Question

Identify the conjugate acid-base pairs in the following reactions. (a) \(\mathrm{HSO}_{4}^{-}+\mathrm{SO}_{3}^{2-} \rightleftharpoons \mathrm{HSO}_{3}^{-}+\mathrm{SO}_{4}^{2-}\) (b) \(\mathrm{S}^{2-}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{HS}^{-}+\mathrm{OH}^{-}\) (c) \(\mathrm{CN}^{-}+\mathrm{H}_{3} \mathrm{O}^{+} \rightleftharpoons \mathrm{HCN}+\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{H}_{2} \mathrm{Se}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{HSe}^{-}+\mathrm{H}_{3} \mathrm{O}^{+}\)

Short answer

(a) Conjugate pairs: \(\mathrm{HSO}_{4}^{-}/\mathrm{SO}_{4}^{2-}\), \(\mathrm{SO}_{3}^{2-}/\mathrm{HSO}_{3}^{-}\). (b) Conjugate pairs: \(\mathrm{S}^{2-}/\mathrm{HS}^{-}\), \(\mathrm{H}_{2} \mathrm{O}/\mathrm{OH}^{-}\). (c) Conjugate pairs: \(\mathrm{CN}^{-}/\mathrm{HCN}\), \(\mathrm{H}_{3} \mathrm{O}^{+}/\mathrm{H}_{2} \mathrm{O}\). (d) Conjugate pairs: \(\mathrm{H}_{2} \mathrm{Se}/\mathrm{HSe}^{-}\), \(\mathrm{H}_{2} \mathrm{O}/\mathrm{H}_{3} \mathrm{O}^{+}\).

Step-by-step solution

Understand Conjugate Acid-Base Pairs

A conjugate acid-base pair refers to two substances that transform into each other by gain or loss of a proton (H+). An acid donates a proton to become its conjugate base, while a base accepts a proton to become its conjugate acid.

Identify Conjugate Pairs in Reaction (a)

In reaction (a), \(\mathrm{HSO}_{4}^{-}\) donates a proton to become \(\mathrm{SO}_{4}^{2-}\), making them a conjugate acid-base pair. \(\mathrm{SO}_{3}^{2-}\) accepts a proton to become \(\mathrm{HSO}_{3}^{-}\), making them the other conjugate acid-base pair.

Identify Conjugate Pairs in Reaction (b)

In reaction (b), \(\mathrm{S}^{2-}\) accepts a proton to become \(\mathrm{HS}^{-}\), making them a conjugate base-acid pair. \(\mathrm{H}_{2} \mathrm{O}\) donates a proton to become \(\mathrm{OH}^{-}\), making them the other conjugate acid-base pair.

Identify Conjugate Pairs in Reaction (c)

In reaction (c), \(\mathrm{CN}^{-}\) accepts a proton to become \(\mathrm{HCN}\), making them a conjugate base-acid pair. \(\mathrm{H}_{3} \mathrm{O}^{+}\) donates a proton to become \(\mathrm{H}_{2} \mathrm{O}\), making them the other conjugate acid-base pair.

Identify Conjugate Pairs in Reaction (d)

In reaction (d), \(\mathrm{H}_{2} \mathrm{Se}\) donates a proton to become \(\mathrm{HSe}^{-}\), making them a conjugate acid-base pair. \(\mathrm{H}_{2} \mathrm{O}\) accepts a proton to become \(\mathrm{H}_{3} \mathrm{O}^{+}\), making them the other conjugate acid-base pair.

Key concepts

Acid-Base Chemistry

Acid-base chemistry is a fundamental concept in chemistry that deals with the reactions between acids and bases. An acid is a substance that can donate a proton (H+ ion), while a base is a substance that can accept a proton. When an acid donates a proton, it forms its conjugate base, which is the substance remaining after the proton is lost. Conversely, when a base gains a proton, it forms its conjugate acid. These pairs are called conjugate acid-base pairs.

The strength of an acid or base is typically determined by its tendency to donate or accept protons. Strong acids readily donate protons, while strong bases readily accept them. Contrarily, weak acids and bases do not partake in proton transfer as freely.

Understanding these concepts is vital because they are essential to many processes in biology, chemistry, and environmental science, such as the buffering capacity of blood, the operation of batteries, and the neutralization of pollution.

Brønsted-Lowry Theory

The Brønsted-Lowry theory is an acid-base theory which was proposed by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. This theory defines an acid as a proton (hydrogen ion, H+) donor and a base as a proton acceptor. It expands on the traditional concept of acids and bases and applies to a broader range of substances.

According to this theory, water can act both as an acid (when it donates a proton to form hydroxide, OH-) or as a base (when it accepts a proton to form hydronium, H3O+). The versatility of water as an acid or base makes it an excellent solvent for acid-base reactions.

In any acid-base reaction, the acid and base react to form their conjugate counterparts, meaning the original base forms the conjugate acid and the original acid forms the conjugate base. These two pairs remain in equilibrium, which is an essential concept in understanding how acid-base reactions proceed.

Proton Transfer Reactions

Proton transfer reactions are at the heart of acid-base chemistry, where a proton is transferred from one species to another. These are also known as protonation and deprotonation reactions. In protonation, a base accepts a proton from an acid, becoming a conjugate acid. In deprotonation, an acid loses a proton to a base, becoming a conjugate base.

For example, in the reaction of ammonia (NH3) with water, ammonia accepts a proton from water, forming ammonium (NH4+) and a hydroxide ion (OH-). Ammonia and ammonium here illustrate a conjugate acid-base pair linked by the transfer of a proton.

Understanding how these reactions take place is crucial for predicting the direction and extent of the reaction, which is influenced by the relative strengths of the acids and bases involved. This knowledge is also pivotal for realizing the behavior of substances in biological systems and industrial processes.

Chemical Equilibrium

Chemical equilibrium refers to the state in a chemical reaction where the rate of the forward reaction equals the rate of the reverse reaction. At equilibrium, the concentrations of reactants and products remain constant over time, but they are not necessarily equal.

In the context of conjugate acid-base pairs, equilibrium is established when the rates at which the acid donates protons and the base accepts protons are balanced. The equilibrium position is influenced by the strength of the acids and bases involved. The equilibrium constant (K), expressed as Kc for concentration or Kp for pressure, gives the ratio of the concentrations of the products to the concentrations of the reactants, each raised to the power of their stoichiometric coefficients.

Le Châtelier's Principle

Le Châtelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions (such as concentration, temperature, or pressure), the position of equilibrium will shift to counteract the change. This principle helps chemists to understand how changes in conditions affect the concentrations of acids, bases, and their conjugate pairs at equilibrium.

Recognizing the factors that affect chemical equilibrium is critical in both academic studies and industrial applications, such as the synthesis of pharmaceuticals or the production of ammonia in the Haber process.