1. Gibbs Free Energy
A decrease in enthalpy, ΔH = (-), favours a spontaneous reaction (i.e. a reaction is more likely to occur if the products formed are more stable than the reactants)
An increase in entropy, ΔS = (+), favours a spontaneous reaction (i.e. if the products formed are more random or disorganized than the reactants were, then the reaction is more likely to happen)
Therefore it easy to see that reactions in which ΔH = (-) and ΔS = (+) should always be spontaneous
But what about if ΔH = (-) and ΔS = (-), or ΔH = (+) and ΔS = (-), or ΔH = (+) and ΔS = (+)? Will these reactions occur?
We need to consider another factor: temperature. For example, the melting of ice is a spontaneous reaction, but only at temperatures greater than 0oC
To more accurately predict whether a reaction will occur, we need to look at a third value, Gibb’s Free Energy (G)

2. Gibbs Free Energy: G
Is energy that becomes “free” or available to do useful work
All spontaneous reactions are accompanied by a loss in free energy
i.e. ΔG < 0  spontaneous reaction
This means that any reaction that is accompanied by an increase in free energy won’t work
i.e. ΔG > 0  non-spontaneous reaction

3. Calculating changes in free energy
The following equation can be used to determine whether ΔG is negative at a given temperature, and therefore whether that reaction will be spontaneous at a that temperature:
ΔG = ΔHo – TΔSo (T must be in K)
A reaction in which:
ΔHo = -100 kJ ΔSo = 100 J T = 293K
ΔG = (-100) – 293(0.100) = kJ
would occur since ΔG is negative

4. Calculations using ΔG = ΔHo – TΔSo
Example 1:
Determine ΔGo for the reaction:
4Fe(s) + 3O2(g)  2Fe2O3(s)
Is the reaction spontaneous (at 25oC)?
ΔH = 2(-826) = kJ
ΔS = [2(90)] – [4(27) + 3(205)] = -543 J = kJ
ΔG = – (298)(-0.543) = kJ
There is a loss in free energy therefore the reaction is spontaneous.

5. Example 2
2SO3(g)  2SO2(g) + O2(g) Is the reaction spontaneous at 300oC? When will it be spontaneous?

6. Example 3
What is the temperature range that the following reaction will occur at? 3NO2(g) + H2O(l)  2HNO3(l) + NO(g) What temperature range that could prevent this reaction?

7. Example 4
At what temperature will steam decompose?

8. ΔGrxn = ΣnΔGfo Products - ΣnΔGfo Reactants
Will the following reaction occur at room temperature? N2(g) + 3H2(g)  2NH3(g)

9. In general: ΔS ΔH + -