Question on: JAMB Chemistry - 2018
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
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Correct Option: D
Formula for enthalpy change: 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.
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