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The ionic compounds are usually present in the solid form, and their molecular force seems very strong. The molecules present in the ionic solids have been arranged in the 3-D grid-like structure known as the lattice structure. The energy required to separate one mole of the solid ionic compound into the constituent gaseous ions is lattice enthalpy. This article will discuss the method of calculating lattice enthalpy and include some examples for better understanding.
EXPLANATION OF LATTICE ENTHALPY
The lattice enthalpy is defined as the measure of the strength of the ionic compound. There is a need to understand that when there is a deduction of electrons, it requires energy, and while adding electrons, it exerts power. It can be said that the strength of the ionic compound depends on how effectively positive ions and negative ions are formed from their specific neutral atoms. The method that helps create the positive ions and negative ions are termed the ionisation enthalpy, and electron affinity is discussed below.
- Electrons are removed to create the positive ions; however, the term “enthalpy” determines what amount of energy will be needed to separate an electron from an atom. It can be therefore said that atoms comprising the lower ionisation enthalpy have higher lattice energy.
- Electrons are added for the creation of the negative ions, the process of adding enthalpy to electron can be defined as the energy exerted when an atom receives an electron; therefore higher the electron affinity, the larger will be lattice energy,
Formula
The Lattice enthalpy formula is written as
ΔU = ΔH – pΔVm
Where,
ΔU = lattice energy of the mole
ΔH = lattice enthalpy of the mole
ΔVm = change of volume per mole
P = outer pressure
FACTORS AFFECTING LATTICE ENTHALPY
Two important factors influence the lattice enthalpy, i.e., 1) charge of the ion and 2) atom size
- Charge of Ion:
The force is available in the lattice ions, which leads ions to get attracted to each other. We need to know that the force available in the lattice crystal seems to be directly proportional to the charge of magnitude. Therefore, it can be said that the higher the charge magnitude, the higher will be the force.
- Size of the Atom:
A small atom has a small interatomic distance between them, and the inter-atomic distance among them seems to include stronger binding forces, which need higher lattice enthalpy.
Lattice enthalpy can be understood in two ways, creation of the solid compounds through the gaseous ions and 2) separating solid into gaseous ions. Along with such, we need to understand that the energy needed to separate 1 mole of a solid crystal into gaseous ions is called lattice dissociation enthalpies, and it always remains positive. NaCl lattice dissociation enthalpies are +787 KJ mol -1. However, on the other hand, when energy is needed to create lattice from the gaseous ions, it is termed as the lattice formation enthalpy. ( always negative).
HOW TO CALCULATE THE LATTICE ENTHALPY?
The lattice enthalpy of the ionic bond can be measured in two methods. The first way is the born Haber cycle or Hess Law cycle, and the second refers to the physics style.
- Born Haber Cycle:
Born Haber cycle, also known as the Hess Law cycle, is the method that helps calculate the changes in the enthalpy. The lattice enthalpies measured through this method are known to be experimental values.
- Physics style:
This procedure emphasises how much energy will be released to dissociate the atom from solid to gaseous forms. The measurement is concerned with the lattice energies, and values found from such a procedure are theoretical values.
For finding the lattice enthalpy by using the Born Haber Cycle method, you need to follow the below steps
- First, we have to determine the heat of formation
- Now, discover the dissociation energy and heat of atomisation.
- Determine the electron affinities and ionisation energies.
- Lastly, find the lattice energy of the ionic compound through steps 2 & 3 from step 1.
CONCLUSION
We have provided the information for calculating lattice enthalpy using the Hess Law cycle with examples. We hope you have found the article helpful; if you want to share your experience or have any doubts, do let us know by commenting below!
