When you think of chemistry, you might imagine a scientist in a laboratory creating an explosive reaction. Some chemical reactions release energy in the form of heat. Physical processes also involve energy. For example, when ice melts, it requires energy to change from a solid state to a liquid. Thermodynamics is all about the energy changes involved in physical and chemical processes. In chemical thermodynamics, we focus specifically on the thermodynamics of chemical systems.
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Jetzt kostenlos anmeldenWhen you think of chemistry, you might imagine a scientist in a laboratory creating an explosive reaction. Some chemical reactions release energy in the form of heat. Physical processes also involve energy. For example, when ice melts, it requires energy to change from a solid state to a liquid. Thermodynamics is all about the energy changes involved in physical and chemical processes. In chemical thermodynamics, we focus specifically on the thermodynamics of chemical systems.
Chemical thermodynamics is the study of thermal energy (heat) in chemical and physical processes, such as chemical reactions and changes of state. It deals with how thermal energy converts to other kinds of energy and how this affects the properties of a system.
Let's break that definition down a bit.
To understand thermodynamics, we need to talk about energy. What is energy? Scientists struggle to define it. Here is a simple definition:
Energy is the capacity to do work or to transfer heat.
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:
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.
We also need to consider systems. In thermodynamics, we separate the universe into two parts, in order to simplify our calculations:
For example, if a reaction takes place in a jar, the jar is the system. Everything outside the jar is the 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.
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.
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.
Enthalpy (H) is a measure of the heat energy in a chemical system. It is typically measured in kJ mol-1.
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.
Check out Enthalpy Changes for examples of enthalpy in chemical reactions.
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.
Entropy (S) is a measure of the disorder of a system. The greater the disorder, the higher the entropy. It is typically measured in J K-1 mol-1.
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.
We've learned about two of the laws of thermodynamics. Let's now consider how they apply to real-world chemical processes.
We can combine principles explored in the first two laws of thermodynamics, enthalpy and entropy, to predict whether reactions are spontaneous or not.
Spontaneous reactions are reactions that occur without outside intervention, such as the input of energy. They are also called feasible reactions.
Examples of spontaneous reactions include salts dissolving, iron rusting, or ice melting.
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:
Entropy is typically measured in J K-1 mol-1. Make sure you convert it into kJ K-1 mol-1 by dividing by 1000.
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.
You might have already found out about Hess' Law earlier on in your chemistry journey.
Hess' law states that the enthalpy change of a reaction is always the same, regardless of the route taken.
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.
We express Hess’ Law by the following equation:
ΔHr = ΔH1 + ΔH2
In this equation:
Enthalpy change is typically measured in kJ mol-1, but provided you keep all of your units consistent, it is ok to use J mol-1 in your calculations.
A great application of Hess' law is calculating lattice enthalpy.
Lattice enthalpy (ΔLEH°), 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 (often written without the hyphen) is a theoretical model based on Hess' law that we use to calculate 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.
We'll now look at the importance of chemical thermodynamics. Here are some of its advantages:
Finally, we'll consider some of the limitations of chemical thermodynamics:
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.
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.
Chemical energy is the energy released when chemical substances undergo a reaction. It is often transformed into thermal energy.
Thermodynamics allows us to predict whether a reaction will occur and work out unknown enthalpy changes.
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.
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.
What is lattice enthalpy?
Lattice enthalpy is the enthalpy change involved in the formation of 1 mole of an ionic lattice from gaseous ions under standard state conditions.
Which of the following represents lattice enthalpy?
ΔatH°
What is enthalpy?
Enthalpy (H) is the thermal energy stored in a system.
What is entropy?
Entropy (S) is a measure of the disorder of a system.
What is the second law of thermodynamics?
In spontaneous changes, the universe tends toward a state of greater disorder.
What is a spontaneous reaction?
A spontaneous reaction is one that happens all by itself, without input of energy from the outside. In a spontaneous reaction, change in enthalpy (ΔH) decreases and entropy increases (ΔS).
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