JEE Challenger
More from Thermodynamics

Standard Gibbs Free Energy Change at Chemical Equilibrium

STATEMENT-1 : For every chemical reaction at equilibrium, standard Gibbs energy of reaction is zero.
and
STATEMENT-2 : At constant temperature and pressure, chemical reactions are spontaneous in the direction of decreasing Gibbs energy.

Options

A

STATEMENT-1 is True, STATEMENT-2 is True; STATEMENT-2 is a correct explanation for STATEMENT-1

B

STATEMENT-1 is True, STATEMENT-2 is True; STATEMENT-2 is NOT a correct explanation for STATEMENT-1

C

STATEMENT-1 is True, STATEMENT-2 is False

D

STATEMENT-1 is False, STATEMENT-2 is True

Correct

Step-by-Step Solution

To evaluate the truth value of each statement:

  1. Analysis of STATEMENT-1:
    The relationship between the Gibbs free energy change (ΔG\Delta G) and the standard Gibbs free energy change (ΔG∘\Delta G^\circ) of a reaction at temperature TT is given by:
    ΔG=ΔG∘+RTln⁡Q\Delta G = \Delta G^\circ + RT \ln Q
    where QQ is the reaction quotient and RR is the universal gas constant.

    At chemical equilibrium, the condition for dynamic equilibrium is:
    ΔG=0andQ=Keq\Delta G = 0 \quad \text{and} \quad Q = K_{\text{eq}}
    Substituting these values into the thermodynamic relation gives:
    ΔG∘=−RTln⁡Keq\Delta G^\circ = -RT \ln K_{\text{eq}}
    The standard Gibbs energy of reaction, ΔG∘\Delta G^\circ, is zero if and only if the equilibrium constant Keq=1K_{\text{eq}} = 1. For an arbitrary reaction at equilibrium, KeqK_{\text{eq}} can have any positive value, meaning ΔG∘\Delta G^\circ is generally non-zero.
    Therefore, STATEMENT-1 is False.

  2. Analysis of STATEMENT-2:
    According to the Second Law of Thermodynamics, the criterion for spontaneity of a process occurring at constant temperature (TT) and pressure (PP) is given by:
    (ΔG)T,P<0(\Delta G)_{T, P} < 0
    A spontaneous chemical change proceeds in the direction where the total Gibbs energy of the system decreases until it reaches a minimum at equilibrium (where (ΔG)T,P=0(\Delta G)_{T, P} = 0).
    Therefore, STATEMENT-2 is True.

Conclusion:
STATEMENT-1 is False, and STATEMENT-2 is True.

Thus, the correct option is D.