Identify the correct statement for change of Gibbs energy for a system $\left(\Delta \mathrm{G}_{\text…
Identify the correct statement for change of Gibbs energy for a system $\left(\Delta \mathrm{G}_{\text {system }}\right)$ at constant temperature and pressure:
If $\Delta \mathrm{G}_{\text {system }} < 0$, the process is not spontaneous
If $\Delta \mathrm{G}_{\text {system }} > 0$, the process is spontaneous
If $\Delta \mathrm{G}_{\text {system }}=0$, the system has attained equilibrium
If $\Delta \mathrm{G}_{\text {system }}=0$, the system is still moving in a particular direction
Solution
If the Gibbs free energy for a system $\left(\Delta \mathrm{G}_{\text {system }}\right)$ is equal to zero, then the system is in equilibrium at constant temperature and pressure.
Related Theory
Free energy change criteria for predicting spontaneity is better than entropy change criteria because the former requires free energy change of system only whereas the latter requires entropy change of system and surroundings.
$\Delta G > 0$; the reaction is non-spontaneous and endergonic
$\Delta G < 0 ;$ the reaction is spontaneous and exergonic
$\Delta G=0 ;$ reaction is at equilibrium
A Caution
According to the second law of thermodynamics entropy of the universe always increases for a spontaneous process. $\Delta G$ determines the direction and extent of chemical change.