Gibbs energy is a thermodynamic state function that is used to measure the spontaneity of a process at constant temperature and pressure.
As explained in an earlier article, the change in total entropy can be used to determine if a process in an isolated adiabatic system is spontaneous or not. However, most reactions occurs at constant pressure and temperature, meaning the reaction system is in thermal contact with a constant temperature bath. The constant temperature of the system is maintained by the transfer of thermal energy from the bath to the system, and the closed system is no longer adiabatic.
Therefore, we need another state function to predict if such reactions are spontaneous or not. Consider a closed system that is in thermal equilibrium with a constant temperature bath. From eq139, . Let
and substitute
in the differential form of eq24 to give
If the system only does pV work,
Since and
are constant and the change in enthalpy at constant pressure is defined as
Since ,
and
are all state functions, we can define a new state function:
where is called the Gibbs energy.
Substituting eq143 in eq142,
Since we have derived eq144 using eq139, where the equality identifies with irreversible processes and the
relation, with reversible processes, the
equality and
relation in eq144 must also identify with the respective processes, where
Question
Why is an irreversible process a spontaneous process?
Answer
See this article for explanation.
As mentioned in the article on reversible and irreversible processes, a reversible process is an idealisation, which makes all real processes irreversible or spontaneous. Hence, the Gibbs energy of a system undergoing a real process at constant and
decreases until such time when the process attains equilibrium.
Although all real processes are spontaneous, they may not occur within a finite timeframe. Whether a real process occurs is also dependent on chemical kinetics.
The integrated form of eq144 is:
where
A useful equation can be derived from eq143 as follows:
If the closed system only involves pV work, we can substitute eq121 in the above equation to give:
Eq148 becomes for a reversible process carried out at constant pressure and temperature, which is consistent with eq144. Eq148 also shows that the measure of the change in Gibbs energy of a process in general is not restricted to the process being carried out at constant pressure and constant temperature. However, becomes a reflection of whether a process is reversible or irreversible if the process occurs at constant pressure and constant temperature.
Finally, Gibbs energy is previously called Gibbs free energy, where the term ‘free’ refers to energy that is “free to do non-pV work”. However in 1988, IUPAC dropped the term to avoid confusion.