A. ΔH is positive and TΔ S is zero
B. ΔH is positive and T Δ S is negative
C. ΔH is negative and TΔ S is zero
D. ΔH is zero and T Δ S is positive
Correct Answer:
Option D – ΔH is zero and T Δ S is positive
Explanation
The definition of enthalpy change is
ΔH = ΔU + Pex(ΔV)
The enthalpy change is zero because both terms on the right are zero in free expansion of an ideal gas.
There is no external pressure in a free expansion, so P (ex) = 0. And the internal energy change, ΔU of free expansion in a closed system is zero since no intermolecular forces have to be overcome. The temperature remains constant and no heat is absorbed or released.
