Chemist and physician Germain Henry Hess (1802-1850) developed important work in the field of thermochemistry.

THE *Hess's Law* It is an experimental law and states that the enthalpy variation of a chemical reaction depends only on the initial and final states of the reaction.

THE *Hess's Law* can also be called *law of the sum of the reaction heats*.

It is a way of calculating the enthalpy variation through the heat of the intermediate reactions. They can be infinite enthalpy variations.

Example:

What is the value of the enthalpy variation of the following reaction?

Data (intermediate equations):

Resolution:

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Note that the ΔH_{1} and ΔH_{2} are summed to obtain the value of the enthalpy variation. The chemical equations are also summed, giving the overall reaction.

To assemble the equations and apply Hess's Law, we can make some mathematical changes by following these rules:

1 °) the intermediate equations must be in agreement with the global reaction. Put the equations (data) in the order they react or are produced. If they do not agree, the ΔH signal is changed;

2 °) adjust the coefficients also according to the overall reaction. If the equation is multiplied, ΔH must also be multiplied by the same number.

3 ° perform the summation to mount the overall reaction;

4 °) add the ΔH values of the intermediate equations to find the ΔH of the overall reaction.

Example:

Calculate the enthalpy variation of the following reaction by Hess's Law:

Dice:

Resolution:

You must write all intermediate equations (data) according to the overall reaction. In the first equation, what is in common is C_{(graphite)}. Then it should be written the same way (as reagent and 1mol).

The second equation has in common with the global reaction H_{2}(g). In the data, this chemical species is not exactly the same as in the global one. You must multiply the whole equation by 2, including the ΔH_{2}.

The third equation has in common with the overall reaction CH_{4}(g). You must invert the position of this equation and therefore change the sign of ΔH_{3}.

Here's how it should be done: