Page 33 - Soil and water contamination, 2nd edition

20 Soil and Water Contamination

H U PV (2.5)
2 -2
2 -2
where H = the enthalpy [M L T ], U = the internal energy [M L T ], P = the pressure [M
-1
-1
-2
3
L T ], V = the volume [L ]. The unit for enthalpy in Equation (2.5) is kJ kg , but most
-1
often the enthalpy is expressed in kJ mol .
The first law of thermodynamics , also known as the law of conservation of energy, states
that the total energy of a system and its surroundings remains constant. This means that, in
general, the formation of molecules takes energy and the reaction is endothermic (enthalpy
increases); for example, the formation of glucose from carbon dioxide and water through
photosynthesis in green plants requires energy in the form of sunlight. Conversely, the energy
released by the breaking of chemical bonds is converted to other forms of energy (e.g. heat,
electricity, or formation of other chemical bonds), and the reaction is exothermic (enthalpy
decreases). There are a few exceptions to this general rule: for example, the formation of O
2
from two oxygen atoms and the formation of N from two nitrogen atoms are exothermic
2
reactions.
The breakdown of molecules also implies an increase in the level of disorder, which is
referred to as entropy . Thus, a disorganised, more random system of molecules or atoms
is reflected in an increased entropy. Gases have more entropy than liquids, which in turn
have more entropy than solids. The second law of thermodynamics states that in a system
at constant temperature and pressure, the entropy increases or remains the same. Eventually,
the energy becomes evenly distributed over the system. Thus, the distribution of energy in
a given system through reactions and phase changes seeks equilibrium between a minimum
level of enthalpy and a maximum level of disorder (entropy ). This equilibrium is represented
by a minimum of the Gibbs free energy , which depends on the chemical composition,
pressure, and temperature. At constant temperature and pressure, the Gibbs free energy
(further referred to as ‘free energy’) is quantitatively expressed as the difference between
enthalpy and the entropy, multiplied by the absolute temperature:

G H TS (2.6)

2 -2
2 -2
where G = the free energy expressed in Joules (J) [M L T ], H = the enthalpy [M L T ], T =
2 -2
-1
the absolute temperature [θ] (in Kelvin (K) = 273.15 + °C), S = the entropy [ML T θ ].
This is a general statement of the third law of thermodynamics , which stipulates that the
entropy of a pure perfect crystal is zero at absolute zero (0 K). Entropy is more difficult to
evaluate and observe quantitatively than the free energy that is released or consumed in a
phase transition or chemical reaction. Therefore, it is more convenient to express Equation
(2.6) in terms of the change in free energy (ΔG):
G H T S (2.7)

At equilibrium , the change in free energy is zero. If the change in free energy is negative,
the reaction or phase transition is spontaneous; if the change in free energy is positive, the
reaction is non-spontaneous.

Example 2.3 Change in the Gibbs free energy

Solid calcium carbonate is formed from the reaction between solid calcium oxide and

carbon dioxide gas :
CaO (s) + CO (g) → CaCO (g)
2 3

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