D25.3 Acid-Base Reactions

Mixing a solution of an acid with a solution of a base results in an acid-base neutralization reaction that produces a salt and water. Thermodynamics dictates that the side with lower energy species (the side with weaker acid and weaker base) is favored at equilibrium. In other words, if the weaker acid and weaker base are on the reactant side, the reaction is reactant-favored at equilibrium; if the weaker acid and weaker base are on the product side, the reaction is product-favored at equilibrium. Strengths of acids and bases are quantitatively comparable by their K_a and K_b values, which can be obtained from a reference table.

A strong acid reacting with a strong base forms a neutral solution containing equal concentrations of H₃O⁺ and OH‾, provided that stoichiometrically equivalent quantities of acid and base are mixed. For example:

The salt formed, NaCl(aq), consists of Na⁺(aq) and Cl⁻(aq) , both with negligible acid or base strength. Hence, this equilibrium heavily favors the product side and goes essentially to completion. (Note that any soluble salt consists of aqueous ions, so the formula NaCl(aq) represents an aqueous solution consisting of Na⁺(aq) and Cl⁻(aq) ions.)

If the mixture has an excess of one of the reactants, then the concentration of leftover acid (HCl) or base (NaOH) determines the pH of the solution.

A weak acid reacting with a strong base forms a salt that contains the conjugate base of the weak acid, which is usually a weak base. For example:

The equilibrium of this reaction, driven by the reactivity of the strong base NaOH, favors the product side, and the reaction can be approximated as going to completion. Mixing stoichiometrically equivalent amounts of reactants gives a solution containing Na⁺(aq), which has no effect on the pH of the solution, and CH₃COO^–(aq), the conjugate base of acetic acid. Because the acetate anion is a weak base, the solution pH would be >7. The acetate anion reaction is:

The equilibrium constant for this reaction is the ionization constant, K_b, for the acetate anion. (Some reference tables only report ionization constants for acids; K_b can be calculated from K_w and K_a of the conjugate acid—acetic acid in this case.) Generalizing this example, when a strong base reacts stoichiometrically with a weak acid, the solution that results is basic.

A strong acid reacting with a weak base forms a salt containing the conjugate acid of the weak base, which is usually a weak acid. For example:

The equilibrium of this reaction, driven by the reactivity of the strong acid HCl, favors the product side, and the reaction can be approximated as going to completion. Mixing stoichiometrically equivalent amounts of reactants gives a solution that contains Cl⁻(aq), which has no effect on the pH of the solution, and NH₄⁺(aq), the conjugate acid of ammonia. Because the ammonium ion is a weak acid, the solution pH would be <7. The ammonium reaction is:

The equilibrium constant for this reaction is the ionization constant, K_a, for the acid NH₄⁺. Generalizing this example, when a weak base reacts stoichiometrically with a strong acid, the solution that results is acidic.

To predict the pH of a solution resulting from the reaction between a weak acid and a weak base, we must know both the K_a of the weak acid and the K_b of the weak base. If K_a > K_b, the solution is acidic; if K_b > K_a, the solution is basic.