Aqueousphase Chemical Equilibria

On dissolution in water a number of species dissociate into ions. This dissociation is a reversible reaction that reaches equilibrium rapidly. In Chapter 12 we will discuss the timescale needed to achieve such equilibrium.

7.3.1 Water

Water itself ionizes to form a hydrogen ion, H+, and a hydroxide ion, OH-:

At equilibrium, we obtain

where K'w = 1.82 x 10 16 M at 298 K. However, the concentration of H20 molecules is so large (around 55.5 M), and so few ions are formed, that [H20] is virtually constant. Therefore the molar concentration of pure water is incorporated into the equilibrium constant to give

TABLE 7.4 Thermodynamic Data for Aqueous Equilibrium Constants

Equilibrium

K at 298 K (M)

AHA at 298 K, kcal moF1

h2o ^ h+ +

oh

1.0 x 10"14

13.35

co2 • h2o ^

h+ + hco3

4.3 x 10"1

1.83

hcoj h+

+ co2"

4.7 x 10-"

3.55

so2 • h2o ^

h+ + hsoj

1.3 x 10"2

-4.16

hsoj ^ h+

6.6 x 10"8

-2.23

nh3 ■ h2o ^

nh+ + oh

1.7 x 10"5

8.65

where Kw = <K[H20] = 1.0 x 10"14 M2 at 298 K (Table 7.4). For pure water, each water molecule dissociates, producing one hydrogen and one hydroxide ion and therefore [H+] = [OH"]. Thus at 298 K, [H+] = [OH"] = 1.0 x 10~7M. Note that the total concentration of ions in pure water is only 0.2 pM compared to 55.5 M for pure water itself. Therefore there are roughly 300 million water molecules for each ion in pure water. As a result of this small fraction of dissociated water molecules, pure water is considered a very weak electrolyte and has a very small electrical conductivity.

Defining the pH of water as we see that pH = 7.0 for pure water at 298 K. 7.3.2 Carbon Dioxide-Water Equilibrium

On dissolution, C02 hydrolyzes; that is, a molecule of carbon dioxide combines with a water molecule to form one molecule of C02 • H20. C02 (aq) and C02 ■ H20 are alternative notations for the same chemical species corresponding to A(aq) in (7.2). Dissolved carbon dioxide, C02 • H20, dissociates twice to form the carbonate and bicarbonate ions:

The equilibrium constants for these reactions are

Pc o2

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