An Introduction to Environmental Chemistry

of material is greater on steeper slopes. Conversely, the potential for dissolution

and transport of dissolved material is lower on steep slopes because the contact

time between soil water and mineral solids is lower. The form of a slope—whether

it is linear, concave or convex—also influences water movement, and potentially

98 Chapter Four

Box 4.8 Chemical energy

The study of energy change is called

thermodynamics. For example, the

combustion of graphite carbon yields energy:

eqn. 1

The total energy released, or the energy

change in going from reactants to products, is

termed the change in Gibbs free energy (DG),

which is measured in kilojoules per mole (kJ

mol-^1 ). If energy is released, i.e. the products

have lower free energy than the reactants,

DGis considered negative. DG° for the

burning of graphite under standard tempera-

ture (25°C) and pressure (1 atm), indicated by

the superscript °, is -394.4 kJ mol-^1. Tables of

DG° for various reactions are widely available

and values of DG° for different reactions can

be calculated by simple arithmetic combina-

tion of tabulated values. Any reaction with a

negative DGvalue will in theory proceed

spontaneously—the chemical equivalent of

water flowing downhill—releasing energy.

The reverse reaction requires an input of

energy, i.e.:

eqn. 2

Since an energetically favoured reaction

proceeds from reactants to products, there is

a relationship between DGand the

equilibrium constant (K) for a reaction.

eqn. 3

where Tis the absolute temperature

(measured in Kelvin (K)) and Ris the

universal gas constant (8.314 J mol-^1 K-^1 ),

relating pressure, volume and temperature

for an ideal gas (see Box 3.1).

Converting equation 3 to decimal

logarithms gives:

eqn. 4

which at 25°C (298 K) yields:

DGRT=- 2 303. log 10 K

DGRT=- lnK

CO 22 ()gÆ+C(graphite)O()g G∞+D 394 4.kJ mol-^1

COCO(graphite)+Æ 22 ()g ()g

eqn. 5

or:

eqn. 6

The total energy released in a chemical

reaction has two components, enthalpy and

entropy. Change in enthalpy (DH, measured

in J mol-^1 ) is a direct measure of the energy

emitted or absorbed by a reaction. Change in

entropy (DS, measured in J mol-^1 K-^1 ) is a

measure of the degree of disorder. Most

reactions proceed to increase disorder, for

example by splitting a compound into

constituent ions or atoms. Enthalpy and

entropy are related:

eqn. 7

In most reactions the enthalpy term

dominates, but in some reactions the entropy

term is important. For example, the

dissolution of the soluble fertilizer potassium

nitrate (KNO 3 ) occurs spontaneously.

However, DHfor the reaction

eqn. 8

is +35 kJ and the solution absorbs heat (gets

colder) as KNO 3 dissolves. Despite the positive

enthalpy, the large increase in disorder

(entropy) in moving from a crystalline solid to

ions in a solution, gives an overall favourable

energy balance or negative DGfor the

reaction.

Electrode potentials (E°, Box 4.3) are a

measure of energy transfer and so can be

related to G:

eqn. 9

where nis the number of electrons trans-

ferred and Fis the universal Faraday constant

(the quantity of electricity equivalent to one

mole of electrons=6.02¥ 1023 e-).

G∞=-nFE∞

KNO 33 ()sÆ+K()+aq NO-()aq

DD DGHTS=-

log

.

10

5 707

KG=-D ∞

DGK∞=-5 707. log 10