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Chemical Thermodynamics

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Chemical Thermodynamics

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.

  • This article is about chemical thermodynamics.
  • We'll define chemical thermodynamics before exploring two of its key laws.
  • We'll then look at the applications of chemical thermodynamics.
  • After that, we'll consider both the importance and limitations of chemical thermodynamics.

Chemical thermodynamics definition

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.

Energy

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:

  • 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.

chemical thermodynamics, system and its surroundings, StudySmarterA 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.

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.

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.

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.

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.

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.

Chemical thermodynamics, iron rusting spontaneous reaction, StudySmarterIron rusting - an example of a spontaneous reaction. Image credits: pixabay.com

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.

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.

Born-Haber cycles

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.


Chemical Thermodynamics, Hess' cycle, StudySmarterHess' Cycle. StudySmarter Original

We express Hess’ Law by the following equation:

ΔHr = ΔH1 + ΔH2

In this equation:

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

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 (ΔLE), 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.

Born-Haber cycle

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.

Chemical Thermodynamics, Born-haber cycle lithium fluoride LiF, StudySmarterBorn-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.

Chemical Thermodynamics - Key takeaways

  • 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.
  • Enthalpy (H) is a measure of the heat energy in a chemical system.
  • Entropy (S) is a measure of the disorder of a system.
  • The laws of thermodynamics are based on enthalpy and entropy and dictate the reactions in the world around us:
    • The first law of thermodynamics states that energy can't be created or destroyed, and so that the total energy in the universe remains constant.
    • The second law of thermodynamics states that in spontaneous changes, the universe tends towards a state of greater entropy.
  • Chemical thermodynamics has many applications:
    • Born-Haber cycles are a theoretical model we use to calculate lattice enthalpy. The principle is the same as 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.
    • Gibbs free energy (ΔG) relates enthalpy and entropy and is used to predict whether a reaction is spontaneous or not.
  • Like all fields of science, chemical thermodynamics has importance as well as limitations.

Frequently Asked Questions about Chemical Thermodynamics

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.

Final Chemical Thermodynamics Quiz

Question

​​​​​​​​​​​​​​​What is lattice enthalpy?

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Answer

Lattice enthalpy is the enthalpy change involved in the formation of 1 mole of an ionic lattice from gaseous ions under standard state conditions.  

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Question

Which of the following represents lattice enthalpy?

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Answer

ΔatH° 

Show question

Question

What is enthalpy?



Show answer

Answer

Enthalpy (H) is the thermal energy stored in a system.

Show question

Question

What is entropy?

Show answer

Answer

Entropy (S) is a measure of the disorder of a system.

Show question

Question

What is the second law of thermodynamics?

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Answer

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

Show question

Question

What is a spontaneous reaction?

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Answer

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).

Show question

Question

What is the equation for Gibbs free energy?

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Answer

ΔG = ΔH - T ΔS

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Question

What do we use Born-Haber cycles for?

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Answer

Calculating lattice enthalpy

Show question

Question

What is Hess’ Law?

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Answer

Enthalpy change in a chemical reaction is independent of the route by which the chemical change occurs.

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Question

What is the difference between lattice enthalpy of dissociation and lattice enthalpy of formation?

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Answer

Enthalpy of lattice formation is the enthalpy change involved in forming one mole of an ionic lattice from gaseous ions under standard state conditions.


Enthalpy of lattice dissociation is the enthalpy change involved when one mole of an ionic lattice is broken up to form its scattered gaseous ions under standard state conditions.


Show question

Question

Order the steps to draw a Born-Haber cycle

  • First ionisation energy of the metal
  • Subsequent ionisation enthalpies if applicable
  • Enthalpy of atomisation of each element
  • Subsequent electron affinities if applicable
  • First electron affinity of the non-metal


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Answer

  1. Enthalpy of atomisation of each element
  2. First ionisation energy of the metal
  3. Subsequent ionisation enthalpies if applicable
  4. First electron affinity of the non-metal
  5. Subsequent electron affinities if applicable


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Question

What is electron affinity?

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Answer

The energy released when one mole of gaseous atoms gain an electron to make one mole of gaseous ions.

