Which of the following is always valid? Choose only ONE answer.

1. W = - P ∫ dV, for the calculation of work in a mechanically-reversible piston-cylinder arrangement

2. For any fluid undergoing a change in pressure, Cp = dH/dT.

3. ΔH = ΔU + Δ(PV)

4. ΔU = Q + W

5. ΔH = ΔU + P.ΔV