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Hess’s Law was stated by the Swiss chemist Hermain Hess and highlights the fact that enthalpy is a state function. The statement of this law reads:
« The enthalpy change (ΔH) of a chemical reaction in which a set of reactants are converted to products is the same regardless of whether the process is carried out in a single step or in a series of consecutive steps ».
Put another way, the enthalpy change of a reaction is independent of the path from reactants to products. This is a consequence of the fact that enthalpy ( H , not ΔH) is a state function. This means that its value depends solely on the current state of a system, and not on how the system got to it.
Hess’s Law represents one of the fundamental laws of thermochemistry and allows the establishment of a relative scale of measurement of the enthalpy of different chemical substances from certain reference states, which correspond to elemental substances in their most natural states. stable under standard conditions, as will be seen later.
Explanation of Hess’s Law
Since ΔH is given by the difference between the enthalpy of the products and that of the reactants, and each of these enthalpies will only depend on the state in which the respective chemical substances are found; then the difference between both enthalpies will also be independent of how the transformation is carried out.
There are many analogies that allow us to understand this concept in a simple way. An example is looking at the enthalpy of a substance as the balance in a savings account. There is a balance (or an enthalpy) in the reactants, before the chemical reaction occurs, and there will be a balance after the reaction has occurred. The difference between the two balances is independent of how many deposits or withdrawals were made. You could have made a single deposit, or you could have made multiple deposits and withdrawals, but once you get to the products and get the final balance, it will be the same no matter how you got there. Since in all cases we are starting from the same initial state, the balance change (ΔH) will always be the same.
Applications of Hess’s Law
The most important application of Hess’s Law is that it allows us to know the reaction enthalpies of practically any reaction indirectly through the combination of other, simpler chemical reactions. There are two particularly important examples of this:
Determination of enthalpies of reaction from enthalpies of formation
All pure substances in nature are made up of atoms of one or more chemical elements. Therefore, we can always write an equation for the reaction in which a pure substance is formed from its elements in their most stable natural state under standard conditions of temperature and pressure .
These types of chemical reactions are called formation reactions. Some examples of formation reactions are:
- Formation reaction of liquid water:
- Gaseous ozone formation reaction:
- Ferric oxide formation reaction:
Because of the way formation reactions are defined, every other imaginable chemical reaction can be written as a combination of formation reactions; some go forward and others go in reverse. Thanks to Hess’s Law, we can say that the enthalpy change to transform the reactants of a reaction directly into the products in a single step, is equal to the enthalpy of all these formation reactions, which is summarized in the following equation:
In this equation, ν represents the stoichiometric coefficient of the balanced chemical equation.
Born-Haber cycle of lattice energy
The Born-Haber cycle is another typical example of the application of Hess’s Law. In this case, the enthalpies of processes such as fusion, vaporization, bond dissociation, as well as other reaction heats such as enthalpies of formation, ionization energies, and electron affinities are used to determine the lattice energy of the compounds. ionic. This corresponds to the enthalpy of the process by which a crystalline ionic solid is separated into its ions in the gaseous state.
Thanks to Hess’s Law, we can determine this energy indirectly, using the fact that the enthalpy change of the direct reaction in a single stage is equal to the sum of the enthalpies of any other set of reactions that takes place from the same stage. initial state to the same final state.
References
Atkins, P., & dePaula, J. (2014). Atkins’ Physical Chemistry (rev. ed.). Oxford, United Kingdom: Oxford University Press.
Chang, R. (2008). Physical Chemistry (3rd ed.). New York City, New York: McGraw Hill.
Chang, R., Manzo, Á. R., Lopez, PS, & Herranz, ZR (2020). Chemistry (10th ed.). New York City, NY: MCGRAW-HILL.
Suárez, T., Fontal, B., Meyes, M., Bellandi, F., Contreras, R., Romero, I. (2005). Principles of Thermochemistry. Retrieved from http://www.saber.ula.ve/
