Ok, I'll get pretty straight-forward. So, the usual assumptions: Energy given to the system: > 0 Energy taken from the system: < 0 Enthalpy is defined as: ΔH = ΔU + pΔV, so ΔU = ΔH - pΔV Let's say that during some chemical reaction there is heat produced (Energy taken from the system): ΔH = -49 kJ ... And a gas was compressed (Energy given to the system): W = 5 kJ To calculate the change in internal energy ΔU = ΔH - pΔV ΔU = -49 kJ - 5 kJ = -54 kJ However, this does not make sense to me. Shoudn't it be -44 kJ by common sense? Is it the sign convention is intentionally switched in the first term? Thanks.