Chemical Thermodynamics

Chemical Thermodynamics

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Chemistry often conjures up images of scientists in labs creating explosions. But did you know that physical processes, like melting ice, also involve energy? That's where thermodynamics comes in – it's all about the energy changes in physical and chemical processes. Chemical thermodynamics is a specific branch that focuses on the energy changes in chemical systems.

In this article, we'll break down chemical thermodynamics and explore two important laws. We'll also dive into the real-world applications of chemical thermodynamics – it's not just theoretical! Finally, we'll take a look at the significance of chemical thermodynamics, as well as its limitations. So, buckle up and get ready to learn about the fascinating world of chemical thermodynamics!

Chemical thermodynamics definition

Chemical thermodynamics is the study of the relationship between thermal energy, which is heat, and chemical and physical processes. It looks at how thermal energy is transformed into other forms of energy and how this transformation affects the properties of a system. Chemical thermodynamics is concerned with understanding how energy changes occur during chemical reactions and changes of state, as well as the effects of these changes on the system being studied


To understand thermodynamics, we first need to understand energy. Energy is the capacity to do work or transfer heat. In chemistry, work occurs when a force acts on something to make it move, while heat is the transfer of energy through thermal interactions like radiation or conduction.

Everything in the universe is made of energy, and this means that everything has the potential to do work or transfer heat. Energy is stored in two basic types: kinetic energy and potential energy. Kinetic energy is the energy an object has due to its motion, while potential energy is the energy an object has based on its relative position, either to different parts of itself or to other objects.

All forms of energy can be classified under these two fundamental types. For example, thermal energy is a form of kinetic energy, but it can also be converted into other forms, such as chemical energy or electrical energy. By understanding the different types and conversions of energy, we can better understand the principles of thermodynamics.


When studying thermodynamics, it's important to consider the concept of a system. A system refers to a substance or collection of substances and energy that are being studied. In thermodynamics, systems can be categorized as open, closed, or isolated.

An open system refers to a system that can exchange both energy and matter with its surroundings. A closed system, on the other hand, can exchange energy with its surroundings but not matter. Finally, an isolated system is a system that cannot exchange either energy or matter with its surroundings.

In any thermodynamic process, the system under study is separated from the surroundings in order to simplify our calculations. This allows us to focus on the changes taking place within the system and to determine how these changes affect the system's energy and properties. The surroundings are simply everything else that is not in the system, and often play a crucial role in affecting the system's behavior. For example, in a chemical reaction taking place in a jar, the jar is the system, while everything outside the jar is the surroundings.

A system and its surroundings
A system and its surroundings

So, in summary, thermodynamics is the study of how thermal energy is converted into other types of energy inside specific environments called systems. In chemical thermodynamics, we focus specifically on how thermal energy changes into chemical energy, and how this affects chemical reactions or changes in state.

Laws of chemical thermodynamics

Chemical thermodynamics is based on the four laws of thermodynamics. These four basic principles, discovered by scientists like Isaac Newton and James Joule,  help us understand how energy moves and they govern the study of thermodynamics. In this article, we'll consider the first and second laws.

First law of thermodynamics

In thermodynamics, the first law states that energy cannot be created or destroyed, only converted from one form to another. This law is based on the conservation of energy and tells us that the total amount of energy in the universe is constant. Enthalpy is a measure of the heat energy in a chemical system and is typically measured in kJ mol-1.

The second law of thermodynamics explains why not all heat energy is converted into useful energy. It tells us that the energy in natural systems tends to move in the direction of increasing entropy, or more disorder. Entropy is a measure of the disorder of a system and is typically measured in J K-1 mol-1.

We can use the principles of enthalpy and entropy to predict whether chemical reactions are spontaneous or not. Spontaneous reactions occur without outside intervention and are also called feasible reactions. Gibbs Free Energy (ΔG) is used to determine whether a reaction is spontaneous or not. Highly exothermic reactions or reactions with a large increase in entropy tend to be spontaneous.

Born-Haber cycles are useful for calculating the enthalpy of formation of ionic compounds. Hess' Law tells us that the enthalpy change of a reaction is always the same, regardless of the route taken. This means that as long as we start with the same reactants and end with the same products, the enthalpy change is the same, regardless of the number of steps taken.


Hess' Cycle
Hess' Cycle

Hess' law is expressed by the equation ΔHr = ΔH1 + ΔH2, where ΔHr is the enthalpy change of the direct route reaction and ΔH1 and ΔH2 are enthalpy changes involved in the indirect route. Enthalpy change is typically measured in kJ mol-1, but it is okay to use J mol-1 as long as all units are consistent.

Lattice enthalpy, also known as enthalpy of lattice formation, is the enthalpy change when one mole of an ionic lattice is formed from its gaseous ions under standard conditions. A Born-Haber cycle is a theoretical model based on Hess' law that we use to calculate lattice enthalpy. By creating an indirect route that includes enthalpy changes we know, we can use the equation for Hess' Law to find the lattice enthalpy.

In a Born-Haber cycle, we take a reaction we want to find the enthalpy change of and create an indirect route that starts with the same reactants and ends with the same products. Each point in the direct and indirect route is represented by a line showing its enthalpy, and the height difference between lines represents the enthalpy change between these points. By writing in the known enthalpy changes of the indirect route, we can calculate the unknown enthalpy change of the direct route.

Born-Haber cycles are particularly useful for calculating the enthalpy of formation of ionic compounds, as they allow us to determine the enthalpy change involved in the formation of an ionic lattice from its constituent gaseous ions.

Born-Haber cycle for the lattice enthalpy of lithium fluoride

Chemical thermodynamics is an essential field of science as it helps us understand how and why many everyday reactions take place. It allows us to determine unknown enthalpy changes, predict reaction feasibility, and optimize chemical processes to improve energy efficiency. Thermodynamics also explains why reactions reach chemical equilibrium. However, there are limitations to chemical thermodynamics. It does not provide information about the rate of a reaction or the individual particles within a system. Despite these limitations, chemical thermodynamics remains a valuable tool for chemists and scientists in understanding and optimizing chemical reactions.

In summary, chemical thermodynamics is the study of thermal energy in chemical and physical processes. Enthalpy and entropy are key measures in this field, and the laws of thermodynamics dictate the reactions in the world around us. Chemical thermodynamics has many practical applications, such as calculating lattice enthalpy using Born-Haber cycles and predicting the spontaneity of a reaction using Gibbs free energy. Despite its usefulness, chemical thermodynamics has its limitations, such as its inability to provide information about reaction rates or individual particles within a system.

Chemical Thermodynamics

What is a chemical thermodynamics system?

A system is a substance or collection of substances and energy. Depending on the type of system, they can exchange energy or matter, or perhaps both or neither, with the surrounding environment. An example of a closed chemical system is a reaction inside a sealed beaker.

What is chemical energy in thermodynamics?

Chemical energy is the energy released when chemical substances undergo a reaction. It is often transformed into thermal energy.

What is the importance of thermodynamics in chemistry?

Thermodynamics allows us to predict whether a reaction will occur and work out unknown enthalpy changes.

What are the elements of chemical thermodynamics?

Chemical thermodynamics is based on the same laws as conventional thermodynamics. For example, the first law of thermodynamics tells us that energy can't be created or destroyed, whilst the second law of thermodynamics states that spontaneous reactions tend towards a state of greater entropy.

Are physical and chemical thermodynamics the same?

Physical and chemical thermodynamics are both based on the same fundamental laws. However, chemical thermodynamics pays particular attention to the thermodynamics of chemical systems, and how thermal energy is converted into chemical energy and vice versa.

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