Question 23:
The component of the total energy of a system that can do work at constant temperature and pressure is called:
Free Energy Thermodynamics Work
Gibbs free energy measures the maximum useful work available from a system at constant temperature and pressure. The correct answer is option (C).
Correct Answer
(C) Free energy
Gibbs free energy (G = H – TS) represents the portion of total energy available for work under constant T and P, excluding entropy-unavailable energy.
Thermodynamic Concepts
Free energy change (ΔG) predicts spontaneity: ΔG < 0 favors forward reaction. At equilibrium, ΔG = 0. This distinguishes it from total energy measures.
Option Analysis
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(A) Enthalpy: Total heat content (H = U + PV); includes work potential but not T,S correction for constant T,P conditions.
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(B) Entropy: Disorder measure (S); opposes work availability, subtracted in free energy equation.
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(C) Free energy: Correct; specifically Gibbs free energy for constant T,P work capacity.
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(D) Kinetic energy: Motion-based energy (½mv²); not a thermodynamic state function for system work potential.
| Option | Definition | Constant T,P Work? | Status |
|---|---|---|---|
| A Enthalpy | Total heat content | No | Wrong |
| B Entropy | Disorder measure | No | Wrong |
| C Free energy | Available work potential | Yes | Correct |
| D Kinetic | Molecular motion energy | No | Wrong |
Exam Preparation Tips
Memorize: Free energy = enthalpy minus T×entropy. Link to biology: ATP hydrolysis ΔG°’ = -30.5 kJ/mol drives endergonic reactions.
