Untitled - Kelly Walsh High School

254 CHEMISTRY FOR THE UTTERLY CONFUSED
• If ∆G < 0, the reaction is spontaneous; energy will be released
• If ∆G 0, the reaction is at equilibrium; there will be no gain or loss of energy
Just like with the enthalpy and entropy, the standard Gibbs free energy change,
(∆G°), is calculated:
∆G ∆G products ∆G° reactants
The ∆G f of an element in its standard state is zero.
17-4 Free Energy and Reactions
Be Careful!
We may also calculate ∆G° for a reaction by using the standard enthalpy and
standard entropy of reaction:
∆G ∆H rxn T∆S rxn
It is possible to use this equation when the temperature is not standard. The
∆H rxn and the ∆S rxn values vary slightly with temperature. This slight variation
allows an approximation of ∆G° at temperatures that are not standard.
In other cases, where the conditions are not standard, you should use the
relationship:
∆G ∆G° RT ln Q
The ∆G symbol refers to the nonstandard Gibbs free energy value, ∆G° is the
standard value, R is the gas constant (8.314 J/molK), T is the temperature (K),
and Q is the reaction quotient first seen in Chapter 14. At equilibrium, this
equation becomes:
0 ∆G° RT ln K or ∆G° = RT ln K
K is the equilibrium constant.
The value of R uses joules and the value of the Gibbs free energy is typically in
kilojoules. You will often need to change joules to kilojoules or kilojoules to joules.