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Hess's law

Hess's law states that in going from one particular set of reactants to one particular set of products it doesn't matter whether the reaction takes place in one or multiple steps: The change in enthalpy will be the same.

Two water molecules and one carbon dioxide molecule are placed above the first horizontal line, indicating the reactant molecules. The arrow pointing upwards indicates the endothermic reaction of bond breaking, which is equal to minus sum of change of enthalpy of formation of the reactants, and leads to another horizontal line, above which two molecules of oxygen, two molecules of hydrogen and one atom of carbon are situated and named ‘elements in their standard state’. From there, the arrow pointing downwards indicates the exothermic reaction of bond making, which is equal to plus sum of change of enthalpy of formation of the products, and leads to last and lowest horizontal line, above which one molecule of methane and two molecules of oxygen are situated and named ‘product molecules’. The vertical line between the reactant molecules line above and product molecules below is named ‘change of the enthalpy of reaction.

Figure 1: Hess's law is illustrated by this reaction, going from the reactants to the products over the constituent elements in their standard states.

It follows from Hess' Law, that the change in enthalpy of a reaction, ΔHr, can be calculated by subtracting the sum of the enthalpies of formation of the reactants from the sum of the enthalpies of formation of the products, according to following equation:

ΔHr = ΣΔHf(products) - ΣΔHf(reactants)

Referred from:

Article
Enthalpies of formation and reaction