Question

What is the strongest type of intermolecular force between solute and solvent in each solution? (a) \(\mathrm{C}_{6} \mathrm{H}_{14}(l)\) in \(\mathrm{C}_{8} \mathrm{H}_{18}(l)\) (b) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{O}(g)\) in \(\mathrm{CH}_{3} \mathrm{OH}(l)\) (c) \(\operatorname{Br}_{2}(l)\) in \(\mathrm{CCl}_{4}(l)\)

Short answer

(a) London dispersion forces(b) Hydrogen bonding(c) London dispersion forces

Step-by-step solution

Identify the molecules

First, identify the molecules involved in each solution. (a) \(\mathrm{C}_{6}\mathrm{H}_{14}(l)\) and \(\mathrm{C}_{8}\mathrm{H}_{18}(l)\) (b) \(\mathrm{H}_{2}\mathrm{C}=\mathrm{O}(g)\) and \(\mathrm{CH}_{3}\mathrm{OH}(l)\) (c) \(\operatorname{Br}_{2}(l)\) and \(\mathrm{CCl}_{4}(l)\)

Identify the type of molecules

Determine whether the molecules are polar or nonpolar. (a) Both \(\mathrm{C}_{6}\mathrm{H}_{14}\) and \(\mathrm{C}_{8}\mathrm{H}_{18}\) are nonpolar.(b) \(\mathrm{H}_{2}\mathrm{C}=\mathrm{O}\) is polar and \(\mathrm{CH}_{3}\mathrm{OH}\) is polar.(c) Both \(\operatorname{Br}_{2}\) and \(\mathrm{CCl}_{4}\) are nonpolar.

Determine intermolecular forces

Identify the strongest type of intermolecular force for each solution. (a) In the solution of \(\mathrm{C}_{6}\mathrm{H}_{14}\) and \(\mathrm{C}_{8}\mathrm{H}_{18}\), the strongest force between nonpolar molecules is London dispersion forces. (b) In the solution of \(\mathrm{H}_{2}\mathrm{C}=\mathrm{O}\) and \(\mathrm{CH}_{3}\mathrm{OH}\), hydrogen bonding occurs because \(\mathrm{CH}_{3}\mathrm{OH}\) can form hydrogen bonds with the oxygen of \(\mathrm{H}_{2}\mathrm{C}=\mathrm{O}\). (c) In the solution of \(\operatorname{Br}_{2}\) and \(\mathrm{CCl}_{4}\), the strongest force is London dispersion forces since both are nonpolar.

Key concepts

London Dispersion Forces

London dispersion forces are a type of intermolecular force that arise from the momentary fluctuations in electron density in a molecule. These fluctuations create temporary dipoles that induce corresponding dipoles in neighboring molecules, leading to attractions.
These forces are present in all molecules, whether they are polar or nonpolar, but they are the only type of intermolecular force in nonpolar molecules.
The strength of London dispersion forces increases with the size of the molecule and the number of electrons, as larger electron clouds are more easily polarizable.
This is why in solutions like \(\text{{\tiny C}_{6} \text{{\tiny H}_{14}}\) with \(\text{{\tiny C}_{8} \text{{\tiny H}_{18}}\) and \(\text{{\tiny Br}_{2}\) with \(\text{{\tiny CCl}_{4}}\), where both components are nonpolar, the London dispersion forces are the dominant interaction.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs when hydrogen is bound to electronegative atoms like nitrogen, oxygen, or fluorine.
The partial positive charge of hydrogen is attracted to the partial negative charge on a neighboring electronegative atom, creating a strong intermolecular force.
In the example of \(\text{{\tiny H_{2}C}=\text{{\tiny O}}\) in \(\text{{\tiny CH}_{3}\text{{\tiny OH}}\), the oxygen in \(\text{{\tiny H_{2}C}=\text{{\tiny O}}\) can form hydrogen bonds with the hydrogen atoms in \(\text{{\tiny CH}_{3}\text{{\tiny OH}}\).
This results in a very effective intermolecular force which is crucial for the solubility and interaction between these molecules.

Nonpolar Molecules

Nonpolar molecules are characterized by an even distribution of electrons across the molecule, resulting in no permanent dipole moment.
They typically consist of atoms with similar electronegativities, meaning the electrons are shared equally.
Examples of nonpolar molecules include \(\text{{\tiny C}_{6} \text{{\tiny H}_{14}}\), \(\text{{\tiny C}_{8} \text{{\tiny H}_{18}}\), \(\text{{\tiny Br}_{2}}\), and \(\text{{\tiny CCl}_{4}}\).
In these molecules, the strongest intermolecular force is the London dispersion force, which is responsible for holding the molecules together in the liquid state despite the lack of any permanent charges or dipoles.

Polar Molecules

Polar molecules have regions of partial positive and negative charge due to the uneven distribution of electrons in the molecule.
This occurs because the atoms involved have different electronegativities, leading to unequal sharing of electrons and creating dipoles.
An example provided in the exercise is \(\text{{\tiny H_{2}C}=\text{{\tiny O}}\), which has a polar carbon-oxygen double bond.
Polar molecules can participate in stronger intermolecular forces such as dipole-dipole interactions and hydrogen bonding.
These forces play a significant role in determining the physical properties of the substance, such as boiling point, solubility, and melting point.