Question

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

Short answer

SO₄²⁻ has the strongest conjugate acid, HSO₄⁻.

Step-by-step solution

Identify the Base of Each Compound/Ion

The conjugate acid of a compound or ion is formed when it gains a proton (H+). We need to first identify the base in each case: - For (a) CN, its conjugate acid is HCN. - For (b) NH₃, its conjugate acid is NH₄⁺. - For (c) SO₄²⁻, its conjugate acid is HSO₄⁻.

Determine Strength of Conjugate Acids

Conjugate acid strength can often be determined by considering the stability and reactivity of the compound. - HCN is a weak acid, which means CN is a comparatively strong conjugate base. - NH₄⁺ is a relatively weak acid, indicating that NH₃ is a strong base. - HSO₄⁻ is a strong acid because it easily donates its proton, meaning SO₄²⁻ is a weak base. The strength of the conjugate acid is inversely related to the strength of the conjugate base.

Compare and Conclude Conjugate Acid Strengths

From Step 2, we know that the stronger the base, the weaker its conjugate acid. Conversely, the weaker the base, the stronger the conjugate acid. Since SO₄²⁻ is a weak base, its conjugate acid, HSO₄⁻, is stronger than those of the other options. Thus, among the choices, SO₄²⁻ has the strongest conjugate acid.

Key concepts

Acid Strength

Acid strength refers to an acid's ability to donate protons (hydrogen ions, H⁺) in water. This ability determines how easily an acid can dissociate to produce these ions. Strong acids dissociate completely in water, releasing a large amount of H⁺ ions, while weak acids only partially dissociate.

When considering the strength of conjugate acids, it is crucial to remember that it is inversely related to the strength of their conjugate bases. Meaning, if a base readily accepts a proton, its conjugate acid is weak, because it will not easily lose that proton. Therefore:

• A weak base forms a strong conjugate acid, as seen with SO₄²⁻, whose conjugate acid HSO₄⁻ has significant proton-donating capability.
• Conversely, a strong base like CN results in a weaker conjugate acid, such as HCN, due to its weak proton-releasing tendency.

Understanding these relationships can help predict chemical behavior of solutions in different settings.

Protonation

Protonation is the process where a base accepts a proton (H⁺) to become a conjugate acid. This transformation is essential in understanding acid-base chemistry, as it reveals how compounds interact in aqueous solutions. During protonation, the base gains an additional hydrogen, turning into its conjugate acid form.

For example, when NH₃ accepts a proton, it becomes NH₄⁺, its conjugate acid. The same happens when CN accepts a proton to form HCN. These reactions are essential in determining how easily a compound can shift between its base and conjugate acid forms. Several factors influence protonation:

• Availability of electrons in the base to attract a proton.
• Stability of the resulting conjugate acid.
• The solvent environment, which can facilitate or hinder the transfer of protons.

A strong understanding of protonation aids in predicting the reactivity and potential applications of chemical species in numerous fields.

Chemical Stability

Chemical stability refers to the tendency of a chemical compound to maintain its original form without undergoing a chemical change. A stable molecule often sees minimal shifts in its structure under various conditions, which is especially apparent when examining conjugate acids and bases.

In the context of conjugate acids like HSO₄⁻, this ion is relatively stable because it doesn't easily revert back to SO₄²⁻ by losing its proton. The stability of a conjugate acid is crucial for its acid strength, as stable acids can exist in solution without decomposing or reacting further. This stability also dictates:

• The ability of an acid to donate a proton effectively.
• The shift in balance between the acid and its conjugate base.
• How the compound interacts with other chemicals and environments.

Analyzing chemical stability can illuminate why certain reactions occur over others and guide the development of new materials and their applications in technology and industry.