The overall enthalpy change that accompanies a chemical reaction is independent of the route by which the reaction takes place, provided the initial and final states are the same. It is a statement of the 'Law of Conservation of Energy' since, for example, if the reverse of a reaction was less endothermic than the forward reaction was exothermic, energy could be created.
The precise value of DH depends on the temperature and pressure at which it is measured, and therefore standard conditions have to be defined so that DH values can be compared. Standard temperature is 298 K and standard pressure is 1 atm (101 325 Nm-2). Standard values are given in chemical data books.
See also Enthalpy of Combustion | Enthalpy of Formation.