Page 34 - Soil and water contamination, 2nd edition

Basic environmental chemistry 21

Calculate the free energy change for the formation of one mole of calcium carbonate at
25 °C according to this reaction, given ΔH = -178.2 kJ per mole and ΔS = -160.6 J per
mole. Is this reaction spontaneous?

Solution
This is a straightforward application of Equation (2.7):
G H T S 178 2 . 298 . 15 . 0 1606 130 kJ mol -1

Note that the J–kJ conversion must be accounted for in the calculation.
The value of ΔG is negative, indicating that the reaction proceeds spontaneously.
From Equation (2.7) it can also be seen that ΔG becomes zero at a temperature above
25 °C. Thus, if the temperature is sufficiently high, the above reaction will proceed in the
reverse direction, i.e. calcium carbonate will decompose into calcium oxide and carbon
dioxide gas.

2.5 PHASES AND PHASE TRANSITIONS
2.5.1 Phases

Substances can occur in three states: solid, liquid, and gas. A phase is a distinct and
homogeneous state of a material with no visible boundary separating it into parts. As a
rule, there is always only one gas phase , as gaseous substances mix fully. There may be more

than one liquid phase (e.g. oil and water). Liquids of one particular substance always mix
fully, except for liquid helium that may occur in two phases. Many substances may occur

in different solid phases ; for example, carbon may occur in both the diamond and graphite
phase, and silicon dioxide may occur in a crystalline (quartz ) or amorphous , glassy phase.

Chemicals that are dissolved in water occur in the so-called dissolved phase , which is also
called the soluble phase or aqueous phase. Sometimes the term liquid phase is used to refer

to the dissolved phase , but it should be noted that this term ignores the difference between
pure liquids and aqueous solutions. The solid phase is a generic but ill-defined term for all
chemicals that are in soil and sediment solids, or in suspended solids in water. There are also
various other terms specifying particular components of the solid phase. The term adsorbed
phase refers to chemicals adsorbed to solid surfaces or suspended matter. Substances
associated with suspended solids in water, i.e. adsorbed onto or incorporated in suspended
particles, are also referred to as the particulate phase or suspended phase.

2.5.2 Thermodynamic considerations on phase transitions

Chemicals may occur in multiple phases at the same time. The conversion between phases is
called a phase transition. Table 2.4 shows the most important phase transitions of chemicals
in soil, water, and air. Chemicals tend to establish equilibrium among the different phases,
which ultimately results in an equilibrium distribution. As seen in the previous section, this
equilibrium state is characterised by a zero change in free energy (ΔG = 0). From Equation
(2.7) it follows that:
H T S (2.8)

The enthalpy changes primarily due to a change in volume (see Equation 2.5). Because
gases have more entropy than liquids, which, in turn, have more entropy than solids, it takes

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