Brønsted Acids and Bases

In chemistry, the Brønsted-Lowry theory is an acid-base theory, independently proposed by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. In this system, acids and bases are defined as follows: an acid is any species that is able to donate a hydrogen cation (H⁺, a proton); a base is any species with the ability to accept a hydrogen cation (H⁺). To that end, if a compound is to behave as an acid by donating a proton, there must be a base to accept that proton; the Brønsted-Lowry concept is therefore defined by the reaction:

acid + base ⇌ conjugate base + conjugate acid

The conjugate base is the ion or molecule that remains after the acid has donated its proton, and the conjugate acid is the species created after the base accepts the proton. The reaction can proceed either forward backward; in each case, the acid donates a proton to the base.

Advantages of Brønsted-Lowry Theory

The Brønsted-Lowry acid-base theory has several advantages over the Arrhenius theory. Recall that the Arrhenius theory defines an acid as any species that increases the concentration of H⁺/H₃O⁺ in solution. Consider the following reactions of acetic acid (CH₃COOH), the organic acid that gives vinegar its characteristic taste:

1. CH₃COOH + H₂O ⇌ CH₃COO^- + H₃O⁺

2. CH₃COOH + NH₃ ⇌ CH3COO^- + NH₄⁺

Both theories easily describe the first reaction: CH₃COOH acts as an Arrhenius acid because it acts as a source of H₃O⁺ when dissolved in water, and it acts as a Brønsted acid by donating a proton to water. In the second example CH₃COOH undergoes the same transformation, in this case donating a proton to ammonia (NH₃); this cannot be described using the Arrhenius definition of an acid, however, because the reaction does not produce H₃O⁺.

Amphoterism of Water

Water is amphoteric, which means it can act as either an acid or a base. In the reaction between acetic acid, CH₃CO₂H, and water, H₂O, water acts as a base. The acetate ion CH₃CO₂^- is the conjugate base of acetic acid, and the hydronium ion H₃O⁺ is the conjugate acid of the base, water:

CH₃COOH + H₂O ⇌ CH₃COO^- + H₃O⁺

Water can also act as an acid, as when it reacts with ammonia. The equation given for this reaction is:

H₂O + NH₃ ⇌ OH^- + NH₄⁺

Here, H₂O donates a proton to NH₃. The hydroxide ion is the conjugate base of water, which acts as an acid, and the ammonium ion is the conjugate acid of the base, ammonia.