a) Rank the following ionic compounds by the magnitude of their

lattice energy. Rank from highest to lowest magnitude of lattice

energy. Highest magnitude to Lowest Magnitude LiCl, MgO, Na2O, BeO,

Na2s b) Given the following thermodynamic data, calculate the

lattice energy of CaBr2(s). Term Value (kJ/mol) ΔH∘f[CaBr2(s)] -675

ΔH∘f[Ca(g)] 179 ΔH∘f[Br(g)] 112 I1(Ca) 590. I2(Ca) 1145 E(Br) -325

Express your answer as an integer, and include the appropriate

units.

## Answer

## General guidance

Lattice energy is the energy released when a crystal lattice is formed. It can also be defined as the energy required to break the crystal lattice into gaseous ions. Usually, it is the energy released so it is taken as negative. Lattice energy depends on the distance between the ions and charge on the ions. As the ionic charge increases, the lattice energy also increases and becomes more negative. And as the ions get closer, then also there is an increase in lattice energy.

## Step-by-step

### Step 1 of 2

**(A)**

Lattice energy depends on the distance between ions and charge on ions.

Depending on this condition the order is as follows:

The decreasing order of the ionic compounds by the magnitude of their lattice energies is as follows:

Since Beryllium has the smallest ionic radius amongst the given ionic compounds, its lattice energy is the highest. Lithium ion and chloride ion carry the lowest charge; therefore, the lattice energy of lithium chloride is the lowest.

### Step 2 of 2

**(2)**

Calculate the lattice energy as follows:

Here, and represent the ionization energy, EA represents the electron affinity, LE represents lattice energy.

Substituting the values in the above equation:

Therefore, the lattice energy of is .

The lattice energy of is .

Enthalpy of formation is the sum of all the enthalpies that includes enthalpy of atomization, electron affinity and lattice energy.

### Answer

The decreasing order of the ionic compounds by the magnitude of their lattice energies is as follows:

The lattice energy of is .

### Answer only

The decreasing order of the ionic compounds by the magnitude of their lattice energies is as follows:

The lattice energy of is .