Transformation of energy

Chemical Thermodynamics: Definition of Thermochemistry

Chemical thermodynamics: definition of thermochemistry

Chemical thermodynamics is a fundamental branch of chemistry that focuses on the study of energy transformations that occur during chemical reactions. This discipline provides the tools and concepts necessary to understand how energy is exchanged between chemical systems and how the course of a reaction can be predicted. This concept is also known as thermochemistry. 

Therefore, chemical thermodynamics refers to the conversions of chemical energy into thermal energy and vice versa, which occur during a reaction between substances with chemical affinity and studies the variables connected to them.

The relationship between thermodynamics and energy includes the physical changes of matter. All these conversions are carried out within the limits of the laws of thermodynamics.

Chemical thermodynamics involves not only laboratory measurements of various thermodynamic properties, but also the application of mathematical methods to the study of chemical questions and the spontaneity of processes.

The beginnings of chemical thermodynamics arise in the work of Josiah Willard Gibbs "On the equilibrium of heterogeneous substances" (1878).

Basic Concepts of Thermochemistry

Chemical thermodynamics: definition of thermochemistryChemical thermodynamics is based on a series of fundamental concepts and laws that are essential to understanding chemical reactions and their energetic implications.

Some of the key concepts include:

1. Internal Energy (U)

The internal energy of a chemical system refers to the total energy contained in the system, including the kinetic and potential energy of the particles that compose it. The change in the internal energy of a system during a reaction is called ΔU.

2. Enthalpy (H)

Enthalpy is a thermodynamic function that represents the amount of thermal energy absorbed or released by a system during a reaction at constant pressure. It is denoted as H and is related to the internal energy by the equation:


Where P is the pressure and V is the volume of the system.

3. Law of Conservation of Energy

This law, also known as the first law of thermodynamics, states that energy is neither created nor destroyed, but is transferred or converted from one form to another. In the context of chemical thermodynamics, this law applies to chemical reactions, where energy is not lost, but is redistributed between the substances involved.

4. Hess's Law

Hess's law states that the enthalpy change in a chemical reaction depends only on the initial and final states of the reactants and products, and not on the route followed to reach those states. This allows ΔH to be calculated for a chemical reaction from the enthalpies of formation of the reactants and products.

Thermodynamics and Chemical Reactions

Chemical thermodynamics is often applied to the study of chemical reactions, allowing us to determine whether a reaction is energetically feasible and under what conditions. Some key concepts related to chemical reactions and thermodynamics are:

  • Activation energy: Activation energy is the minimum energy that particles must have for a chemical reaction to occur. Thermodynamics focuses on the difference between the activation energy and the total energy released or absorbed in a reaction.

  • Chemical equilibrium: In a system at equilibrium, the free energy is minimal and there are no net changes in the concentrations of the reactants and products.

  • Le Chatelier's Law: This law describes how a system at equilibrium responds to changes in concentration, pressure, or temperature.

  • Gibbs Free Energy (ΔG): Gibbs free energy is a measure of the availability of energy to do useful work during a chemical reaction. ΔG is an indicator of whether a reaction is spontaneous (ΔG negative) or not (ΔG positive).


Published: September 18, 2019
Last review: October 11, 2023