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Law Of Heat Summation

According to the law of heat summation put forth by Henri Hess, we can find the total enthalpy of a reaction by adding the enthalpy changes of the individual steps in a reaction. It doesn't really matter if the reaction occurs in many steps or in one step. As long as the enthalpy values that we sum up are correct, we will get the overall enthalpy change.

Law of heat summation: The total enthalpy of a reaction can be derived by adding the enthalpy changes of the individual steps in the reaction.



Calculate the ΔH of the reaction between HCl and fluorine, forming HF and chlorine. The enthalpies of the related reactions are also given.

image\14723 ch 10.png

(Reaction 1)

(Reaction 2)

(Reaction 3)
image\14753 ch 10.png

image\14715 ch 10.png

The individual reactions that are given have to be reversed if necessary, so that the sum of the sub-reactions will add up to the overall equation. Reaction 1 should be multiplied by 2. The second and third reactions should be reversed. This applies to the corresponding enthalpy values as well. That means, if the reaction is reversed the sign of the corresponding change in enthalpy should also be reversed. The enthalpy of the overall reaction is –988.4 kJ/mol.

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