>> View the other Key Equations and Concepts in this chapter

Brønsted–Lowry Acid–Base Reactions

>> Parts of this equation/concept include:

A -	Identifying Acids and Bases
B -	Conjugate Acids and Bases

Brønsted–Lowry acids and bases are defined based on the reaction. The substance that loses an H⁺ is the acid; the substance that gains it is the base.

>> Example 1

Identify the acid and base in the following reactions.

1. NH₃ + HNO₂ NH₄⁺ + NO₂^–
2. CH₃COOH + OH^– H₂O + CH₃CO₂^–
3. H₂O + HSO₄^– H₃O⁺ + SO₄^2–
4. H₂O + HS^– OH^– + H₂S
5. CO₃^2– + HS^– HCO₃^– + S^2–

Solution:

1. NH₃ gains an H⁺ to become NH₄⁺, making it the base.

   HNO₂ loses the H⁺ to become NO₂^–, making it the acid.

2. CH₃COOH loses the proton on the end, making it the acid.

   OH^– gains an H⁺ to become water, making it the base.

3. H₂O gains H⁺ to become H₃O⁺, so it is the base.

   HSO₄^– loses its H⁺ to become SO₄^2–, so it is the acid.

4. In this case H₂O is the acid, losing H⁺ to be OH^–. (It depends on the reaction!)

   HS^– is the base, gaining H⁺ to be H₂S.

5. HS^– is the acid. It loses H⁺ to become S^2–.

   CO₃^2– is the base; it gains H⁺.

  >> back to the Top of the Page

A conjugate acid–base pair differs by an H⁺. Removing an H⁺ creates the conjugate base; adding an H⁺ creates the conjugate acid. Common mistakes are to add (or subtract) more than one H⁺ and to not change the charge along with the formula.

>> Example 1

What is the conjugate acid of the following?

1. H₂O
2. OH^–
3. (CH₃)₃N
4. S^2–

Solution:

1. Since water is normally involved in acid–base reactions, its conjugate acid and base are important. Adding H⁺ creates H₃O⁺.
2. The conjugate acid is H₂O. (This is common enough that you should recognize it.)
3. Since the H⁺ bonds with the nitrogen of the amine, it is customary to add it next to the nitrogen. (CH₃)₃NH⁺.
4. Adding only one H⁺, the answer is HS^–.

>> Example 2

What is the conjugate base of the following?

1. H₂O
2. H₃O⁺
3. CH₃COOH
4. H₂S

Solution:

1. Removing H⁺ leaves OH^–.

2. Removing H⁺ leaves H₂O.

   You will probably memorize all the water acid–base combinations, even if you don't want to.

3. With organic acids, the acidic proton (H⁺) is the one at the end of the COOH. Therefore the conjugate base is CH₃CO₂^–.
4. Only one H⁺ is removed from H₂S, HS^–.

>> View the other Key Equations and Concepts in this chapter