How does hess's law use intermediate reactions to calculate the enthalpy of a desired reaction?

By adding the enthalpies of the intermediate reactions together to get the enthalpy of the desired reaction

Answer : Hess's law uses the intermediate reactions to calculate the enthalpy by the law of Constant Heat Summation which states that regardless of the multiple stages occurring in the reaction, the total enthalpy change for the reaction is the sum of all changes.

This law was a manifestation that enthalpy is a state function i. e it is irrespective of the path followed to reach the final state.

The application of Hess’s law, uses the standard heats of formation of the reactants and products to indirectly calculate the heat of reaction (enthalpy) for any reaction that occurs at standard conditions.

An enthalpy change which occurs specifically under standard conditions is called the standard enthalpy of reaction and is given the symbol ΔH°.

The standard heat of reaction can be calculated by using the given equation., i. e.;

ΔH° = Σn ΔH°f (products) - ΣnΔH°f (reactants)

Here, Σ represents "the sum of"

The standard heat of reaction is equal to the sum of all the standard heats of formation of the products subtracted by the sum of all the standard heats of formation of the reactants.

The letter "n" represents that each heat of formation must first be multiplied by its coefficient in the balanced equation.