Arrange the following substances in order of decreasing boiling point.
(a) CH3CH2CH2CH3, H2NCH2CH2NH2,
CH3CH2CH2NH2
(b) ICl, Br2, N2
(c) LiCl, CO2, CS2
The Correct Answer and Explanation is :
Let’s break this down and analyze each set of compounds to arrange them in decreasing boiling point order.
(a) CH3CH2CH2CH3, H2NCH2CH2NH2, CH3CH2CH2NH2
We are comparing three organic compounds with different types of intermolecular forces. Boiling points are significantly affected by the type of intermolecular forces present in a substance. Stronger intermolecular forces lead to higher boiling points.
- CH3CH2CH2CH3 (Butane): This is a nonpolar molecule, and the primary intermolecular force is London dispersion forces (induced dipole-induced dipole interactions). It is the weakest of the intermolecular forces.
- CH3CH2CH2NH2 (Propylamine): This is a polar molecule due to the presence of an amine group (-NH2). The molecule can form hydrogen bonds (a relatively strong intermolecular force). This leads to a higher boiling point than that of butane.
- H2NCH2CH2NH2 (Ethylamine): This is a polar molecule as well, and like propylamine, it has hydrogen bonding. However, the presence of two -NH2 groups allows for stronger hydrogen bonding and increased surface area compared to propylamine, which generally results in a higher boiling point than that of propylamine.
Order of boiling points (decreasing):
- H2NCH2CH2NH2 (strong hydrogen bonding)
- CH3CH2CH2NH2 (hydrogen bonding)
- CH3CH2CH2CH3 (only London dispersion forces)
(b) ICl, Br2, N2
Here, we have two halogen compounds and a nonpolar molecule.
- ICl: This is a polar molecule due to the difference in electronegativity between iodine and chlorine. The major intermolecular force in ICl is dipole-dipole interactions, but it also has London dispersion forces.
- Br2: This is a nonpolar molecule, but it is relatively large and thus has strong London dispersion forces. As a result, its boiling point is higher than that of N2, but it is lower than that of ICl.
- N2: Nitrogen is a nonpolar molecule, and the only intermolecular force present is London dispersion forces, which are relatively weak for such small molecules.
Order of boiling points (decreasing):
- ICl (polar, dipole-dipole interactions + dispersion forces)
- Br2 (nonpolar, large molecule, strong dispersion forces)
- N2 (nonpolar, small molecule, weak dispersion forces)
(c) LiCl, CO2, CS2
These are a mix of ionic and molecular compounds, and the boiling points are influenced by the types of forces involved.
- LiCl: This is an ionic compound, and ionic bonds are very strong. The strong electrostatic forces between the Li+ and Cl- ions lead to a high boiling point.
- CS2: This is a nonpolar molecule, but it is relatively large. The main intermolecular forces are London dispersion forces, which are stronger than in smaller molecules but still weaker than ionic bonds.
- CO2: This is a nonpolar molecule with London dispersion forces. However, it is smaller than CS2, so its boiling point will be lower than that of CS2.
Order of boiling points (decreasing):
- LiCl (ionic bonds, strongest intermolecular forces)
- CS2 (nonpolar, large molecule with relatively strong dispersion forces)
- CO2 (nonpolar, small molecule with weak dispersion forces)
Final Answer:
- (a) H2NCH2CH2NH2 > CH3CH2CH2NH2 > CH3CH2CH2CH3
- (b) ICl > Br2 > N2
- (c) LiCl > CS2 > CO2
Explanation:
The boiling point is influenced by the strength of the intermolecular forces. Ionic compounds, like LiCl, have very strong forces (ionic bonds), leading to higher boiling points. Molecules with hydrogen bonding, like H2NCH2CH2NH2, also tend to have higher boiling points. Nonpolar molecules like Br2, CS2, and CO2 experience London dispersion forces, which are weaker than other intermolecular forces, resulting in lower boiling points.