Page 44 - Basic physical chemistry for the atmospheric sciences

30 Basic physical chemistry

stant temperature and pressure (e.g. , phase transitions at atmospheric
pressure), we have from Eqs. (2. 2 8) and (2 3 1 )
.
dh
ds = ­ (2. 3 3)
T

2.6 Standard free energy changes
The Gibbs free energy of a substance depends upon its state. By
convention, the following are considered standard sta t es: (I) for a
solid, the pure solid at l atm and 25°C; (2) for a liquid, the pure liquid
at l atm pressure and 25°C; (3) for a gas, an ideal gas at l atm partial
pressure and 25°C; and (4) for a solution, an ideal solution with a
concentration of l mole of solute per liter of solution (i. e . , I M)
at 25°C.
The change in Gibbs free energy of a system, when reactants in
their standard states are converted to products in their standard states,
is called the molar standard r ee energy change (LlCo) for the reaction.
f
The superscript zero to the G indicates the standard state and the
overbar indicates that the molar amounts of the reactants and products
given by the numerical coefficients in the balanced chemical equation
for the reaction are involved. For the forward reaction of the general
chemical reaction I .5)
(
LlGJ = [g LlGl(G) + h LlGl(H) + . . . ] - [a LlGl(A) + b LlGl(B) + . . . ] (2.34)
where LlGjl(X), which is called the molar standard Gibbs f r ee energy
o f f o rmation (or simply, the standard f r ee energy o f f o rmation) of
X
compound , is the change in the Gibbs free energy when l mole of X
is formed from its elements. By convention, the standard free energies
of formation of the elements in their most stable forms at 1 atm are
taken to be zero. The temperature chosen for tabulating values of
LlOl(X) is usually 25°C. A selection of standard free energies of forma­
V
tion is given in Appendix .
i
I t follows from the above definitions and Eq. (2.32) that f LlGjl(X)
for a reaction is negative, the reactants in their standard states will be
converted spontaneously into the products in their standard states.
If LlGJ is positive, the conversion will not be spontaneous, but the
corresponding reverse reaction will be. However, even when LlGJ i s
positive, some products can form u t in concentrations below that of
b
their standard states.