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A Born Haber cycle (also known as a Born-Haber cycle, which is what we'll call it from now on) is a theoretical model we use to calculate lattice enthalpy. We do this by comparing enthalpy changes involved in forming an ionic lattice from its gaseous ions to the standard enthalpy of formation of the ionic compound. Keep reading to find out how this works!
We call the measure of the strength between the bonds of the ions in an ionic lattice, lattice enthalpy.
Lattice enthalpy () is the enthalpy change involved in forming one mole of an ionic lattice from gaseous ions under standard state conditions.
However, you can also say:
Lattice enthalpy () is the enthalpy change involved when one mole of an ionic lattice breaks up to form its scattered gaseous ions under standard state conditions.
The thermodynamic standard state of a substance is its most pure and stable form under standard pressure (1 atm) and at 25℃ (298 K). We represent standard states with the symbol 0 or 𝛉. Sometimes standard states are called standard conditions. Don’t confuse this with STP!
Why do we have two definitions for lattice enthalpy? Remember, enthalpy changes not only when bonds break, but also when they form. We consider the bonds in an ionic compound completely broken only when the ions are in a gaseous state. The particles are so far apart we view them as having negligible interactive forces. So in an enthalpy diagram that would look like this:
One definition considers the formation of an ionic bond from gaseous ions, and the other one looks at breaking an ionic bond to make gaseous ions. So we don’t get confused, we say this instead:
Lattice formation enthalpy is the enthalpy change involved in forming one mole of an ionic lattice from gaseous ions under standard state conditions.
Lattice dissociation enthalpy 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.
You only need to know one of these definitions of lattice enthalpy- either lattice formation enthalpy, or lattice dissociation enthalpy.
Lattice enthalpy helps scientists predict how soluble an ionic compound might be in water. We cannot directly measure change in lattice enthalpy. Since it’s impossible to measure lattice enthalpies, we say they are experimental values. This is because we calculate lattice enthalpy using enthalpy changes we can measure. Let us discuss these enthalpy changes and how we use them in a Born-Haber cycle.
Have a look at the Born-Haber cycle below. How many different enthalpy changes can you spot?
You may have spotted the following enthalpy changes:
Enthalpy of formation ()
When we draw a Born-Haber cycle, we aim to fill in as many of these values as we can. By using Hess’ Law we can use them to calculate lattice enthalpy as long as we start at the same place in the cycle:
Lattice enthalpy (direct route) = Enthalpy of formation + Enthalpy of atomisation - Ionisation energy - Bond enthalpy - Electron affinity (indirect route)
In summary, Born-Haber cycles use enthalpy changes that can be measured to calculate lattice enthalpy, a change that can’t be measured. We have covered three of these enthalpies already: ionisation energy, electron affinity and bond enthalpy. In order to calculate Born-Haber cycles, you will also need to know about enthalpy change of formation, as well as enthalpy of atomisation.
Standard molar enthalpy of formation () is the enthalpy change when one mole of a compound is formed from its elements in their standard states. We also call it standard enthalpy change of formation.
For example, the enthalpy of formation of water would be the energy change when hydrogen and oxygen bond to make one mole of .
You write an equation for the enthalpy of formation as shown below:
When you write an equation for enthalpy change of formation you must end up with one mole of the compound. If you have to write a fraction on the left side of the equation to do this, it’s okay!
The standard enthalpy of atomisation () is the enthalpy change when one mole of gaseous atoms is formed from its element in its standard state.
Before you can form gaseous ions in a Born-Haber cycle, you will need to atomise the elements that make up the compound. That means you take the elements in their standard states and turn them into monatomic gases as shown below:
½Cl2(g) → Cl(g) 𝚫Hat𝛉 = +122 kj mol-1
Enthalpy of atomisation values are always positive because you need energy to break the bonds between the atoms and turn them into gaseous atoms. In other words: 𝚫Hat𝛉 is always endothermic.
As you know, atoms become ions by losing or gaining electrons. They do this to achieve a complete valence shell. You have also learned that the energy required to remove one electron from the outer shell of an atom is called the first ionisation energy. Similarly, we call the energy released when an atom gains an electron, electron affinity.
You must include the change in energy when an atom loses an electron (ionisation energy) and the change in energy when an atom gains an electron (electron affinity) as individual steps in a Born-Haber cycle. Here’s an example:
As with ionisation energy and electron affinity, we already know the standard enthalpies of formation and enthalpies of atomisation for many compounds. They appear on a table in your exam. You put these values into the Born-Haber cycle when calculating lattice enthalpy.
Bond enthalpy is the amount of energy needed to break a specific covalent bond in one mole of a molecule into separate atoms in the gas phase.
You must know this definition for your exams, but you will not be expected to include bond enthalpy values in your Born-Haber diagrams.
As you can see, there are various enthalpy changes involved when an ionic lattice is split into its gaseous ions. When we draw Born-Haber cycles we must show these enthalpy changes in the following order:
Why do we draw the energy changes in that order? You will remember that ionisation energy turns gaseous atoms into gaseous ions. So, the first ionisation energy cannot come before atomisation enthalpy. Similarly, electron affinity must come after ionisation energy. For the non-metal to gain an electron, the metal has to lose one first.
That sounds like a lot to remember! To help you see how it all comes together, let us draw a Born-Haber cycle.
We will construct a Born-Haber cycle for the lattice formation enthalpy of potassium chloride (KCl). We start with KCl, and go round the cycle filling in all the different enthalpies until we arrive at the beginning again. We use downward arrows for exothermic enthalpies and upwards arrows for endothermic enthalpy changes.
Separate potassium chloride (KCl) into the atoms of the elements using enthalpy of formation.
Atomise potassium (K) using enthalpy of atomisation
Atomise chlorine (Cl) using enthalpy of atomisation.
Ionise potassium using first ionisation energy.
Ionise chlorine using first electron affinity.
Complete the cycle with the lattice enthalpy.
Well done! You’ve completed the Born-Haber cycle for potassium chloride. Notice how we followed the steps from enthalpy of formation to the last electron affinity. The steps to draw a Born-Haber cycle are always the same. As a recap here is the order of enthalpy changes:
A Born-Haber cycle is a theoretical model we use to calculate lattice enthalpy. We do this by comparing enthalpy changes involved in forming an ionic lattice from its gaseous ions to the standard enthalpy of formation of the ionic compound.
What is the difference between lattice enthalpy of dissociation and lattice enthalpy of formation?
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.
Order the steps to draw a Born-Haber cycle
What is electron affinity?
The energy released when one mole of gaseous atoms gain an electron to make one mole of gaseous ions.
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