Bronsted Acids and Bases

Learning Goals

You will now learn another model for acids and bases and apply it in a number of situations as we broaden our understanding of acids and bases and their reactions.

Synopsis

Bronsted in Denmark and Lowry in England actually simultaneously and independently suggested this definition of acids and bases. They suggested that an acid is a substance that can donate a proton to another substance, and a base is a substance that can accept a donated proton. These acids and bases can be neutral compounds such as HCl or they can be ions such as OH^-. The acids can also be monoprotic (HCl) or polyprotic (H₂SO₄). The polyprotic acids will have more than one dissociation step in a water solution:

H₂SO₄ + H₂O --> HSO₄^- + H₃O⁺ and then HSO₄^- + H₂O --> SO₂^2- + H₃O⁺.

The bases can also be polyprotic such as SO₄^2- and they also will have multiple steps in accepting protons in a water solution:

SO₄^2- + H₂O --> HSO₄^- + OH^- and then HSO4₄ + H₂O --> H₂SO₄>

Now note something very interesting in the above two reaction sequences. Water acted in the first sequence as a proton acceptor (as a base) and in the second sequence as a proton donor (as an acid)! Substances that act both as an acid and a base are said to be amphiprotic.

Also note in the above reactions that one of the reactants is acting as an acid and one is acting as a base. But also note the products. You will note that one of the products can function as an acid and the other can function as a base. Also note that the product that can function as a base comes from the reactant that functions as an acid. Also note that the product that can function as an acid comes from the reactant that functions as a base. Because one type of functionality comes from an opposite functionality we call these two acids and bases conjugate acid-base pairs. In the first sulfuric acid step above, H₂SO₄₂ is the acid and HSO₄^- is the conjugate base because it can accept a proton.

A strong acid is one that is almost completely ionized in solution. Hydrochloric acid is a very strong acid in this sense and so we no longer use the double arrow notation for this reaction:

HCl(aq) + H₂O(l) --> Cl^-(aq)+ H₃O⁺(aq)

A strong base is one that no longer exists in water. Such a strong base is the O₂^- ion:

O₂^- (aq) + H₂O(l) --> 2 OH^- (aq)

Now in each of the above two reactions we have an acid and a base forming a conjugate acid and a conjugate base. The strength of the conjugate acid or base is opposite to that of the parent base or acid. Thus HCl is a strong acid and the Cl^- ion is a weak base and in the same reaction water is a weak base so H₃O⁺ is a strong acid. Also for a strong acid reaction such as the above the acids and bases on the reactant side are stronger acids and bases than the conjugate acids and bases and so the reaction proceeds virtually completely to the product side. All proton transfer reactions proceed in the direction from the stronger acid-base pair toward the weaker acid-base pair. Table 17.3 in your text orders the relative strengths of some acids and bases.

We can now use the relative acid-base strengths to predict the direction of an acid-base reaction. Consider the reaction

HSO₄^- (aq) + NH₃(aq) --> NH₄⁺(aq) + SO₄^-(aq)

Which direction is favored? Look on the relative strength table and compare the acid HSO₄^- with the conjugate acid NH₄⁺ and you will see that HSO₄^- is a stronger acid than the conjugate acid NH₄⁺. Now look on the table again and you will see that NH₃ is a stronger base than the conjugate base SO₄^2-. Thus the reactant acids and bases are stronger than the product acids and bases, and the product direction is favored.

Review Questions

See your CD-ROM screen 17.2 for more information on these definitions.
Work Exercise 17.1 on page 696 in your text.