Behavior of Polyprotic Acids in Water The tartaric acid, H2C4H4O6, in grapes promotes a crisp flavor and graceful aging in wine. Its Ka values are given in Table 13.6. (a) Write equations that show the ionization of tartaric acid in water. (b) Besides water, which ion or molecule has the

Question

Behavior of Polyprotic Acids in Water

The tartaric acid, H₂C₄H₄O₆, in grapes promotes a crisp flavor and graceful aging in wine. Its K_a values are given in Table 13.6.

TABLE 13.6 | Acid Ionization Constants for Some Polyprotic Acids (at 25°C)

K_a3	K_a2	K_a1	Name	Formula
	4.7 × 10^–11	4.5 × 10^–7	carbonic acid	H₃CO₃
4.0 × 10^–7	1.7 × 10^–5	7.4 × 10^–4	citric acid	H₃C₆H₅O₇
	1.0 × 10^–19	8.9 × 10^–8	hydrosulfuric acid	H₂S
	1.5 × 10^–4	5.6 × 10^–2	oxalic acid	H₂C₂O₄
4.8 × 10^–13	6.2 × 10^–8	6.9 × 10^–3	phosphoric acid	H₃PO₄
	1.2 × 10^–2	strong	sulfuric acid	H₂SO₄
4.3 × 10^–5	1.0 × 10^–3		tartaric acid	H₂C₄H₄O₆

(a) Write equations that show the ionization of tartaric acid in water.

(b) Besides water, which ion or molecule has the highest concentration in solution when tartaric acid is added to water?

Accepted Answer

(a) Tartaric acid has two K_a values, so it has two acidic hydrogen atoms. Two equations are required to describe its behavior in water. The first shows the ionization of the first H⁺. The K_a that corresponds to this process is the largest K_a value because the first H⁺ always ionizes to the greater extent.

[latex]H_{2}C_{4}H_{4}O_{6}(aq)+H_{2}O(l) \xrightleftharpoons[]{} HC_{4}H_{4}O^{-}_{6}(aq)+H_{3}O^{+}(aq) [/latex]

K_a1= 1.0 × 10^–3

[latex]HC_{4}H_{4}O^{-}_{6}(aq)+H_{2}O(l) \xrightleftharpoons[]{} C_{4}H_{4}O^{2-}_{6}(aq)+H_{3}O^{+}(aq) [/latex] K_a2 = 4.3 × 10^–5

(b) The K_a1 value for H₂C₄H₄O₆ shows that it is a weak acid and ionizes only to a small extent. That means that most of the H₂C₄H₄O₆ does not ionize and is present in water in the highest concentration.

Question 13.7: Behavior of Polyprotic Acids in Water The tartaric acid, H2...

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