Transformation of energy

What Is the Thermodynamics Chemistry Definition?

Chemical thermodynamics is the study of the relationship between chemistry and thermodynamics. This concept is also known as thermochemistry. 

What is the thermodynamics chemistry definition?

Therefore, chemical thermodynamics refers to converting chemical energy into thermal energy and vice versa, which occurs during a reaction between substances with chemical affinity and studies the variables. The relationship between thermodynamics and energy includes the physical changes of matter.

It involves laboratory measurements of several properties and the use of maths in the study of chemical questions and the spontaneity of processes.

The beginnings of chemical thermodynamics arise in Josiah Willard Gibbs's work "On the Equilibrium of Heterogeneous Substances" (1878).

An example of chemical thermodynamics is the heat generated during the charging and discharging of solar batteries in a photovoltaic (PV) system.

What Is a Chemical Reaction?

A chemical reaction is a process that transforms one set of chemical substances into another. In a chemical reaction, atoms or groups of atoms are rearranged to form new compounds. The reactants in a chemical reaction are called reactants, while the products are called products.

In order for a chemical reaction to occur, there must be a change in the chemical bonds between atoms. In other words, a chemical reaction is a process that involves the breaking and forming of chemical bonds.

In most cases of interest in chemical thermodynamics, there are internal degrees of freedom and processes, such as chemical reactions and phase transitions. These chemical reactions always create entropy unless they are in equilibrium or are maintained in a continuous balance.

Even for homogeneous materials, free energy functions depend on the composition, as do all extensive thermodynamic potentials, including internal energy.

What Is Thermodynamics?

Thermodynamics is the study of heat and energy. It is a branch of physics that deals with the behavior of matter and energy under different conditions. The main goal of thermodynamics is to find out how energy can be converted into work and how work can be converted into heat.

What Are the Laws of Thermochemistry?

All these conversions are carried out within the limits of the laws of thermodynamics.

In addition to these laws, other laws govern this entire discipline:

  • Law of Lavoisier and Laplace (formulated in 1780): the heat transfer that accompanies a given chemical reaction is equal and opposite to the heat transfer of the opposite reaction;

  • Hess's Law (formulated in 1840): the change in the enthalpy of reaction is the same as the reaction occurs in one or more successive and independent stages (even purely hypothetical).

  • The ideal gas law (PV = nRT) relates to the macroscopic properties of ideal gases. An ideal gas is a gas in which the particles (a) neither attract nor repel each other and (b) do not take up space (have any volume). Of course, no gas is truly ideal, but the ideal gas law does provide a good approximation of the actual behavior of gases under many conditions.

The two first laws were derived empirically and stated before the first law of thermodynamics. However, you can prove that they are direct effects of it and that the enthalpy H and the internal energy U are functions of the state.

Description of Chemical Thermodynamics

The main objective of chemical thermodynamics is to establish a criterion to determine the feasibility or spontaneity of a given transformation. In this way, chemical thermodynamics is typically used to predict the energy exchanges that occur in the following processes: chemical reactions, phase changes, and solution formation.

Thermochemical reactions can be produced by different thermodynamic processes such as:

The main thermodynamic processes in a reaction between the most important chemical substances are the following:

  • The isobaric process takes place at constant pressure.

  • The isochoric process takes place at constant volume.

  • The isothermal process takes place at a constant temperature.

  • Adiabatic process: it is a process in which the amount of heat remains constant. Therefore, there is no heat transfer between the system and the surroundings.

  • The isentropic process takes place at constant entropy.

The following state functions are of primary interest in this topic: internal energy (U), enthalpy (H), entropy (S), and Gibbs free energy (G).

The structure of chemical thermodynamics is based on the first two laws of thermodynamics. From the first law of thermodynamics and the second law of thermodynamics, four equations called "fundamental Gibbs equations" can be derived.

From these four, many equations can be derived relating the thermodynamic properties of the system using relatively simple maths.

What Is a Thermodynamic System?

A thermodynamic system is the specific portion of the universe that is being studied. Everything that is outside the system is considered the environment or surroundings. A system can be:

  • A (completely) isolated thermodynamic system that cannot exchange energy or matter with the surroundings, such as an isolated bomb calorimeter.

  • A thermally isolated system can exchange mechanical work but not heat or matter, such as an isolated closed piston or balloon.

  • A mechanically isolated system can exchange heat but not work or mechanical matter, such as a non-insulated bomb calorimeter.

  • A closed system can exchange energy but not matter, like a closed balloon or piston without insulation.

  • An open system can exchange matter and energy with the surroundings, like a pot of boiling water.

Gibbs' Free Energy and Spontaneity

The second law of thermodynamics helps us determine whether a process will be spontaneous. The use of so-called Gibbs free energy changes can predict whether a reaction will be spontaneous one way or the other (or at equilibrium).  

In chemistry, a spontaneous process occurs without the supply of external energy. A spontaneous process can happen quickly or slowly since spontaneity is not related to the kinetics or rate of reaction.

In order to better understand the thermodynamics chemistry definition, it is helpful to look at some examples.

Examples of Thermodynamics Chemistry

One example of thermodynamics is when a cup of hot coffee is left out on a cold winter day. The heat from the coffee will eventually be transferred to the surrounding air, and the coffee will become cold. It is an example of entropy or the transfer of energy from a system (the hot coffee) to its surroundings (the cold air).


Another example of thermodynamics is when a fire is lit in a fireplace. The heat from the fire will travel up the chimney and into the atmosphere. This process is called convection and is essential to how heat travels through our environment.


The third example has to do with chemical reactions. In order for a reaction to occur, there must be a transfer of energy between the reactants and products. This transfer of energy can be either endothermic or exothermic. Endothermic reactions absorb heat from their surroundings, while exothermic reactions release heat into their surroundings.


History of Chemical Thermodynamics

In 1865, the German physicist Rudolf Clausius suggested that the principles of thermochemistry, for example, the heat generated in combustion reactions, could be applied to the principles of thermodynamics. He made his proposal in his Mechanical Theory of Heat.

Based on the work of Clausius, between 1873-76, the American physicist Willard Gibbs published a series of three articles. The most famous one was the document on the balance of heterogeneous substances.

In these articles, Gibbs showed how the first two laws of thermodynamics could be measured to know the equilibrium of chemical reactions and their tendencies to occur or move forward. Gibbs' collection of documents provided the first unified body of thermodynamic theorems from the principles developed by others, such as Clausius and Sadi Carnot.

At the beginning of the 20th century, two essential publications successfully applied the principles developed by Gibbs to chemical processes and thus established the foundations of the science of chemical thermodynamics. The first was the 1923 textbook Thermodynamics and Chemical Free Energy by Gilbert N. Lewis and Merle Randall. This book was responsible for supplanting chemical affinity with the term free energy in the English-speaking world.

The second was the 1933 book Modern Thermodynamics by the methods of Willard Gibbs, written by EA Guggenheim. Thus, Lewis, Randall, and Guggenheim are considered the founders of modern chemical thermodynamics due to the outstanding contribution of these two books to Unify the application of thermodynamics to chemistry.


Published: September 18, 2019
Last review: October 15, 2022