Question

In each of the following chemical equations, identify the conjugate acid-base pairs. a. \(\mathrm{HF}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{F}^{-}(a q)+\mathrm{H}_{3} \mathrm{O}^{+}(a q)\) b. \(\mathrm{CN}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HCN}(a q)+\mathrm{OH}^{-}(a q)\) c. \(\mathrm{HCO}_{3}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{H}_{2} \mathrm{CO}_{3}(a q)+\mathrm{OH}^{-}(a q)\)

Short answer

In each given chemical equation, the conjugate acid-base pairs are: a. HF (Acid) and F⁻ (Conjugate base); H2O (Base) and H3O⁺ (Conjugate acid) b. H2O (Acid) and OH⁻ (Conjugate base); CN⁻ (Base) and HCN (Conjugate acid) c. H2O (Acid) and OH⁻ (Conjugate base); HCO3⁻ (Base) and H2CO3 (Conjugate acid)

Step-by-step solution

1. Identify Conjugate Acid-Base Pairs for Equation (a)

For the chemical equation: a. \(\mathrm{HF}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{F}^{-}(a q)+\mathrm{H}_{3} \mathrm{O}^{+}(a q)\) We can observe that the reaction involves the transfer of a hydrogen ion (H+) from HF to H2O. Therefore, the conjugate pairs are: - HF (Acid) and F⁻ (Conjugate base) - H2O (Base) and H3O⁺ (Conjugate acid)

2. Identify Conjugate Acid-Base Pairs for Equation (b)

For the chemical equation: b. \(\mathrm{CN}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HCN}(a q)+\mathrm{OH}^{-}(a q)\) We can observe that the reaction involves the transfer of a hydrogen ion (H+) from H2O to CN⁻. So, the conjugate pairs are: - H2O (Acid) and OH⁻ (Conjugate base) - CN⁻ (Base) and HCN (Conjugate acid)

3. Identify Conjugate Acid-Base Pairs for Equation (c)

For the chemical equation: c. \(\mathrm{HCO}_{3}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{H}_{2} \mathrm{CO}_{3}(a q)+\mathrm{OH}^{-}(a q)\) We can observe that the reaction involves the transfer of a hydrogen ion (H+) from H2O to HCO3⁻. Thus, the conjugate pairs are: - H2O (Acid) and OH⁻ (Conjugate base) - HCO3⁻ (Base) and H2CO3 (Conjugate acid)

Key concepts

Chemical Equations

Chemical equations are symbolic representations that show the reactants and products in a chemical reaction. They provide critical information such as the substances involved, their phase (solid, liquid, gas, or aqueous), and the direction of the reaction.
For example, in the equation \(\mathrm{HF}(aq) + \mathrm{H}_2\mathrm{O}(l) \rightleftharpoons \mathrm{F}^{-}(aq) + \mathrm{H}_3\mathrm{O}^{+}(aq)\), we can see that hydrofluoric acid (HF) reacts with water (H2O) to form fluoride ion (F⁻) and hydronium ion (H3O⁺). The double arrow (\(\rightleftharpoons\)) indicates that the reaction can occur in both forward and reverse directions, which is a characteristic of a dynamic equilibrium.
To read and write chemical equations effectively, one must understand the conventions used, such as the states of the substances (denoted by the letters s, l, g, and aq for solid, liquid, gas, and aqueous, respectively) and the stoichiometry, which is the numerical relationship between the amounts of reactants and products.

Acid-Base Reaction

Acid-base reactions involve the transfer of a hydrogen ion (H⁺), also known as a proton, from one species to another. These reactions are central to the concept of pH, which measures the acidity or basicity of a solution.
In the context of conjugate acid-base pairs, the original acid donates a proton, forming its conjugate base, while the original base receives a proton, forming its conjugate acid. This can be seen in the given equations where conjugate pairs like HF and F⁻, H2O and H3O⁺, H2O and OH⁻, CN⁻ and HCN, as well as HCO3⁻ and H2CO3, are formed.
Understanding this concept is crucial as it helps predict the outcome of reactions and the strength of acids and bases. A strong acid will have a weak conjugate base, and a strong base will have a weak conjugate acid, reflecting the tendency of the stronger substance to donate or accept protons.

Hydrogen Ion Transfer

The transfer of a hydrogen ion, or proton, is what defines an acid-base reaction. When an acid loses a hydrogen ion, it becomes a base, termed the 'conjugate base' of the acid. Conversely, when a base gains a hydrogen ion, it becomes an acid, known as the 'conjugate acid' of the base.
This proton transfer is easily observed in the given equations. For instance, when HF (acid) loses a hydrogen ion to H2O (base), it forms its conjugate base, F⁻, and the water becomes its conjugate acid, H3O⁺. In this context, it's also important to note that water can act as both an acid (proton donor) and a base (proton acceptor), a property called 'amphoteric'.
The idea of conjugate acid-base pairs is fundamental in understanding the balance in chemical reactions, and it reflects the reversible nature of acid-base chemistry. Through the recognition of these pairs, one can predict the direction of the reaction and the relative strengths of acids and bases involved.