Lattice Enthalpy of an Ionic Solid

A chemical reaction is a process that involves a rearrangement of the molecular or ionic structure of a substance. In this article, we shall discuss ionic solids and what is lattice enthalpy. The crystalline structure of ionic compounds is strong and rigid. It takes enough energy to break their bond. Ionic compounds are solids that have high boiling and melting points.  The strong bonds between oppositely charged ions lock them into a network or lattice within the crystalline structure. Lattice enthalpy is the change when this structure is formed or is broken. 

What is Lattice Enthalpy?

In simple words, Lattice enthalpy is the change in Enthalpy connected with the formation of one mole or an ionic compound from its gaseous ions, other things remaining standard. It is the energy required to entirely separate one mole of ionic compound into gaseous ions. We can even say that Lattice enthalpy is a measure of the strength of an ionic compound. Lattice is a strong network, mesh, or web and enthalpy is the heat content, total heat or a thermodynamic quantity equal to the internal energy of a system and the product of its volume and pressure. 

We can describe lattice enthalpy in the following two ways:

1 – We can say that the enthalpy is the change when 1 mole of sodium chloride (or any other substance) was formed from its scattered gaseous ions. In the case of sodium chloride case, that would be -787kJ mol⁻¹.   

2 – The second definition describes it as the enthalpy when 1 mole of sodium chloride (or any other substance) is broken up to form its scattered gaseous ions. In the case of sodium chloride case, it would be +787kJ mol⁻¹.  

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So, the breaking up and the formation both get a reference as lattice enthalpy. Fig.1.

What are the Factors of Lattice Enthalpy?  

As you know by now that lattice enthalpy is the change in enthalpy associated with the formation of one mole of an ionic compound from its gaseous ions under normal conditions. One of the key lattice enthalpy questions is the two principal factors that affect it. They are -

Charges on the ions - Sodium chloride (NaCl) and magnesium oxide (MgO) have the same arrangement of ions in the lattice. However, their lattice enthalpies differ. The lattice enthalpy of MgO is greater than that of NaCl. The reason is in MgO, two +ions attract two –ions, whereas in NaCl, the attraction is between one +ion and one –ion.   

The radius of the ions- the lattice enthalpy of MgO is increased in relation to NaCl as the magnesium ions are smaller than sodium ions. It means that the ions are close together in the lattice of MgO, and this increases the strength of the attractions.  Note that oxide ions are always smaller than chloride ions. Ref Fig.2 

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Can Lattice Enthalpy be Positive or Negative? 

As mentioned earlier, lattice enthalpy is about forming up as well as breaking down.  If we talk about the requirement of energy to split up the lattice into scattered gaseous ions- it is breaking down. In NaCl, the break-down or lattice dissociation enthalpy is +787 kJ mol⁻¹. 

When talking about lattice formation, the energy released when a lattice is created from its scattered gaseous state- it is forming up. In the case of NaCl, the build-up or lattice formation enthalpy is -787 kJ mol⁻¹. 

Hence, the lattice dissociation enthalpies are always positive and the lattice formation enthalpies are always negative.  

How to Calculate Lattice Enthalpy? 

It is essential to note that you cannot measure the enthalpy change starting from a solid crystal that converts it into its gaseous ions.  It is tougher to calculate the reverse scenario where you start with the gaseous ions and measure the change when the ions convert to solid-state. However, you can calculate lattice enthalpies in two different ways: 

First, you can use a Hess law cycle or Born-Haber cycle that involves enthalpy changes that can be measured. Lattice enthalpies obtained through this method are known as experimental values. 

Secondly, you can do it the physics way by working out how much energy would be released to make a lattice when ions (as point charges) come together to form a lattice.  The calculation is about lattice energies and values obtained are known as theoretical values.

FAQ (Frequently Asked Questions)

Q1. What is Lattice Energy and How is it Determined by the Born - Haber cycle?

Lattice energy is defined in two ways-

The energy that is needed for breaking an ionic solid into its component atoms and the conversion of component atoms into gaseous ions is known as lattice energy. Here, the reaction is endothermic and the lattice energy is positive. Secondly, the energy released when the gaseous ions combine to form ionic solid is known as lattice energy. Here, the reaction is exothermic and lattice energy is negative. 

Born-Haber cycle for the formation of ionic solid has 5 steps - the sublimation of a solid metal to gaseous state, ionizations to cation, bond dissociation of 2nd gaseous molecule, the formation of anion by gaining of 1 electron, and combining of both gaseous ions to form ionic solid.

Q2. What are the Practical Applications of Lattice Enthalpies?

The applications of lattice energy are in use to evaluate the electron relations and fluoride relationships, estimation of standard enthalpies formation and lattice energy is in use to determine the strength of ionic solids.  A detailed explanation is – 

Lattice energy factor helps to find about the released energy from a compound when a number of ions come together to make that particular compound. 

It is primarily is used to find about the strength of ionic compounds or solids.  

Typically, the ionic solids or compounds have high lattice energy that allows a solid to attain more stability which makes them tough to break the bond between them.