If a process written as the sum of several stepwise processes, the enthalpy change of the total process equals the sum of the enthalpy changes of the various steps. Hess’s law is valid because enthalpy is a state function.

How does Hess's law allow you to determine the enthalpy of a reaction without carrying out the reaction?

Hess’s law allows us to calculate ΔH values for reactions that are difficult to carry out directly by adding together the known ΔH values for individual steps that give the overall reaction, even though the overall reaction may not actually occur via those steps.

What is thermodynamic basis of Hess's law?

Hess’s law states that the energy change in an overall chemical reaction is equal to the sum of the energy changes in the individual reactions comprising it. … The law is a variation of the first law of thermodynamics and conservation of energy.