And simply change in enthalpy of the system is equal to change in internal energy or simply change in energy. The PV term disappears because it's zero. Now thirdly, reactions are reversible, which means that if this wasn't true, the equation would break down as well. Now, since reactions are reversible and enthalpy is a state function, hess's law tells us the change in enthalpy of the four reaction is equal to the negative change in enthalpy of the reverse reaction. So if we look at this reversible reaction here, a plus B is equal to C plus D, where the change in enthalpy is negative ten. And the reverse reaction, according to Hess's Law, is C plus D equal A plus B, and this is negative of this.