Gibbs Energy Change And Criteria For Equilibrium

The maximum (or reversible) work that can be done by a thermodynamic system at constant temperature and pressure is known as Gibbs energy. The reversible work in thermodynamics means a special method in which work is carried out such that the system is in perfect equilibrium with all its surroundings. In terms of chemical reactions, the word reversible means that the reaction can be carried out in either direction simultaneously and a dynamic equilibrium is always maintained. This further means that reactions in both the directions should proceed with a decrease in free energy, which seems impossible. This is only possible if at equilibrium the Gibbs energy of the system reaches its minimum value. Otherwise the system will spontaneously change to configuration of lower free energy.

The conditions of equilibrium

A thermodynamic system is said to be in equilibrium if it’s intensive properties (temperature, pressure) and extensive properties (U, G, A) are constant. Or the total change in any of the property is zero. Looking at the following equation we can say, if the reaction is reversible and the Gibbs free energy is zero then the system is said to be in equilibrium.
\( A + B ⇌ C + D\); \(\triangle_r G\) = \(0\)

The Gibbs energy for a reaction which is in standard state, \(\triangle_r Gᶿ\) is related to equilibrium constant as follows:

\(0\) = \(\triangle_r Gᶿ + RT~ ln~ K\)
or \(\triangle_r Gᶿ\) = \(- RT ~ln ~K\)
or \(\triangle_r Gᶿ\) = \(-2.303~ RT~ log~ K\)

It is also known that:

\(\triangle_r Gᶿ\) =\( \triangle_rHᶿ – T\triangle_r Sᶿ\) =\(- RT ~ln ~K\)
For endothermic reactions the value of \(\triangle_rHᶿ\) maybe large and positive; if the value of K is less than 1 then it is unlikely to form much of the product. In case of exothermic reaction the value of \(\triangle_rHᶿ\) is large and negative, even the value of \(\triangle_rGᶿ\) is likely to be large and negative. In these cases the value of K will be much larger than 1. It can be said that strong exothermic reactions will have a larger value of K.

So far we have read Gibbs free energy and the conditions that affect the equilibrium of the system. For any further query on these topics kindly install Byju’s the learning app.


Practise This Question

Silane, SiH4 like methane, burns in air. The product silica (silicon dioxide) is solid quite unlike carbondioxide.
SiH4(g)+2O2(g)SiO2(s)+2H2O(g) standard Gibbs energy of formation of SiO2(s), H2O and SiH4 are 805, – 228.6 and + 52.3 (all is KJ mol1) respectively. Calculate the value of ΔrG.