Problem 20 - Chemistry and Chemical Reactivity

Question

Which of the following compounds or ions has the weakest conjugate base? Briefly explain your choice (a) HCN (b) HClO (c) \(\mathrm{NH}_{4}^{+}\)

Step-by-Step Solution

Answer

The weakest conjugate base is ClO⁻, as it's derived from the strongest acid, HClO.

Step 1: Identify the Conjugate Bases

To find the weakest conjugate base, we first need to identify the conjugate base of each given acid or ion. The conjugate base is formed when the acid donates a proton (H⁺). For (a) HCN, the conjugate base is CN⁻. For (b) HClO, the conjugate base is ClO⁻. For (c) NH₄⁺, the conjugate base is NH₃.

Step 2: Assess the Acidity of Each Parent Compound

The strength of an acid is inversely related to the stability of its conjugate base. In other words, the stronger the acid, the weaker its conjugate base. HCN is a weak acid, HClO is an intermediate acid, and NH₄⁺ is the conjugate acid of a weak base NH₃.

Step 3: Compare the Strength of Conjugate Bases

Since stronger acids have weaker conjugate bases, we compare the conjugate bases CN⁻, ClO⁻, and NH₃. ClO⁻ is the conjugate base of HClO, which is the strongest acid among the three, making ClO⁻ the weakest base compared to CN⁻ and NH₃.

Step 4: Conclude the Weakest Conjugate Base

Based on the analysis, since ClO⁻ is derived from the strongest acid (HClO), it is the weakest conjugate base among CN⁻, ClO⁻, and NH₃.

Key Concepts

Conjugate Base

In acid-base chemistry, understanding the concept of a conjugate base is key to determining the characteristics of various compounds and ions. A conjugate base forms when an acid donates a proton ( (H⁺) ). Essentially, what remains of the acid becomes the conjugate base. This process helps maintain acid-base equilibrium in reactions. Here is a simple breakdown:

• For HCN, after donating a proton, it becomes its conjugate base: (CN⁻) .
• For HClO, the conjugate base formed is (ClO⁻) .
• In the case of (NH₄^{+}) , once it loses a proton, it turns into NH₃, its conjugate base.

Understanding the formation of conjugate bases is pivotal for reactions. It directly influences the acidity and basicity of a solution because the stability and strength of the conjugate base determine how easily it can capture protons back and reform the acid.

Acid Strength

The strength of an acid is a crucial concept when exploring acid-base chemistry. It indicates how easily an acid can donate a proton. Strong acids readily give up their protons and completely dissociate in water, resulting in weak conjugate bases. Conversely, weak acids do not fully dissociate in water, producing stronger conjugate bases. HClO is notable here because of its status as the strongest acid. Consider these points:

• HClO is relatively more eager to lose its proton, making it more acidic.
• HCN is a weak acid, showing limited proton donation capability.
• The ammonium ion ( (NH₄^{+}) ) is the conjugate acid of ammonia, which is a weak base. Therefore, it donates protons less readily too.

Remembering that stronger acids form weaker conjugate bases helps to rationalize why HClO, being the strongest acid in this context, results in ClO⁻ being the weakest conjugate base.

Chemical Stability

Chemical stability in the context of conjugate bases refers to how unlikely the compound is to participate in further reactions. This stability impacts the strength of the conjugate base itself.
A more stable conjugate base is generally weaker because it does not easily attract protons to reform the corresponding acid. Here’s a closer look:

• The conjugate base (ClO⁻) of HClO is quite stable, hence making it weak in capturing protons back.
• Conversely, (CN⁻) and NH₃ are less stable than ClO⁻, thus stronger bases, meaning they tend to pick up protons more readily.
• Stability often arises from factors like electronegativity and resonance. More electronegative atoms or resonance stabilization contribute to a more stable, and consequently weaker, conjugate base.

By recognizing how chemical stability influences conjugate base strength, students can predict reactivity and equilibrium positions better in acid-base reactions.