Chemical Thermodynamics

Chemical thermodynamics definition

Let's break that definition down a bit.

Energy

To understand thermodynamics, we need to talk about energy. What is energy? Scientists struggle to define it. Here is a simple definition:

In chemistry, Work (w or W) is when a force acts on something to make it move. So if there is no motion, no work is done. Heat (q or Q) means the transfer of energy through thermal interactions like Radiation or conduction.

Everything in the universe is made of energy. This means that everything has the potential to do work or transfer heat. Energy is stored as two basic types, which can be converted from one to the other:

• Kinetic Energy is the energy an object has due to its motion.
• Potential Energy is the energy an object has thanks to its relative position, either to different parts of itself or to other objects.

All forms of energy come under these two fundamental types. For example, thermal energy is a form of kinetic energy. However, like all energy, it can be converted into other forms, such as chemical energy or electrical energy.

Systems

We also need to consider systems. In thermodynamics, we separate the universe into two parts, in order to simplify our calculations:

• A system is a substance or a collection of substances and energy. Systems can be open, closed, or isolated, and this determines whether they can exchange energy or matter with each other.
• The surroundings are simply everything else that is not in the system.

For example, if a reaction takes place in a jar, the jar is the system. Everything outside the jar is the surroundings.

A system and its surroundings. StudySmarter Originals

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

Previously, you learned about the law of conservation of energy. This law states:

"Energy cannot be created or destroyed, it only converts from one form to another."

The first law of thermodynamics is based on the conservation of energy. However, we add an extra sentence:

"The total amount of energy in the universe is constant."

We call the total amount of energy in a particular system its enthalpy.

Energy can change within a system from one form to another. It can also be transferred from the system to its surroundings. However, the total amount of energy in the entire universe always remains the same.

Second law of thermodynamics

The first law of thermodynamics tells us that energy can't be created or destroyed, and explains many everyday processes, such as how an electrical current powers a light bulb or glucose gives us energy to move. But although energy can be used over and over again, it isn't always used efficiently. In fact, lots of it is wasted. This helps explain some of the more random phenomena we see in the universe around us and forms the basis of the second law of thermodynamics:

"Not all heat energy is converted into useful energy."

But where does this energy go? It contributes to something called entropy.

This leads us to the next part of the second law of thermodynamics:

"In spontaneous changes, the universe tends toward a state of greater entropy."

-The Second Law of Thermodynamics

The second law tells us that the energy in natural systems tends to move in the direction of increasing entropy - or in other words, more disorder. It explains why energy moves in one direction and not in the other. Examples of increasing entropy include solids dissolving or gas mixing.

Applications of chemical thermodynamics

We've learned about two of the laws of thermodynamics. Let's now consider how they apply to real-world chemical processes.

Spontaneous reactions

We can combine principles explored in the first two laws of thermodynamics, enthalpy and entropy, to predict whether reactions are spontaneous or not.

Examples of spontaneous reactions include salts dissolving, iron rusting, or ice melting.

Iron rusting - an example of a spontaneous reaction. Image credits: Wikimedia commons, Public domain

We use a quantity called Gibbs Free Energy (ΔG) to determine whether a reaction is spontaneous or not. It relates enthalpy and entropy with the following equation:

ΔG = ΔH - TΔS

Note the following:

• ΔG is the change in Gibbs free energy, measured in kJ mol^-1.
• ΔH is the change in enthalpy, measured in kJ mol^-1.
• T is the temperature, measured in K.
• ΔS is the change in entropy, measured in kJ K^-1 mol^-1.

If ΔG is negative, then the reaction is spontaneous. From this equation, we can infer that highly exothermic reactions, or reactions with a large increase in entropy, tend to be spontaneous.

Born-Haber cycles

You might have already found out about Hess' Law earlier on in your chemistry journey.

As long as you start with the same reactants and end with the same products, the enthalpy change is the same. It doesn’t matter whether you do it in one step, two steps, or fifteen steps.

Hess' Cycle. StudySmarter Original

We express Hess’ Law by the following equation:

ΔH_r = ΔH₁ + ΔH₂

In this equation:

• ΔH_r is the enthalpy change of the direct route reaction.
• ΔH₁ and ΔH₂ are enthalpy changes involved in the indirect route.

A great application of Hess' law is calculating lattice enthalpy.

The principle here is the same as the one we use in Hess’ Law cycles: If we create an indirect route to the gaseous ions, we can use the equation for Hess’ Law to find the lattice enthalpy. For example, we might not know the direct route for lattice enthalpy. However, we can work it out using an indirect route that includes enthalpy changes that we do know. Here’s an overview of how the cycles work:

• We take a reaction that 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. In this case, we want to find the enthalpy of lattice formation.

• We represent each point in the direct and indirect route as a line showing its enthalpy.

• The height difference between lines represents the enthalpy change between these points.

• We write in the known enthalpy changes of the indirect route, and use them to calculate the unknown enthalpy change of the direct route.

Here's an example. Don't worry if it seems a little confusing - we'll go through each term in more detail in Born Haber Cycles. You'll also be able to practice using Born-Haber cycles in Born Haber Cycles Calculations.

Born-Haber cycle for the lattice enthalpy of lithium fluoride, LiF. StudySmarter Originals

Importance of chemical thermodynamics

We'll now look at the importance of chemical thermodynamics. Here are some of its advantages:

• It is an essential field of science because it explains how and why many everyday reactions take place.
• It allows us to work out unknown enthalpy changes and predict whether a reaction will occur or not.
• Thermodynamics also allows us to optimise chemical processes and improve the efficiency of energy transfer. For example, it can help us save on our energy bills and reduce the costs of industrial reactions.
• Thermodynamics explains why reactions reach Chemical Equilibrium.

Limitations of chemical thermodynamics

Finally, we'll consider some of the limitations of chemical thermodynamics:

• It doesn't tell us anything about the rate of a reaction or how long the reaction takes to go to completion.
• It deals with systems as a whole, and doesn't give us any information about the individual particles within them.

That's the end of this article. You should now understand what we mean by the term chemical thermodynamics and how enthalpy and entropy relate to its first two laws. You should also know some of the applications of thermodynamics, such as calculating enthalpy changes and predicting the feasibility of reactions, and why thermodynamics is a useful field of science. Finally, you should be able to consider why thermodynamics has its limitations.