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Entropy and Free Energy 253
17-2 Entropy
The qualitative entropy change (increase or decrease of entropy) for a system
can sometimes be determined using a few simple rules:
1. Entropy increases when the number of molecules increases during a reaction.
2. Entropy increases with an increase in temperature.
3. Entropy increases when a gas forms from either a liquid or solid.
4. Entropy increases when a liquid forms from a solid.
In much the same fashion as the ∆H was tabulated, the standard molar
entropies (S) of elements and compounds are tabulated. This is the entropy
associated with 1 mol of a substance in its standard state. Unlike the enthalpies,
the entropies of elements are not zero. For a reaction, it is possible to calculate
the standard entropy change in the same fashion as the enthalpies of reaction:
∆S ∆S products ∆S reactants
17-3 Gibbs Free Energy
One of the goals of chemists is to be able to predict if a reaction will be spontaneous.
A reaction may be spontaneous if its ∆H is negative or if its ∆S is positive,
but neither one is a reliable predictor by itself about whether or not a reaction
will be spontaneous. Temperature also plays a part. A thermodynamic factor
that takes into account the entropy, enthalpy, and temperature of the reaction
would be the best indicator of spontaneity. This factor is the Gibbs free energy.
The Gibbs free energy (G) is:
G H TS where T is the Kelvin temperature
Like most thermodynamic functions, it is only possible to measure the change
in Gibbs free energy, so the relationship becomes:
∆G ∆H T∆S
If there is a ∆G associated with a reaction and we reverse that reaction, the sign
of the ∆G changes.
∆G is the best indicator chemists have as to whether or not a reaction is
spontaneous:
• If ∆G > 0, the reaction is not spontaneous; energy must be supplied to cause
the reaction to occur