If enthalpy change for a reaction is zero, then ΔG∘ equals to
(1) – T ΔS∘
(3) – ΔH∘
(2) TΔS∘
(4) In keq
Correct Answer:
(1) ΔG∘=−TΔS
Explanation:
The Gibbs free energy change (ΔG∘) is given by the equation:
ΔG∘ = ΔH∘ -TΔS∘
where:
- ΔG∘ = Standard Gibbs free energy change
- ΔH∘ = Standard enthalpy change
- T = Absolute temperature (Kelvin)
- ΔS∘ = Standard entropy change
If enthalpy change is zero (ΔH∘=0), the equation simplifies to:
ΔG∘=−TΔS∘
which is option (1).
Why Other Options Are Incorrect?
Option (3): −ΔH∘ is incorrect because we are given that ΔH∘=0, so this term doesn’t contribute to ΔG∘.
Option (2): TΔS∘ is incorrect because the correct expression has a negative sign (−TΔS∘).
Option (4): lnKeq is incorrect directly, but it relates to ΔG∘ using:
ΔG∘=−RTlnKeq
However, this is not the direct expression we need in the given case.
Final Answer:
Since ΔG∘=−TΔS∘ when ΔH∘=0, the correct answer is:
(1) −TΔS∘.