How is Hess's law used to calculate the enthalpy of a reaction?
A. The enthalpy is obtained from the difference in final and initial reactions in a path.
B. Enthalpies from similar equations are substituted for unknown reaction enthalpies. C. The final equation in a reaction path provides the enthalpy for the desired reaction.
D. The desired enthalpy is obtained through adding intermediate reactions together.

Hess's law, also called Law can also be called Law of additivity of the enthalpies of reaction, it is a useful method when it is not possible to calculate the enthalpies of reaction from the enthalpies of formation, or in reactions in which the reaction enthalpy cannot be determined experimentally because it is very slow or very explosive.

Hess's law states that when reactants are converted to products, the enthalpy change is the same whether the reaction takes place in one step or in a series of steps.

In this way, Hess's Law determines that when a chemical reaction can be expressed as an algebraic sum of other reactions, its reaction enthalpy is equal to the sum of the enthalpies of the partial reactions. In this way, it allows thermochemical equations to be treated as algebraic equations, and can be added, subtracted, or multiplied by a number to find the desired thermochemical equation.

Summary

Hess's Law states that the sum of the ∆H of each stage of the reaction will give us a value equal to the ∆H of the reaction when it occurs in a single stage.

That is, the desired enthalpy is obtained through adding intermediate reactions together. (option D)

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