The enthalpy changes associated with reaction are dependent upon the temperature. They are also dependent upon the amounts, and the pressure and states of the reactants and products. For this reason it is convenient to specify a standard state for a substance. The standard state for a substance is defined as being the pure substance at 1 atmosphere pressure, and at a specified temperature
Because enthalpy is a state function and it follows that the absolute enthalpy associated with the products and reactants in a reaction are independent of the process by which they were formed. Consequently, the enthalpy changes during the course of a reaction and given by ΣHreactants-ΣHproducts is independent of the reaction pathway. The law of constant heat summation is recognition of this fact, and states that:
'The overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual steps in the reaction measured at the same temperature'.
Enthalpy change associated with the burning of carbon to form carbon monoxide. Enthalpy is impossible to prevent formation of some carbon dioxide if the enthalpy change is measured directly. The reaction may be written as either a direct (one-step) or an indirect (two-step) process (Fig. 1). Hess's law indicates that the total enthalpy change by either path is identical in which case ΔH1=ΔH2+ΔH3, so allowing to be obtained a value for ΔH1 without the need for direct measurement
Fig.1. Two possible chemical pathways to the formation of CO from its elements.