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Question

Among the following which one term does not represent Gibbs free energy ?

This question was previously asked in
CUET PG 2026 Agri-Business Management Question Paper (25-Mar-2026) (Shift 2)
The correct answer is
$G=U+TS$

Gibbs Free Energy Formulas Explained

Gibbs free energy ($G$) represents the maximum amount of non-expansion work that can be extracted from a closed system at constant temperature and pressure. It is a thermodynamic potential.

We need to identify which given expression does not correctly represent Gibbs free energy ($G$).

Thermodynamic Relationships

Key thermodynamic relationships are:

  • Internal Energy: $U$
  • Enthalpy: $H = U + PV$
  • Helmholtz Free Energy: $F = U - TS$
  • Gibbs Free Energy: $G = H - TS$

Analyzing the Options

Let's examine each option based on the definition $G = H - TS$ and the relationship $H = U + PV$.

  1. Option 1: $G = U + TS$
    This expression is not a standard formula for Gibbs free energy. It resembles the Helmholtz free energy formula ($F = U - TS$) but with a positive sign for the $TS$ term.
  2. Option 2: $G = F + PV$
    Substitute $F = U - TS$: $G = (U - TS) + PV$ $G = U - TS + PV$ This matches a known form of Gibbs free energy.
  3. Option 3: $G = H - TS$
    This is the fundamental definition of Gibbs free energy.
  4. Option 4: $G = U - TS + PV$
    Substitute $H = U + PV$ into the definition $G = H - TS$: $G = (U + PV) - TS$ $G = U - TS + PV$ This is a valid expression for Gibbs free energy.

Conclusion on Gibbs Free Energy Representation

Options 2, 3, and 4 represent valid forms of Gibbs free energy ($G$). Option 1, $G = U + TS$, does not represent Gibbs free energy.

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