Question

In each pair, select the stronger acid. (a) Pyruvic acid ( \(\left.\mathrm{p} K_{\mathrm{a}} 2.49\right)\) or lactic acid ( \(\left.\mathrm{p} K_{\mathrm{a}} 3.08\right)\) (b) Citric acid ( \(\mathrm{p} K_{\mathrm{a} 1} 3.08\) ) or phosphoric acid ( \(\mathrm{p} K_{\mathrm{al}} 2.10\) )

Short answer

Answer: The stronger acids in each pair are: (a) Pyruvic acid and (b) Phosphoric acid.

Step-by-step solution

Compare the pKa values in pair (a)

We are given two acids: pyruvic acid with pKa 2.49 and lactic acid with pKa 3.08. Compare these two values to determine which is the stronger acid.

Identify the stronger acid in pair (a)

Since pyruvic acid has a lower pKa value (2.49) compared to lactic acid (3.08), pyruvic acid is the stronger acid in this pair.

Compare the pKa values in pair (b)

We are given another pair of acids: citric acid with pKa1 3.08 and phosphoric acid with pKa1 2.10. Compare these two values to determine which is the stronger acid.

Identify the stronger acid in pair (b)

Since phosphoric acid has a lower pKa value (2.10) compared to citric acid (3.08), phosphoric acid is the stronger acid in this pair.

Conclusion

The stronger acids in each pair are: (a) Pyruvic acid and (b) Phosphoric acid.

Key concepts

pKa Values

Understanding the strength of an acid is paramount in acid-base chemistry, and this is where pKa values come into play. The pKa value is a numerical representation of an acid's dissociation in water; lower values indicate a stronger acid capable of donating a proton (H+) more readily.

In essence, pKa is directly connected to the equilibrium constant (Ka) for the dissociation of an acid into its conjugate base and a proton. The formula used to calculate pKa is given by: \[ pKa = -\text{log}(Ka) \]In comparing acids, such as in our textbook exercise, a lower pKa value signifies a stronger acid. For instance, pyruvic acid with a pKa of 2.49 is stronger than lactic acid, which has a pKa of 3.08. This is because the lower pKa of pyruvic acid indicates a greater tendency to lose a proton, enhancing its acidity. Comprehending pKa values is not only crucial for predicting the outcome of reactions, but also for processes such as buffer preparation in chemistry.

Acid-Base Chemistry

At the heart of understanding acid and base interactions is acid-base chemistry. It is a sub-discipline within chemistry that deals with the properties and reactions of acids and bases, including how they can neutralize each other.

An acid is a substance that can donate a proton (H+), and a base is a substance that can accept a proton. This relationship is characterized by the Bronsted-Lowry acid-base theory. In the context of comparison, the strength of an acid is defined by its ability to donate a proton. This concept links to how we evaluated pyruvic and lactic acids in our example — by considering which acid more freely donates a proton, we can determine which one is stronger.

The pH scale, another critical aspect of acid-base chemistry, measures the acidity or basicity of a solution. A lower pH (typically less than 7) indicates an acidic solution, whereas a higher pH (above 7) indicates a basic solution. Acid-base reactions are fundamental to diverse areas including biochemistry, medicine, and industrial processes.

Organic Acids

Among acids, organic acids are a vast and varied group. They contain one or more carbon atoms and typically have acidic properties due to the presence of carboxyl (-COOH) groups. Examples from our exercise, pyruvic acid and lactic acid, are both organic acids central to metabolic processes in the body.

The structure of organic acids often influences their strength. Factors such as the presence of electronegative elements, the ability to stabilize the resulting anion after deprotonation, and resonance stabilization greatly affect an organic acid's pKa value, and thereby its strength. In our textbook example, differences in pKa between pyruvic and lactic acids can be attributed to their structural differences and the ability of their conjugate bases to stabilize the negative charge after they have donated a proton.

Organic acids play vital roles not only in biological systems but also in industrial applications ranging from food preservation to pharmaceuticals. Understanding their acid strength is essential for both practical applications and theoretical chemical studies.