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By stating that the pathway of a reaction does not matter for the total change in enthalpy, the Hess's law allows us to add the separate changes in enthalpies of separate reactions to get the total change in enthalpy of the sum reaction.
To add on to what everyone said above, Hess's law can be useful for finding the total enthalpy for a reaction if you are given multiple sub-reactions. You then manipulate all the sub-reactions to obtain the one they ask for, and then calculate the total enthalpy by adding all the enthalpies of the sub-reactions you found.
Hess's Law uses the enthalpies of intermediary reactions to calculate the total enthalpy. By using Hess's law , you are manipulating reactions to get rid of the intermediary reactants/products and up with the reaction you want. You then factor those manipulations into your calculations for enthalpy.
Hess's Law states that the total enthalpy is independent of the intermediate reactions. This means you can add up the enthalpy changes of intermediate reactions to find the overall enthalpy change of the reaction. You use it for enthalpy change questions.
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