Dissemination of IT for the Promotion of Materials Science (DoITPoMS)


Chemical Reactions

We will now see why studying the free energy of a system is useful in determining its behaviour.

The free energy change, ΔG, of a chemical reaction is the difference in free energy between the products of the reaction and the reactants. If the free energy of the products is less than the free energy of the reactants there will be a driving force for the reaction to occur.

For the reaction

$${\rm{A}} + {\rm{B}} \to {\rm{C}}$$,

the free energy change,

\(\Delta G{\rm{ = }}{{\rm{G}}_{\rm{C}}}{\rm{ - }}{{\rm{G}}_{\rm{A}}}{\rm{ - }}{{\rm{G}}_{\rm{B}}}{\rm{ }}\)                (13)

\( = \Delta G^\circ + RT\ln \) \(\left( {{{{{{p_c}} \over {{p^0}_c}}} \over {{{{p_A}{p_B}} \over {{p^0}_A{p^0}_B}}}}} \right) \) (14) 
$$ = \Delta G^\circ + RT\ln \left( {{{{p_c}} \over {{p_A}{p_B}}}} \right)$$

if the standard states \(p_A^ \circ = p_B^ \circ = p_C^ \circ = {\rm{1bar}}\). We see that the free energy change of a reaction is determined by the relative quantities of reactants and products.