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The first method is Hess's Law, which you use when you know the deltaH of two different reactions and are able to combine them to find the deltaH of the new reaction. The second method uses bond enthalpies, since in chemical reactions, bonds in reactants and broken and bonds in products are formed. The bond enthalpies for certain elements are known average values that you can add and subtract to find the overall deltaH. The last method uses standard enthalpies of formation, which is the standard deltaH of 1 mol of a substance that is formed from its elements in their most stable form. The last method shows that the sum of all the standard enthalpies of formation of the products minus the sum of all the standard enthalpies of formation of the reactants gives the overall standard enthalpy of the reaction.
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