water (H_2O). The equation for this reaction is: \[ \text{HPO}_4^{2-} + \text{OH}^- \rightarrow \text{PO}_4^{3-} + \text{H}_2\text{O} \]

Key Concepts

Buffer Systems

A buffer system is a solution that resists changes in pH when small amounts of an acid or a base are added. This resistance is essential for maintaining a stable environment in biological systems, where enzymes and other biochemical processes are pH-sensitive. At the heart of buffer action is a conjugate acid-base pair, typically consisting of a weak acid and its conjugate base, which can neutralize added acids (H⁺ ions) or bases (OH^- ions).

The way buffers work is by a process called 'buffering,' wherein the weak acid releases H⁺ ions when the pH rises (becomes more basic), and the conjugate base captures H⁺ ions when the pH drops (becomes more acidic). This exchange helps maintain a relatively constant pH level. An effective buffer has a pKa value close to the desired pH range and sufficient concentrations of both components for adequate capacity to absorb fluctuations in H⁺ or OH^- ions.

Acid-Base Reactions

Acid-base reactions are processes where acids and bases interact, often resulting in the formation of water and a salt. Acids are substances that can donate protons (H⁺ ions), while bases are substances that can accept protons. The strength of an acid or a base is determined by its ability to donate or accept protons, respectively.

In the context of buffer systems, the relevant acid-base reactions involve the transfer of H⁺ ions between the buffer components. The principle guiding these reactions is known as the Brønsted-Lowry theory, where acids are proton donors and bases are proton acceptors. The reactions provided in the exercise demonstrate the dual behavior of a hydrogen phosphate ion (HPO₄^2-) as it can act both as a base, by accepting H⁺ (to form H₂PO₄^-), and as an acid, by donating H⁺ (to form PO₄^3- and water). These reactions are fundamental to understanding how this ion contributes to the buffering capability of phosphate buffer systems.

Phosphate Buffering

Phosphate buffering is a specific type of buffer system that uses phosphate ions in solution to maintain a stable pH. This buffering system is particularly important in biological fluids, such as cytoplasm and blood plasma, where it helps keep the pH within the optimal range for cellular functions. The phosphate buffer system consists of dihydrogen phosphate ions (H₂PO₄^-) as the weak acid and hydrogen phosphate ions (HPO₄^2-) as the conjugate base.

The hydrogen phosphate ion (HPO₄^2-) is versatile—it can react with added acids (H⁺ ions) to form dihydrogen phosphate (H₂PO₄^-), thus acting as a base, or it can interact with added bases (OH^- ions) to produce phosphate ions (PO₄^3-) and water, thereby acting as an acid. This dual capability allows it to buffer against pH changes in both directions, which is why the hydrogen phosphate ion is the focal point of phosphate buffers.