Question

Question 89: By considering the nature of the intermolecular forces in each case, rank the following substances from smallest to largest enthalpy of vaporization: $\mathbf{\text{KBr}}\mathbf{,}\mathbf{\text{Ar}}\mathbf{,}{\mathbf{\text{NH}}}_{\mathbf{3}}$ , and $\mathit{H}\mathit{e}$.Explain your reasoning.

Short answer

The order of enthalpy of vaporization is, $\text{He}<\text{Ar}<{\text{NH}}_3<\text{KBr}$.

Step-by-step solution

Given data

The largest enthalpy of vaporization among${\text{KBr,Ar,NH}}_3\text{and}\mathit{\text{He}}$.

Concept of enthalpy of vaporization

Enthalpy of vaporization:

It is defined as the amount of heatrequired to transform a liquid into a gas.

It is largely dependent on the strength of the bond.

The stronger the bond, the more heat will be needed to break that bond apart

Bonds

Covalent compounds have three different types of intermolecular forces, ranked from weakest to strongest:

- London dispersion forces - the weakest, always present

- Dipole-dipole: the relationship between two polar molecules' positive and negative ends.

- Ion-dipole -between an ion and a neutral molecule that has a dipole

- Hydrogen bonds - between hydrogen and an electronegative atom ($\mathit{F}\mathit{,}\mathit{}\mathit{O}\mathbf{}\mathbf{or}\mathbf{}\mathit{N}$ ).

Explanation of enthalpy of vaporization of Ar and KBr.

KBr is an ionic compound.

This means that is stronger than all of the bonds that are possible in covalent compounds as the ionic bond are electrostatic in nature.

Argon is a noble gas. Its only bond within its molecules is the London dispersion forces. This is the case with all noble gasses

Explanation of enthalpy of vaporization of nitrogen and helium gas

For, $N{H}_3$:

Nitrogen is a highly electronegative element

This means that it will form a hydrogen bond when in touch with hydrogen atoms.

For He,

Helium is a noble gas, and just like argon, its only bonds are the London dispersion forces.

To decide which noble gas has the stronger bond simply look at the molar mass.

The higher the molar mass is the stronger the bond.

Argon has a larger molar mass than helium therefore its forces are stronger.

Arrange them in order of smallest enthalpy of vaporization (weakest bond) to highest.

data-custom-editor="chemistry" $\text{He}<\text{Ar}<{\text{NH}}_3<\text{KBr}$