M15Q1: Arrhenius and Bronsted-Lowry Acids and Bases

Many acids and bases are found in our everyday lives, from the food that we eat to the chemicals we clean with. For example, coffee and soda are acidic solutions, while milk and vegetables are more basic. Vinegar is an acidic cleaning solution, while bleach is a basic solution. The properties of the molecules in these foods and solutions give rise to many interesting properties:

In an earlier chapter on chemical reactions, we defined acids and bases as Carl Axel Arrhenius did: We identified an acid as a compound that dissolves in water to yield hydronium (H₃O⁺) ions and a base as a compound that dissolves in water to yield hydroxide ions (OH⁻). However, this definition is somewhat limited.

Later, we extended the definition of an acid or a base using the more general definition proposed in 1923 by the Danish chemist Johannes Brønsted and the English chemist Thomas Lowry. Their definition centers on the proton, H⁺. A proton is what remains when a normal hydrogen atom loses an electron. A compound that donates a proton to another compound is called a Brønsted-Lowry acid, and a compound that accepts a proton is called a Brønsted-Lowry base. An acid-base reaction is the transfer of a proton from a proton donor (acid) to a proton acceptor (base). In a subsequent chapter of this text we will introduce the most general model of acid-base behavior introduced by the American chemist G. N. Lewis.

Common Acids and Bases

Acids may be compounds such as HCl or H₂SO₄, organic acids like acetic acid (CH₃COOH) or ascorbic acid (vitamin C), or H₂O. Anions (such as HSO₄^–, H₂PO₄^–, HS⁻, and HCO₃^–) and cations (such as H₃O⁺, NH₄⁺, and [Al(H₂O)₆]³⁺) may also act as acids.

Bases fall into the same three categories. Bases may be neutral molecules (such as H₂O, NH₃, and CH₃NH₂), anions (such as OH⁻, HS⁻, HCO₃^–, CO₃^2–, F⁻, and PO₄^3-), or cations (such as [Al(H₂O)₅OH]²⁺]). The most familiar bases are ionic compounds such as NaOH and Ca(OH)₂, which contain the hydroxide ion, OH⁻. The hydroxide ion in these compounds accepts a proton from acids to form water:

H⁺ + OH^–  →  H₂O

When an acid molecule donates a proton, the resulting product is referred to as the acid’s conjugate base. This species is a base because it can accept a proton (to re-form the acid):

acid + H₂O  ⇌  hydronium ion + conjugate base

HF + H₂O  ⇌  H₃O⁺ + F^–
H₂SO₄ + H₂O  ⇌  H₃O⁺ + HSO₄^–
HSO₄^– + H₂O  ⇌  H₃O⁺ + SO₄^-2
NH₄⁺ + H₂O  ⇌  H₃O⁺ + NH₃
H₂O + H₂O  ⇌  H₃O⁺ + OH^–

Similarly, when a base accepts a proton, the resulting product is referred to as the base’s conjugate acid. This species is an acid because it can give up a proton (and thus re-form the base):

base + H₂O  ⇌  hydroxide ion + conjugate acid

NH₃ + H₂O  ⇌  OH^– + NH₄⁺
S^2- + H₂O  ⇌  OH^– + HS^–
CO₃^2- + H₂O  ⇌  OH^– + HCO₃^–
F^– + H₂O  ⇌  OH^– + HF
H₂O + H₂O  ⇌  OH^– + H₃O⁺
OH^– + H₂O  ⇌  OH^– + H₂O

Amphiprotic Species

Like water, many molecules and ions may either gain or lose a proton under the appropriate conditions. Such species are said to be amphiprotic. Another term used to describe such species is amphoteric, which is a more general term for a species that may act either as an acid or a base by any definition (not just the Brønsted-Lowry one). Consider for example the bicarbonate ion, which may either donate or accept a proton as shown here:

HCO₃^–(aq) + H₂O(ℓ)  ⇌  CO₃^2-(aq) + H₃O⁺(aq)
HCO₃^–(aq) + H₂O(ℓ)  ⇌  H₂CO₃(aq) + OH^–(aq)

Key Concepts and Summary

After learning about equilibrium reactions, we are going to dive into one specific type of equilibrium reactions – acid-base reactions. We can use the products formed during a reaction to identify whether a molecule is an acid or a base. The definition of Arrhenius acids and bases are the most limited, but in this course, we will use the Brøntsed-Lowry acid and base definition most often, where an acid donates a proton and a base accepts a proton. After the acid donates a proton, it forms the conjugate base. After a base accepts a proton, it forms the conjugate acid.

Glossary

amphiprotic (amphoteric)

a molecule or ion that can either gain or lose a proton (H⁺)

Arrhenius acid

a compound that produces H₃O⁺ when dissolved in water

Arrhenius base

a compound that produces OH^– when dissolved in water

Brønsted-Lowry acid

a compound that donates a proton (H⁺)

Brønsted-Lowry base

a compound that accepts a proton (H⁺)

conjugate acid

the resulting product after a base accepts a proton (H⁺)

conjugate base

the resulting product after an acid donates a proton (H⁺)