The formation of a compound from its constituent elements has already been defined in terms of both energy and entropy.
In this section we look at the standard free energy of formation of compounds, as derived from the standard enthalpy change of formation and the standard entropy change of formation.
Standard free energy of formation
In previous sections we have seen that the standard enthalpy change for formation of a compound is defined as the energy change when 1 mole of a compound is formed from its constituent elements in their standard states = ΔHf.
The standard entropy change of formation is similarly defined as the change in absolute entropy when 1 mole of a compound is formed from its constituent elements in their standard states = ΔSf.
We have also seen that for any reaction or process, the Gibbs' free energy change is:
ΔG = ΔH - TΔS
If the changes all take place under standard conditions then we can define the Standard Gibbs' free energy change of formation as the free energy change that occurs when 1 mole of a compound is formed from its constituent elements in their standard states:
ΔGf = ΔHf - TΔSf
Example: The standard free energy change of reaction can be calculated from the standard free energy of formation of the individual components of the equation.
The standard free energy changes of formation of some compounds are shown below:
Which statement is correct about the following reaction?
MgO(s) + H2O(l) Mg(OH)2(s)
The reactants must be broken apart and the products formed. This means that the standard free energy of formation of the reactants must be reversed in sign before being added to the standard free energies of formation of the products.
ΔG(reaction) = ΔGf(products) - ΔGf(reactants)
∴ ΔG(reaction) = -834 - (-570 - 237) = -27 kJ the reaction is spontaneous