All we care about is by how much the enthalpy changes , because it is only this enthalpy change that is available to us. In other words, we don't need H inital and H final , but we can calculate H by direct measurement. The enthalpy change for a reaction can make itself known in the surroundings either as work or as heat energy or as some of each. When all of the enthalpy change appears as heat, we have a way to measure the enthalpy change for a reaction, because in this circumstance H is equal to the heat of reaction at constant pressure. The sign of q, defined earlier, is actually determined by the sign of H. If H is negative so q is negative for exothermic changes.
H is positive and so q is positive for endothermic changes.
One important truth has been strongly implied so far, but never stated in so many words - the amount of energy that leaves a system is exactly the same as the amount that goes into the surroundings. No energy is lost. It just transfers from one place to another, and some of it might change from one form to another. The formal statement of this truth is called the law of conservation of energy.