Question

Identify the Bronsted-Lowry acid and base in each of the following reactions: a. \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{OH}^{-}+\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}\) b. \(\mathrm{CH}_{3} \mathrm{COOH}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\) c. \(\mathrm{KOH}+\mathrm{HBr} \longrightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{KBr}\)

Short answer

a. Acid: \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{N}\), Base: \(\mathrm{H}_{2} \mathrm{O}\); b. Acid: \(\mathrm{CH}_{3} \mathrm{COOH}\), Base: \(\mathrm{H}_{2} \mathrm{O}\); c. Acid: \(\mathrm{HBr}\), Base: \(\mathrm{KOH}\).

Step-by-step solution

Identify the Acid and Base in Reaction (a)

For reaction a, the Bronsted-Lowry acid is the species donating a hydrogen ion (proton), and the base is the species accepting it. In this reaction, \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{N}\) donates a hydrogen ion to water, forming \(\mathrm{OH}^{-}\) and \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}\). Thus, \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{N}\) is the acid, and \(\mathrm{H}_{2} \mathrm{O}\) is the base.

Identify the Acid and Base in Reaction (b)

In reaction b, \(\mathrm{CH}_{3} \mathrm{COOH}\) donates a hydrogen ion to form \(\mathrm{H}_{3} \mathrm{O}^{+}\) and \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\). Thus, \(\mathrm{CH}_{3} \mathrm{COOH}\) is the acid for donating a proton, and \(\mathrm{H}_{2} \mathrm{O}\) once again acts as a base by accepting the proton.

Identify the Acid and Base in Reaction (c)

Reaction c involves \(\mathrm{KOH}\) reacting with \(\mathrm{HBr}\). In this case, \(\mathrm{HBr}\) donates a hydrogen ion to form \(\mathrm{H}_{2} \mathrm{O}\), so \(\mathrm{HBr}\) is the acid. \(\mathrm{KOH}\), which produces \(\mathrm{OH}^{-}\) that accepts the proton to form water, is the base in this reaction.

Key concepts

Acid-Base Reactions

Acid-base reactions are fundamental chemical processes in which an acid and a base interact to form a salt and usually water. In this context, according to the Bronsted-Lowry definition, an acid is any substance that can donate a proton (H+ ion), while a base is a substance that can accept a proton. For instance, in reaction (b) above, acetic acid (\textbf{CH}\(_3\)COOH) donates a proton to water, resulting in the formation of the hydronium ion (\textbf{H}\(_3\)O+) and acetate anion (\textbf{C}\(_2\)\textbf{H}\(_3\)\textbf{O}\(_2^-\)), showcasing a typical acid-base reaction.

These reactions are not only limited to simple neutralizations but can be complex and occur in various environments, including in the chemistry of biological organisms and in industrial processes.

Proton Transfer

Proton transfer is the movement of a proton from one molecule (the acid) to another (the base), which is the central event in a Bronsted-Lowry acid-base reaction. Understanding the movement of protons is essential to grasp the nature of acidity and basicity. For example, in reaction (a), \textbf{C}\(_5\)\textbf{H}\(_5\)\textbf{N} acts as a Bronsted-Lowry acid because it donates a proton to \textbf{H}\(_2\)\textbf{O}, which functions as a base by accepting the proton. This proton transfer profoundly affects the chemical properties of the involved species, such as their reactivity and solubility.

Additionally, the strength of an acid or base can be evaluated by its tendency to donate or accept protons, respectively. The easier the transfer of a proton from an acid to a base, the stronger the acid, and correspondingly, the stronger the base when it readily accepts a proton.

Chemical Equilibrium

Chemical equilibrium occurs when the forward and reverse reactions in a chemical process happen at the same rate, leading to a stable ratio of products and reactants over time. This concept is crucial for understanding acid-base reactions, as they often reach a state of dynamic equilibrium. For instance, both reaction (a) and (b) feature a double arrow (\rightleftharpoons), indicating that these reactions are reversible and can reach equilibrium.

At equilibrium, the concentration of all species involved in the reaction remains constant, although they are still reacting with each other. The equilibrium constant (\textbf{K}\(_\text{eq}\)) quantifies the balance between products and reactants. In the context of acids and bases, the equilibrium positions can tell us about the relative strengths of the acids and bases involved in the reaction.

Aqueous Solutions

Many acid-base reactions occur in aqueous solutions, which are water-based solutions. Water itself can act as either an acid or a base, which makes it an amphoteric substance. It acts as a base when it accepts a proton, as it does in reactions (a) and (b), and as an acid when it donates a proton, as it can do in other contexts.

Understanding the behavior of acids, bases, and salts in aqueous solutions requires knowledge of the solvent properties of water and its role in facilitating proton transfer. The ionic product of water (\textbf{K}\(_\text{w}\)) and the pH scale are also significant concepts in aqueous solution chemistry, offering a quantitative measure of acidity and basicity in aqueous solutions.