H and U are state functions but W and q are not state functions. From the equation, ΔH=ΔU+ΔPV At constant volume, ΔH=ΔU+VΔP At constant pressure, ΔP=0,ΔH=qp so, it is a state function. At constant volume, ΔV=0,ΔU=qp so, it is a state function Work done in any adiabatic process is state function. ΔU=q−W (∴q=0) ΔU=−W Work done in isothermal process is not a state function. W=−p (∴ΔT=0,q≠0)