Spontaneity of a reaction
Reactions which release heat (and so increase stability) tend to occur. Reactions which increase entropy (ΔS is positive) tend to occur, but neither can be used to accurately predict spontaneity alone.
Gibbs free energy (G) is defined as a measure of the total entropy of the universe. Hence the change in Gibbs free energy (ΔG) is the change in the total entropy of the universe. The total entropy of the universe must increase for any process to occur.
When heat is released in a reaction (exothermic change) this energy heats up the universe and effectively increases its entropy (there are a greater number of possible energy states that the particles in the universe can adopt).
The total entropy of the universe must increase and consequently exothermic reactions are favourable.
If the entropy of a reaction mixture increases then this is also favourable as the total entropy of the universe also increases.
Gibbs free energy change = ΔH - TΔS
If Gibbs free energy change is negative (convention) then the total entropy of the universe increases and the reaction is spontaneous. Why is the sign negative?
When ΔG is negative, the reaction is spontaneous, when it's positive, the reaction is not.
Gibbs free energy calculations
Enthalpy changes can be calculated indirectly by summing the enthalpy values of related equations using Hess' law. Entropy changes can be calculated in the same way. It follows then that Gibbs free energy changes can be calculated from a knowledge of Gibbs free energy values in related equations.
Spontaneity of reaction
Determined by the relationship
ΔG = ΔH - Temperature(in kelvin) x ΔS
See standard level entropy for a fuller treatment
| Enthalpy change | Entropy change | Gibbs free energy | Spontaneity |
| positive | positive | depends on T, may be + or - | yes, if the temperature is high enough |
| negative | positive | always negative | always spontaneous |
| negative | negative | depends on T, may be + or - | yes, if the temperature is low enough |
| positive | negative | always positive | never spontaneous |
Tips for exam:
GIBBS ENERGY IS ALWAYS IN FINAL EXAMS. Practice this concepts well. Normally, there will be an equation of a reaction whereby you will need to find the change in Gibbs with information provided. If you find a negative Gibbs it is a spontaneous reaction.
Please do understand that Gibbs energy takes into account both entropy and enthalpy. Questions may require you to find entropy and enthalpy in order to find Gibbs. A good example is as below:.................................(full notes will be available upon purchase)
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