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