1. Distinguish between a proton, a hydrated hydrogen ion, and a hydronium ion.
2. Describe a conjugate acid-base pair.

In this chapter we will develop further the Brønsted definition of acids and bases previously introduced in Chapter 4. The concept of equilibrium will be applied to acid-base reactions and the pH scale will be introduced. Later in the chapter the structural features of molecules that make them acids or bases will be investigated.

Brønsted Acids and Bases. According to the Brønsted acid-base theory, an acid is defined as a substance that can donate a proton to another substance. A base is a substance that can accept a proton from another substance. For example, when HBr gas dissolves in water, it donates a proton to the solvent. Therefore, HBr is a Brønsted acid.

HBr(aq)	+	H2O(l)	H3O+(aq) + Br–(aq)
acid		base

The water molecule is the proton acceptor. Therefore, water is a Brønsted base in this reaction. All acid-base reactions, according to Brønsted, involve the transfer of a proton. When a water molecule accepts a proton, a hydronium ion (H₃O⁺) is formed.

There are three forms of the hydrogen ion to keep straight. First is the proton (H⁺) that is transferred in any Brønsted acid-base reaction. Second is the hydrated hydrogen ion H⁺(aq) which is a general purpose representation of the proton in aqueous solution. Third, when a proton bonds to a water molecule a hydronium ion (H₃O⁺) is formed. This is the best representation of the actual structure of the hydrated hydrogen ion.

In the above example, HBr and Br^– are related chemical species. A conjugate acid-base pair consists of two species that differ from each other by the presence of one H⁺ unit. HBr and Br^– ion are a conjugate acid-base pair. Removing a proton (H⁺) from the acid (HBr) gives its conjugate base (Br^–). H₃O⁺ and H₂O are also a conjugate acid-base pair.

Write the formula of the conjugate base of H₂SO₄.

H₃SO₄⁺
H₂SO₄
HSO₄^-
SO₄^2-

          Correct!

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Note: A number in [brackets] indicates a subscript, a number or sign in {braces} indicates a superscript.

For the reaction:

HSO₄^-(aq) + HCO₃^-(aq) SO₄^2-(aq) + H₂CO₃(aq)

1. In the forward reaction, HSO[4]{-} HCO[3]{-} is the acid and HSO[4]{-} HCO[3]{-} is the base. In the reverse reaction SO[4]{2-} H[2]CO[3] is the acid and SO[4]{2-} H[2]CO[3] is the base.

          Correct!

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2. The two conjugate acid-base pairs are

   HSO[4]{-} / HCO[3]{-} HSO[4]{-} / SO[4]{2-} HSO[4]{-} / H[2]CO[3] and HCO[3]{-} / SO[4]{2-} HCO[3]{-} / H[2]CO[3]

          Correct!

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1. In the forward reaction, HSO₄^- ion is the proton donor which makes it an acid. The HCO₃^- ion is the proton acceptor, and is a base. In the reverse reaction, H₂CO₃ is the proton donor (acid), and SO₄^2- is the proton acceptor (base).

2. HSO4-	+	HCO3-		SO42-	+	H2CO3
   acid1		base2		base1		acid2

   the subscripts 1 and 2 designate the two conjugate acid-base pairs.

Substances that can behave as an acid in one proton transfer reaction and as a base in another reaction are called amphoteric. Water is an amphoteric substance. When ammonia dissolves in water for example, a proton transfer reaction takes place. Only this time water is the acid and ammonia is the base.

NH₃(aq) + H₂O(l) NH₄⁺(aq) + OH^–(aq)

The conjugate acid-base pairs are:

NH₄⁺ – NH₃, and H₂O – OH^–

Such pairs are usually labeled as shown below the following equation:

NH3(aq)	+	H2O(l)		NH4+(aq)	+	OH–(aq)
base1		acid2		acid1		base2

The subscript 1 designates one conjugate acid-base pair, and the subscript 2 the other pair. The double arrow indicates that the reaction is reversible. In the reverse reaction OH^– acts as a base and NH₄⁺ acts as an acid.

Complete the following questions to check your understanding of the material. Select the check button to see if you answered correctly.

1. Identify the conjugate base of HCO₃^- in the following acid-base reaction:

   CO₃^2- + CH₃COOH HCO₃^- + CH₃COO^–

2. Identify the Brønsted acid and base on the left side of the following equations, and identify the the conjugate partner of each on the right side.

   1. NH₄⁺(aq) + H₂O(l) NH₃(aq) + H₃O⁺(aq)
   2. HNO₂(aq) + CN^–(aq) NO₂^-(aq) + HCN(aq)