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
jamb 2018chemistry

The correct answer is D. ΔH is zero and T Δ S is positive

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.

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