Question

How is the strength of an acid related to the fact that a competition for protons exists in aqueous solution between water molecules and the anion of the acid?

Short answer

The strength of an acid is directly related to the competition for protons in an aqueous solution between water molecules and the anion of the acid. A strong acid readily donates its protons and has a higher acidity constant (Ka), resulting in a weakly competing anion. In contrast, a weak acid is less likely to donate protons and has a lower Ka, leading to a strongly competing anion. This competition influences the acid's dissociation in the aqueous solution, determining its strength.

Step-by-step solution

Define the terms

Acid Strength: The strength of an acid is determined by how readily it donates its protons (H+ ions) to a solution when dissolved in water. A strong acid will readily donate its protons, while a weak acid will not. Proton (H+): A proton is a hydrogen ion with a positive charge, represented as H+. In the context of acids, a proton is donated by an acid to a solution, which determines the acid's strength. Anion: An anion is a negatively charged ion. In the context of acids, the anion is the remaining part of the acid molecule after it has donated its proton.

Explain acidity constants and the role of water in an aqueous solution

Acidity Constants (Ka): The acidity constant, Ka, is a measure of how readily an acid donates its proton in an aqueous solution. A larger Ka value indicates a stronger acid, while a smaller Ka value indicates a weaker acid. The Ka can be represented mathematically as: \[ K_a = \frac{[\mathrm{H}^+][\mathrm{A}^-]}{[\mathrm{HA}]} \] Water molecules: In an aqueous solution, water molecules can act as both acids and bases, meaning they can both donate and accept protons. This results in the autoionization of water, in which a small amount of water molecules break down into H+ and OH− ions. \[2\;\mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{H}_3\mathrm{O}^+ + \mathrm{OH}^-\] In proton competition, water molecules play a significant role by accepting and donating protons.

Explain the competition for protons in an aqueous solution

When an acid is dissolved in water, it donates its proton (H+) to the solution. This creates an equilibrium between the undissociated acid, the anion formed after donating the proton, and the H3O+ ions resulting from the interaction between the donated protons and water molecules. In this competition for protons, both water molecules and the anions of the acid can accept protons, which influences the overall acidity of the solution.

Relate the competition for protons to the acid's strength

The strength of an acid is determined by how readily it donates its protons to the solution (highly dissociated or not). This is influenced by the competition for protons between water molecules and the anion of the acid. A strong acid will have a higher Ka value, meaning it is more likely to donate protons to the solution and less likely to accept protons from the water molecules. As a result, the anion formed from a strong acid is generally a weak competitor for protons in an aqueous solution. On the other hand, a weak acid will have a lower Ka value, meaning it is less likely to donate protons and more likely to accept protons from the water molecules. The anion formed from a weak acid is generally a strong competitor for protons in an aqueous solution, suppressing the dissociation of the weak acid. In conclusion, the competition for protons in an aqueous solution between water molecules and the anion of the acid is an essential factor that influences the strength of an acid. The acid's strength is directly related to the ability of its anion to compete for protons and thus, the extent to which the acid dissociates in the aqueous solution.