Predict whether a reaction or process will be spontaneous by using the sign of ΔGӨ
The entropy and enthalpy both affect whether a reaction is spontaneous. The balance between these factors is found using Gibbs free energy, symbol G. This indicates whether a reaction will take place on its own.
This equation is derived from the relationship between ΔS and ΔH:
- ΔH < 0 means that the products are more stable than the reactants. The enthalpy change reflects the drive towards stability.
- ΔS > 0 means that the products are more disordered than the reactants. The entropy change reflects the drive towards disorder
- T increases the influence of ΔS and is measured in Kelvin The change in Gibbs free energy is found according to the equation:
The change in Gibbs free energy is found according to the equation:
- If ΔGӨ < 0 the reaction is spontaneous
- If ΔGӨ = 0 the reaction is at equilibrium
- If ΔGӨ > 0 the reaction is not spontaneous
Calculate ΔGӨ for a reaction using the equation ΔGӨ = ΔHӨ – TΔSӨ and by using the values of the standard free energy change of formation ΔGf
When doing these calculations, don’t get caught in the trap of forgetting to check your units. The enthalpy is usually written with kJ, whilst entropy uses J because it tends to be much smaller. You need to convert the enthalpy into J. Make sure the temperature is in Kelvin.
Circumstances for Negative ΔG
Spontaneous Reaction
Exothermic Reaction with Increasing Entropy
Exothermic Reaction with Decreasing Entropy at a Low Temperature
Endothermic Reaction with Increasing Entropy at a High Temperature
Circumstances for Positive ΔG
Non-Spontaneous Reaction
Endothermic Reaction with Decreasing Entropy
Exothermic Reaction with Decreasing Entropy at a High Temperature
Endothermic Reaction with Increasing Entropy at a Low Temperature
You might also be given a table with the standard free energy change of formation, which can then be used to calculate the standard Gibbs free energy of reaction. The equation for this is:
Predict the effect of a change in temperature on the spontaneity of a reaction using standard entropy and enthalpy changes and the equation ΔGӨ = ΔHӨ – TΔSӨ
When the magnitude of temperature increases, this means that the TΔSӨ will also increase in magnitude. If the ΔS is negative, then the increasing temperature will cause the reaction to become less spontaneous.