Bronsted Lowry Theory of Acids and Bases

Acid-Base Reactions Beyond Aqueous Solutions

The Bronsted-Lowry acid-base theory identifies acid-base pairs based on proton transfer.
The Bronsted-Lowry acid-base theory identifies acid-base pairs based on proton transfer. Ann Cutting / Getty Images

 The Brønsted-Lowry acid-base theory (or Bronsted Lowry theory) identifies strong and weak acids and bases based on whether the species accepts or donates protons or H+. According to the theory, an acid and base react with each other, causing the acid to form its conjugate base and the base to form its conjugate acid by exchanging a proton. The theory was proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923.

In essence, Brønsted-Lowry acid-base theory is a general form of the Arrhenius theory of acids and bases. According to the Arrhenius theory, an Arrhenius acid is one that can increase the hydrogen ion (H+) concentration in aqueous solution, while an Arrhenius base is a species that can increase the hydroxide ion (OH-) concentration in water. The Arrhenius theory is limited because it only identifies acid-base reactions in water. The Bronsted-Lowry theory is a more inclusive definition, capable of describing acid-base behavior under a wider range of conditions. Regardless of the solvent, a Bronsted-Lowry acid-base reaction occurs whenever a proton is transferred from one reactant to the other.

Main Points of the Bronsted Lowry Theory

  • A Bronsted-Lowry acid is a chemical species capable of donating a proton or hydrogen cation.
  • A Bronsted-Lowry base is a chemical species capable of accepting a proton. In other words, it is a species that has a lone electron pair available to bond to H+.
  • After a Bronsted-Lowry acid donates a proton, it forms its conjugate base. The conjugate acid of a Bronsted-Lowry base forms once it accepts a proton. The conjugate acid-base pair have the same molecular formula as the original acid-base pair, except the acid has one more H+ compared to the conjugate base.
  • Strong acids and bases are defined as compounds that completely ionize in water or aqueous solution. Weak acids and bases only partially dissociate.
  • According to this theory, water is amphoteric and can act as both a Bronsted-Lowry acid and Bronsted-Lowry base.

Example Identifying Brønsted-Lowry Acids and Bases

Unlike Arrhenius acid and bases, Bronsted-Lowry acids-base pairs can form without a reaction in aqueous solution. For example, ammonia and hydrogen chloride may react to form solid ammonium chloride according to the following reaction:

NH3(g) + HCl(g) → NH4Cl(s)

In this reaction, the Bronsted-Lowry acid is HCl because it donates a hydrogen (proton) to NH3, the Bronsted-Lowry base. Because the reaction does not occur in water and because neither reactant formed H+ or OH-, this would not be an acid-base reaction according to the Arrhenius definition.

For the reaction between hydrochloric acid and water, it's easy to identify the conjugate acid-base pairs:

HCl(aq) + H2O(l) → H3O+ + Cl-(aq)

Hydrochloric acid is the Bronsted-Lowry acid, while water is the Bronsted-Lowry base. The conjugate base for hydrochloric acid is the chloride ion, while the conjugate acid for water is the hydronium ion.

Strong and Weak Lowry-Bronsted Acids and Bases

When asked to identify whether a chemical reaction involves strong acids or bases or weak ones, it helps to look at the arrow between the reactants and the products. A strong acid or base completely dissociates into its ions, leaving no undissociated ions after the reaction is completed. The arrow typically points from left to right.

On the other hand, weak acids and bases don't completely dissociate, so the reaction arrow points both left and right. This indicates a dynamic equilibrium is established in which the weak acid or base and its dissociated form both remain present in the solution.

An example if the dissociation of the weak acid acetic acid to form hydronium ions and acetate ions in water:

CH3COOH(aq) + H2O(l) ⇌ H3O+(aq) + CH3COO-(aq)

In practice, you might be asked to write a reaction rather than have it given to you. It's a good idea to remember the short list of strong acids and strong bases. Other species capable of proton transfer are weak acids and bases.

Some compounds can act as either a weak acid or a weak base, depending on the situation. An example is hydrogen phosphate, HPO42-, which can act as an acid or a base in water. When different reactions are possible, the equilibrium constants and pH are used to determine which way the reaction will proceed.