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Question

Which two factors affect lattice enthalpy?

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Answer

Charge of the ion

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Question

How does the charge on the ion affect lattice enthalpy?

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Answer

Ions with large ionic charges have stronger electrostatic attraction between the ions. This results in a larger lattice enthalpy.

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Question

How does the radius of the ion affect lattice enthalpy? Give an example.


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Answer

Ions with larger radii tend to have smaller lattice enthalpies, because the ions are farther apart in the lattice structure. Electrostatic attraction between smaller ions is stronger, since the strength of ionic attraction depends on the closeness of the centres of the attracted ions


For example the ions in MgO are smaller than the ions in NaCl, so NaCl has a smaller lattice enthalpy.


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Question

Why are physics-style calculations of lattice enthalpy theoretical?

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Answer

They are based on the assumption that a substance is highly ionic with strong electrostatic attraction between the ions.

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Question

What do large differences between the theoretical and experimental values of lattice enthalpy suggest?

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Answer

Large differences between the theoretical and experimental values of lattice enthalpy suggest there is a covalent character in the ionic bond. This is as a result of polarisation due to radius of the ions and the charge on the ions.

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Question

Why are lattice enthalpies calculated using Born-Haber cycles called experimental?

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Answer

Lattice enthalpies calculated using Born-Haber cycles are experimental because they use enthalpy changes that can be measured.

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Question

Why is the enthalpy change of hydration of chloride ions exothermic?


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Answer

Heat energy is released when the ions form bonds with the water molecules, so hydration enthalpy is always exothermic.

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Question

What is entropy?

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Answer

Entropy is the number of possible ways energy can be distributed between the particles in a system.

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Question

What is a spontaneous or feasible change?

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Answer

A spontaneous or feasible change is one that takes place on its own without outside intervention, given enough time.

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Question

What is the second law of thermodynamics?

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Answer

In spontaneous changes, the total entropy change for a system and its surroundings is always positive.

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Question

Select factors that can make a reaction feasible?

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Answer

Negative enthalpy change

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Question

Arrange the following in order of increasing entropy: Gas, solid, liquid.

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Answer

Solid, liquid, gas.

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Question

What is Gibbs free energy?

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Answer

Gibbs free energy is the energy available to do work. It shows the relationship between enthalpy and entropy to determine whether a reaction is feasible, ∆G = ∆H - T∆S.


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Question

What is the equation for free energy?



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Answer

∆G = ∆H - T∆S



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Question

Why does ∆G decrease as T increases?



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Answer

Because the change in entropy (∆S) increases with the temperature. The expression -T∆S will become large enough to overcome ∆H.



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Question

Suggest how values for ∆H, ∆S, and T can be used to predict the feasibility of a reaction.



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Answer

Use the equation for free energy ∆G = ∆H - T∆S to work out the feasibility. If ∆G is less than or equal to 0 then the reaction is feasible.




Show question

Question

What is the relation between free energy and the changes in entropy and enthalpy of a reaction?

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Answer

A bigger increase in entropy (the system becomes more disordered) and a bigger decrease in enthalpy (the system releases more heat) contribute towards a lower (more negative) value for free energy. A lower (more negative) free energy means that the reaction is more feasible.

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Question

What are the units of enthalpy?

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Answer

kJ mol-1

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Question

What are the units of entropy?

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Answer

J K-1 mol-1

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Question

What is the first law of thermodynamics?

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Answer

Energy can't be created and destroyed; the total amount of energy in the universe is constant.

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Question

True or false? Chemical thermodynamics obeys different laws to physical thermodynamics.

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Answer

False

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Question

What are the units of Gibbs free energy?

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Answer

kJ mol-1

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Question

Which has more entropy?

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Answer

Solid

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Question

Which has more entropy?

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Answer

Liquid

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Question

Which has more entropy?

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Answer

Solid

Show question

Question

What is the correct equation for Gibbs Free Energy?

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Answer

ΔG = ΔH - TΔS

Show question

Question

What is Gibbs Free Energy?

Show answer

Answer

The energy that becomes 'free' in a reaction and factors in enthalpy changes, entropy changes and temperature.

Show question

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