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Ionic compounds are generally in the form of a solid and their molecular force is very strong. The molecules in these ionic solids are arranged in a three-dimensional grid like structure also called the lattice structure. The energy needed in order to separate one mole of a solid ionic compound into constituent gaseous ions is called lattice enthalpy. It is the measurement of the strength of an ionic compound. For example, the lattice enthalpy of sodium chloride ( NaCl) is 788kj mol -1 which means it requires 788 energy to separate 1 mole if Na+ (gaseous) and cl-(gaseous) to infinite distance
Explanation of Lattice Enthalpy
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The lattice enthalpy refers to the calculation of the strength of any ionic compound. Whenever there is a deduction of an electron it needs energy and on adding electrons it exerts energy. Therefore the strength that the ionic compound contains is dependent on how easily the positive and negative ions are created from their respective neutral atoms. The process which facilitates in the formation of these positive and negative ions is called the ionization enthalpy and the electrons affinity as given below:
- Removing the electrons leads to the formation of the positive ions. However ionization enthalpy refers to the amount of energy required to separate an electron from an atom. Hence atoms containing lower ionization enthalpy will contain higher lattice energy.
- Addition of an electron leads to the formation of the negative ions. The addition of enthalpy to an electron can be explained as the amount of energy exerted when an atom gains an electron, hence greater the electron affinity, the larger will be the lattice energy.
Formula
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
Lattice enthalpy of NaCl
Factors Affecting lattice enthalpy
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There are two factors that affects the lattice enthalpy are charge of ion and size of the atom which are explained below:
Charge of Ion:
Force is present in ions of lattice crystals, due which ions are attracted to each other. The force of lattice crystal is directly proportional to the magnitude of charge which means the higher the magnitude of charge higher the would-be force and vice versa.
For example, potassium chloride and calcium have the same amount of lattice crystal but calcium has greater force because calcium ions have +2 charges whereas sodium chloride has +1 and as we know the greater the force this calcium has more electrostatic force than sodium chloride.
Size of the Atom:
A small atom has a small interatomic distance between them and they have a stronger binding force between them which requires a higher lattice enthalpy.
For example: the ions present in magnesium ions are small which means the bond between them is strong and thus requires higher lattice enthalpy
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Is Lattice Enthalpy Negative or Positive?
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Lattice enthalpy can be described in two ways, formation of solid compounds from gaseous ions and separating solid into gaseous ions. When energy is required to separate 1 mole of a solid crystal into gaseous ions known as lattice dissociation enthalpies which is always positive
For example, the lattice dissociation enthalpies for Nacl is +787 KJ mol -1
When the energy is required to form lattice from gaseous ions it is known as lattice formation enthalpy which is always negative.
For example, The lattice formation enthalpy of NaCl is -787 KJ mol -1
How to Calculate Lattice Enthalpy?
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The lattice enthalpy could be calculated in two ways. The first method is known as the Hess Law cycle or Born Haber cycle and the second method is called physics style. Both the methods are explained below:
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Born Haber Cycle:
In the Born Haber Cycle method which is also known as the Hess Law Cycle the changes in the enthalpy can be measured. Lattice enthalpies acquired through this procedure are called the experimental values.
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Physics style:
This method focuses on how much energy would be released to dissociate the atom from solid to gaseous. The calculation is regarding the lattice energies and values which when acquired through this method are called the theoretical values.
Sample Questions
Ques. What is the practical application of lattice enthalpy? (1 mark)
Ans. The lattice enthalpy helps in defining the electron relationship and fluoride relationship. It determines the strength of an ionic solid. The higher the lattice enthalpy energy, the more stable the solid molecule, which means it is very tough to break such a compound.
Ques. What is lattice enthalpy and how is it different from the enthalpy of atomization? (2 marks)
Ans. Lattice enthalpy is a term for ionic bonds and is referred to as the energy required to break 1 mole of the solid ionic compound into gaseous ions.
Enthalpy of atomisation is the term used for metallic bonds. It means the energy required to break 1 mole of the metal lattice into neutral atoms of metal.
Ques. What is the difference between lattice enthalpy dissociation and the boiling point of an ionic compound? (1 mark)
Ans. Lattice enthalpy dissociation means the process where a certain amount of energy is required to break the solid crystal into gaseous ions. Whereas the boiling point of an ionic compound is the temperature at which the ionic compound boils.
Ques. What is the difference between lattice enthalpy and lattice energy? (1 mark)
Ans. Lattice enthalpy is the energy required to form solid ions from scattered gaseous ions and also dissociate the solid compound into gaseous ions whereas lattice energy is the energy required to break 1 mole of solid ions into gaseous ions
Ques. How to determine the highest lattice energy? (1 mark)
Ans. As we have already discussed, the size of an atom is the factor of lattice enthalpy. So the smallest atom would have the highest lattice energy as the distance between them is very small and requires more energy to break.
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