The formation of bonds is exothermic or energy is released. Now, from another lecture we saw that change in enthalpy is equal to a change in internal energy plus PD work done or the work done by the system on the environment to create that system. We also saw that if pressure is held constant and the number of moles is held constant, the change in volume is zero. If the change in volume is zero, this term becomes zero. So we can say change in enthalpy is simply equal to change in internal energy. Remember, when bonds are broken, reaction is endothermic.