What is thermochemistry?

The thermochemical handles the study of heat changes take place in the reactions between two or more chemical species. It is considered an essential part of thermodynamics, which studies the transformation of heat and other types of energy to understand the direction in which processes develop and how their energy varies.

Likewise, it is essential to understand that heat involves the transfer of thermal energy that occurs between two bodies, when they are at different temperatures; while thermal energy is that associated with the random movement of atoms and molecules.

Therefore, since in almost all chemical reactions energy is absorbed or released through heat, the analysis of the phenomena that occur through thermochemistry is of great relevance.

What does thermochemistry study?

Thermochemistry studies the energy changes in the form of heat that occur in chemical reactions or when processes that involve physical transformations occur.

In this sense, it is necessary to clarify certain concepts within the subject for a better understanding of it.


For example, the term “system” refers to the specific segment of the universe that is being studied, with “universe” being understood as the consideration of the system and its surroundings (everything external to it).

So, a system generally consists of the species involved in the chemical or physical transformations that occur in the reactions. These systems can be classified into three types: open, closed and isolated:

  • An open system is one that allows the transfer of matter and energy (heat) with its surroundings.
  • In a closed system there is the exchange of energy but not of matter.
  • In an isolated system, there is no transfer of matter or energy in the form of heat. These systems are also known as “adiabatic”.

Laws of thermochemistry

The laws of thermochemistry are closely linked to Laplace and Lavoisier’s law, as well as Hess’s law, which are the precursors of the first law of thermodynamics.

The principle put forward by the French Antoine Lavoisier (important chemist and nobleman) and Pierre-Simon Laplace (famous mathematician, physicist and astronomer) review that “the alteration in energy that manifests itself in any physical or chemical transformation has equal magnitude and meaning contrary to the alteration in the energy of the inverse reaction ”.

Hess’s Law

Simple and general diagram where Hess’s law is applied for four chemical reactions

In the same vein, the law formulated by the Russian chemist originally from Switzerland, Germain Hess, is a cornerstone for the explanation of thermochemistry.

Hess’s law can be enacted this way: “the total enthalpy in a chemical reaction is the same, whether the reaction is carried out in a single step or in a sequence of several steps.”

The total enthalpy is given as the subtraction between the sum of the enthalpy of the products minus the sum of the enthalpy of the reactants.

In the case of the change in the standard enthalpy of a system (under standard conditions of 25 ° C and 1 atm), it can be schematized according to the following reaction:

ΔH reaction = ΣΔH (products) – ΣΔH (reactants)

Another way to explain this principle, knowing that the enthalpy change refers to the heat change in reactions when they occur at constant pressure, is by saying that the change in the net enthalpy of a system does not depend on the path followed. between initial and final state.

First Law of Thermodynamics

This law is so intrinsically linked to thermochemistry that sometimes it is confused which one was the one that inspired the other one; So, to shed light on this law, one must start by saying that it is also rooted in the principle of conservation of energy.

So thermodynamics not only takes into account heat as a form of energy transfer (like thermochemistry), but also involves other forms of energy, such as internal energy ( U ).

So the variation in the internal energy of a system (ΔU) is given by the difference between its initial and final states (as seen in Hess’s law).

Taking into consideration that the internal energy is made up of the kinetic energy (movement of the particles) and the potential energy (interactions between the particles) of the same system, it can be deduced that there are other factors that contribute to the study of the state and properties of each system.

Thermochemistry applications

Thermochemistry has multiple applications, some of these will be mentioned below:

  • Determination of energy changes in certain reactions by using calorimetry (measurement of heat changes in certain isolated systems).
  • Deduction of enthalpy changes in a system, even when these cannot be known by direct measurement.
  • Analysis of experimentally produced heat transfers when organometallic compounds are formed with transition metals.
  • Study of energy transformations (in the form of heat) given in coordination compounds of polyamines with metals.
  • Determination of the enthalpies of the metal-oxygen bond of β-diketones and β-diketonates bound to metals.
  • As in previous applications, thermochemistry can be used to determine a large number of parameters associated with other types of energy or state functions, which are those that define the state of a system at a given time.
  • Thermochemistry is also used in the study of numerous properties of compounds, as in titration calorimetry.

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