Hess's law in thermodynamics is used to indirectly check the heat of reaction, and according to the precursor of this law, the Swiss chemist Germain Henri Hess in 1840 instituted that, if a process of reactants react to give a process of products, the heat of reaction released or absorbed is independent of whether the reaction takes place in one or more periods. That is, the heat of reaction only needs the reactants and products, or also that the heat of reaction is a function of state.

Hess was totally occupied with chemistry and one of the best known works was the Law of the constant sum of heat, which was later named as Hess's Law in his honor; It mainly explained that the enthalpy of a reaction could be achieved by adding algebraically the enthalpies of other reactions, some related to the one that matters. Hess's Law is the use of chemical reactions becoming one of the first principles of thermodynamics.

This principle is an adiabatic closed system, that is, there is no heat exchange with other systems or its environment as if it is isolated, which develops from an initial phase to another final phase. For example:

The heat of formation ðH1 of carbon monoxide, CO:

C + 1/2 O2 = CO AH1

It cannot be established directly in the environment in which it is produced, part of the CO is converted into CO2, but if it can be measured directly with the calorimeter, the heats of reaction of the following processes:

CO + 1/2 O2 = CO2

AH2 = 282´6 kJ / mol

C + O2 = CO2

AH3 = -392´9 kJ / mol

The heat of reaction is the algebraic sum of the heats of these reactions.

The heat of reaction of an established chemical process is constantly the same, whatever the process made by the reaction or its intermediate stages.

Enthalpy is a magnitude of thermodynamics represented by the capital letter H and describes the amount of energy that a system exchanges with its environment. In Hess's law it explains that enthalpy changes are additive, ΔHneta = ΣΔHr and contains three norms:

1. If the chemical equation is reversed, the symbol for ΔH is reversed as well.
2. If the coefficients are multiplied, multiply ΔH by the same factor.
3. If the coefficients are divided, divide ΔH by the same divisor.

For example: The enthalpy of reaction is calculated for the reaction:

2 C (s) + H2 (g) → C2H2 (g)

The data is as follows:



The equations corresponding to the given enthalpies are proposed:



The reactants and products of the chemical reaction sought are located in them:



Now the equations must be adjusted:

• Equation (1) must be inverted (the value of enthalpy is also inverted).
• Equation (2) must be multiplied by 2 (the entire equation is multiplied, both reactants and products and the value of enthalpy, since it is an extensive property.
• equation (3), is left the same.

The sum of the fitted equations should give the problem equation.

• Reactants and products are added or canceled.
• Enthalpies add algebraically.