Page 44 - Soil and water contamination, 2nd edition

Basic environmental chemistry 31

0
found in nature. The reaction enthalpy ΔH is calculated from the formation enthalpies,
r
which are listed in thermodynamic tables.
Example 2.7 Temperature dependency of the equilibrium constant

Calculate the equilibrium constant for the reaction of calcite with carbon dioxide (see
Example 2.6) at 15 °C

The enthalpies of formation for the reactants and reaction products are
ΔH 0 = –1206.9 kJ mol -1
f CaCO 3
ΔH 0 = –393.5 kJ mol -1
f CO 2
ΔH 0 = –285.8 kJ mol -1
f H 2 O
ΔH 0 2+ = –542.8 kJ mol -1
f Ca
ΔH 0 - = –692.0 kJ mol -1
f HCO 3
Solution
The reaction enthalpy is

0
ΔH = ΔH 0 2+ + 2 ΔH 0 - – ΔH 0 – ΔH 0 – ΔH 0
r f Ca f HCO 3 f CaCO 3 f CO 2 f H 2 O
0
ΔH = -542.8 – 2 × – 692.0 + 1206.9 + 393.5 + 285.8 = –40.6 kJ mol -1
r
0
Note that ΔH is negative, so the reaction is exothermic , which implies that heat is lost
r
when calcite dissolves. Use Equation (2.33) to calculate log K at 15 °C:
H 0 1 1
log K log K r
15 oC 25 oC
2. 303 R 298. 15 288. 15
40. 6 1 1
6
log K 15 Co log 1. 10 59
2. 303 8. 3144 10 3 298. 15 288. 15
5. 42

K = 10 -5.42 = 3.80 • 10-6
15°C
The larger value of K at 15 °C implies that the equilibrium shifts to the right with
decreasing temperature . This means that the solubility of calcite increases with decreasing
temperature.

2.6.2 Kinetics

Reaction kinetics refers to the rate at which chemical reactions occur. In the general chemical
reaction in Equation (2.30), the reaction rate is usually assumed to be proportional to the
product of the activity of each substance participating in the reaction raised to the power of an
exponent. Consequently, the rates of the forward, reverse, and entire reactions can be written as:
Rate of forward reaction: r k [A ] [B ] (2.39a)
1 1

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