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 is D
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 inter molecular forces have to be overcome. The temperature remains constant and no heat is absorbed or released.