Problem 18 - Chemistry and Chemical Reactivity

Question

Which of the following compounds or ions has the strongest conjugate acid? Briefly explain your choice. (a) \(\mathrm{CN}^{-}\) (b) \(\mathrm{NH}_{3}\) (c) \(\mathrm{SO}_{4}^{2-}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{HSO}_4^{-}\) is the strongest conjugate acid.

Step 1: Identify the Conjugate Acids

First, identify the conjugate acid for each compound or ion given. The conjugate acid of a base is formed by adding a proton (H\(^+\)) to it. The conjugate acids for the given options are:- (a) \[\mathrm{HCN} -\text{ Conjugate acid of } \mathrm{CN}^{-}\]- (b) \[\mathrm{NH}_4^{+} - \text{ Conjugate acid of } \mathrm{NH}_3\]- (c) \[\mathrm{HSO}_4^{-} - \text{ Conjugate acid of } \mathrm{SO}_4^{2-}\]

Step 2: Assess the Strength of Conjugate Acids

The strength of a conjugate acid is inversely related to the strength of its base. To assess which conjugate acid is strongest, compare their corresponding bases' strengths:- \(\mathrm{CN}^{-}\) is a weak base.- \(\mathrm{NH}_3\) is a moderate base.- \(\mathrm{SO}_{4}^{2-}\) is a very weak base.The weaker the base, the stronger the conjugate acid will be.

Step 3: Compare Conjugate Acids

Since the strongest conjugate acid comes from the weakest base, compare the bases identified:- \(\mathrm{CN}^{-}\) (weak base) -> \(\mathrm{HCN}\) (moderate conjugate acid)- \(\mathrm{NH}_3\) (moderate base) -> \(\mathrm{NH}_4^{+}\) (moderate conjugate acid)- \(\mathrm{SO}_{4}^{2-}\) (very weak base) -> \(\mathrm{HSO}_4^{-}\) (strongest conjugate acid)Thus, \(\mathrm{HSO}_4^{-}\) is the strongest conjugate acid.

Key Concepts

Base Strength

Base strength refers to a base's ability to accept protons (H\(^+\)). Understanding base strength is important because it directly affects the strength of the conjugate acid. In general, bases can be categorized as strong, moderate, or weak based on their tendency to attract protons.

• Strong Bases: These readily accept protons, making their conjugate acids weaker.
• Moderate Bases: These have a fair ability to accept protons, leading to moderately strong conjugate acids.
• Weak Bases: These have a low tendency to accept protons, resulting in strong conjugate acids.

By evaluating the base strength of compounds such as CN\(^{-}\), NH\(_3\), and SO\(_4^{2-}\), we can predict the relative strengths of their conjugate acids.

Acid-Base Chemistry

Acid-base chemistry is central to many chemical reactions and involves the transfer of protons between molecules or ions. This concept helps in understanding how substances like acids and bases interact in solutions.

• Acids are proton donors, meaning they release protons into the solution.
• Bases accept these protons, forming a conjugate acid.
• The relationship between an acid and its conjugate base, or a base and its conjugate acid, is essential for predicting reactions.

In the case of the provided exercise, acid-base chemistry allows us to determine how adding a proton to each base forms its conjugate acid, such as turning CN\(^{-}\) into HCN.

Ion Identification

Ion identification focuses on recognizing and understanding different ions in a reaction or solution. Properly identifying ions is crucial for predicting their behavior in acid-base reactions.

• An ion is an atom or group of atoms with a net electric charge due to the loss or gain of electrons.
• Some ions act as bases. For example, CN\(^{-}\), NH\(_3\), and SO\(_4^{2-}\) are bases in the exercise.
• Each base can attract a proton to form its conjugate acid.

Recognizing these ions and understanding their properties ensures that students can accurately predict outcomes like which conjugate acid will be the strongest, as illustrated by identifying HSO\(_4^{-}\) as the strongest conjugate acid in the provided examples.