The situation obtained when a perfect gas expands into a vacuum is?

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.