The Dependence of Free Energy on Pressure

**In this section we will discuss
how pressure effects free energy. For and ideal gas, enthalpy
does not depend on pressure but entropy does depend on pressure
because of its dependence of volume. Pressure will effect free
energy because of G = H - TS. For example, there is a volume of
10L of an ideal gas. If we made the volume bigger then the
entropy will be greater because it has more positions available
for its molecules. At a given temperature of an ideal gas**

**
S**_{large volume}** > S**_{small
volume}

**and since pressure and volume
are related the**

**
S**_{low pressure}** > S**_{high
pressure}

**
To show how entropy and the free energy of an ideal gas depend on
its pressure, we will consider it through the equation:**

**
****G = ****G° +
RT ln(Q)**

**where Q is the reaction quotient
(from the law of mass action), T is the temperature in Kelvin, R
is the gas law constant 8.3145 J/K/mo, ****G° is
the free energy change for the reaction with all reactants and
products at a pressure of 1 atm and ****G is
free energy for pressures of reactants and products.**