Question

What is the physical significance of free energy?
A) A decrease in free energy is the measure of the useful work done by the system.
B) An increase in free energy is the measure of the useful work done by the system.
C) A decrease in free energy is the measure of the useful heat loss by the system.
D) None of these.

Answer 1

Hint: The energy associated with any chemical reaction which can be used to perform work is known as the free energy of the system. The free energy of the system is the summation of the enthalpy of the system and the product of temperature and entropy of the system.

Complete answer:
The formula for the Gibbs free energy or free energy is,
$\Delta G = \Delta H - T\Delta S$
Where $\Delta G$ is the change in the Gibbs free energy or free energy,
$\Delta H$ is the change in the enthalpy of the system
T is the temperature,
$\Delta S$ is the change in entropy of the system.
If the free energy of the system increases the reaction cannot proceed and no work can be done. If the free energy of the system decreases the reaction can proceed. Thus, work can be done when the free energy of the system decreases.
The free energy is the maximum amount of non-expansion type of work done.
When the system changes from initial state to the final state, the free energy is equal to the work done by the system on its surrounding.
Thus, a decrease in free energy is the measure of the useful work done by the system.

Thus, the correct option is (A) a decrease in free energy is the measure of the useful work done by the system.

Note: When the free energy is negative, a spontaneous reaction occurs. When the free energy is positive, a nonspontaneous reaction occurs. When the free energy is equal to zero, the system has achieved a state of equilibrium.
Thus, the free energy of any system in nature is always negative.
The change in free energy tells us the direction and the extent of the reaction.