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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.
Thermodynamics is the study of thermal energy or heat in chemical and physical processes. It deals with how thermal energy converts to other kinds of energy and how this affects the properties of a system.
In thermodynamics we separate the universe into two parts: the system and the surroundings. We do this to make it easier to work out calculations. A system is a substance or a collection of substances and energy. Everything else that is not in the system, we call the surroundings. For example, if a reaction takes place in a jar, the jar is the system. Everything outside the jar is the surroundings.
Before we get into 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.
Let’s break that down: 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 energy. Even when things are not moving they have the capacity to work or transfer heat. We classify energy into two basic types: kinetic energy and potential energy. Kinetic energy has to do with moving objects, while potential energy is stored energy. All other kinds of energy come under these two basic types.
The laws of thermodynamics help us understand how energy moves. Scientists like Isaac Newton and James Joule discovered four basic principles that govern the study of thermodynamics. We call them the four laws of thermodynamics. In this article we will only consider the first and second laws.
Previously, you learned about the law of conservation of energy which says:
"Energy cannot be created or destroyed, it only converts from one form to another."
In thermodynamics, we know the law of conservation of energy as the first law. However, we add an extra sentence:
"The total amount of energy in the universe is constant."
-The First Law of Thermodynamics
Remember everything in the universe is energy. That energy is never lost, it only changes from one form to another. So, the total amount of energy in the universe remains the same.
We’ll get to the second law of thermodynamics a little later. First, let’s review what you know about enthalpy.
Enthalpy (H) is the thermal energy stored in a system. We also call it heat content.
A glass of water has a specific value of mass, volume and temperature. That same glass of water also has a specific enthalpy. But enthalpy is not as easy to measure as the volume, temperature and mass. We can never know the absolute enthalpy of a system. That is like trying to measure the total volume of water in the ocean - next to impossible! But, if you pour five litres of orange juice into the ocean, you can say that the volume has increased by five litres.
In the same way, chemists are interested in the energy that goes in and out of a system. Some chemical reactions release energy in the form of heat. We call them exothermic reactions.
A heat transfer or chemical change under constant pressure is called enthalpy change or heat of reaction (𝚫H).
1. We use the Greek symbol ‘delta’ 𝚫 to represent change in energy.
2. All chemical reactions occur under a constant pressure.
Fortunately, enthalpy is a state function, or pathway independent. That means it does not matter how we arrive at a value for enthalpy change, we will always get the same value. So if we know the heat value at the beginning and end of an experiment, we can measure the enthalpy change.
We can calculate enthalpy change using an equation called Hess’ Law. So far, you have learned that enthalpy is the thermal energy in a system and that it is pathway independent. A Swiss scientist named Germain Hess summed up this discovery in a law named after him.
"Enthalpy change in a chemical reaction is independent of the route by which the chemical change occurs."
So 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:
: Enthalpy change of a reaction
: Enthalpy change in direct route
: Enthalpy change in indirect route
You have previously learned there is energy stored between the bonds of the atoms in a molecule. We call the amount of energy stored between the bonds of the atoms in a covalent compound bond enthalpy. What about ionic bonds?
You may remember that we call the structure formed by an ionic compound a lattice or crystal lattice. A lattice is a regular, geometrical, 3-dimensional arrangement of atoms or ions.
Lattice enthalpy () is the enthalpy change involved in forming one mole of an ionic lattice from gaseous ions under standard state conditions.
Lattice enthalpy is a measure of the strength of the bonds between the ions in an ionic compound. However, these bonds can only completely break when the ions are in a gaseous state, where they are so far apart we consider their forces to be negligible.
We cannot measure lattice enthalpy - we have to calculate it. We use a type of Hess’ cycles known as Born-Haber Cycles to calculate lattice enthalpy.
A Born-Haber cycle is a theoretical model we use to calculate lattice enthalpy. Here’s an overview of how the cycles work:
We draw lines representing energy levels at different points in the reaction.
The base line represents the ionic solid
The top line is the energy level of the gaseous ions
The height difference represents the lattice enthalpy or the drop in energy as we go from one to the other.
Remember, we cannot measure lattice enthalpy, but we can find the enthalpy of formation experimentally.
is usually smaller than so we draw it as a much smaller drop in energy in the Born-Haber cycle. You also write the species that we are interested in above the line.
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.
Now that you know a little about lattice enthalpy and Born-Haber cycles, let’s get back to the laws of thermodynamics. After you learned the first law, you might have wondered: if all the energy in the universe is constant, why is the universe so random? Why does ice melt and sugar dissolve? Why does popcorn pop all over the place? The second law explains it like this:
"In spontaneous changes, the universe tends toward a state of greater disorder."
-The Second Law of Thermodynamics
The second law explains why energy moves in one direction and not in the other. For example, heat will always flow from a hotter body to a colder one. Ice will always melt and sugar will dissolve. These reactions happen spontaneously. We call this randomness in the universe entropy.
Entropy (S) is a measure of the disorder of a system. The greater the disorder, the higher the entropy.
The energy in natural systems tends to move in the direction of increasing entropy. Entropy also increases as a system changes from a solid to a liquid to a gas. Think about how the particles go from highly ordered in a solid to the random movements and collisions in a gas!
Entropy increases as a system changes from a solid to a liquid to a gas.
You might have guessed that a spontaneous reaction happens all by itself, without the input of energy from outside. Reactions are more likely to occur if there is an increase in entropy. So the particles move from an ordered state to a less ordered state.
From the previous example of melting ice, you can see spontaneous changes do not have to happen immediately. They can be incredibly slow! Another example of a spontaneous reaction is when iron turns to rust.
In a spontaneous reaction, change in enthalpy (ΔH) decreases while entropy increases (ΔS).
The difference between entropy and enthalpy is something we call free energy or Gibbs Free Energy (ΔG). It shows us the route of a reaction.
We express the relationship between entropy and enthalpy in the equation below:
ΔG = ΔH - T ΔS
ΔG: Change in free energy.
ΔH: Change in enthalpy.
ΔS: Change in entropy.
Thermodynamics is the study of thermal energy or heat in chemical and physical processes. It deals with how thermal energy is converted to other kinds of energy and how this affects the properties of a system.
We use the letter Q to symbolise heat in thermodynamics. Heat (q or Q) means the transfer of energy through thermal interactions like radiation or conduction.
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?
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).
What is the equation for Gibbs free energy?
ΔG = ΔH - T ΔS
In what order do we draw Born-Haber cycles?
Draw lines for enthalpies of formation.
Draw top line for gaseous ions.
Calculate lattice enthalpy using Hess’ Law.
Draw base line for ionic solid.
Answer: correct order is D, B, A, C
What is Hess’ Law?
Enthalpy change in a chemical reaction is independent of the route by which the chemical change occurs.
Which is the correct equation for Hess’s Law?
Answer: Both are correct! (The enthalpy change for a reaction is equal to the sum of the enthalpy of formation of all the products minus the sum of the enthalpy of formation of all the reactants.)
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