If volume is constant, then change in volume is zero, because the final and the initial are the same. So the TV work done is zero. And that means that we could approximate change in enthalpy. 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.