Question

What is the strongest interparticle force in each substance? (a) \(\mathrm{CH}_{3} \mathrm{Cl}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) (c) \(\mathrm{NH}_{3}\)

Short answer

(a) Dipole-dipole, (b) London dispersion, (c) Hydrogen bonding.

Step-by-step solution

- Identify Types of Interparticle Forces

Understand the possible types of interparticle forces: London dispersion forces, dipole-dipole interactions, and hydrogen bonding. London dispersion forces are present in all molecules, dipole-dipole interactions occur in polar molecules, and hydrogen bonding occurs in molecules containing H bonded to N, O, or F.

- Analyze Structure of \(\text{CH}_{3}\text{Cl}\)

Determine the intermolecular forces in \(\text{CH}_{3}\text{Cl}\). It is a polar molecule due to the electronegativity difference between Cl and C, leading to dipole-dipole interactions.

- Analyze Structure of \(\text{CH}_{3}\text{CH}_{3}\)

Determine the intermolecular forces in \(\text{CH}_{3}\text{CH}_{3}\). It is a nonpolar molecule and thus only has London dispersion forces.

- Analyze Structure of \(\text{NH}_{3}\)

Determine the intermolecular forces in \(\text{NH}_{3}\). It has a polar bond and contains hydrogen bonded to nitrogen, allowing for hydrogen bonding.

- Compare and Conclude

Identify the strongest interparticle force for each substance: Dipole-dipole interactions for \(\text{CH}_{3}\text{Cl}\), London dispersion forces for \(\text{CH}_{3}\text{CH}_{3}\), and hydrogen bonding for \(\text{NH}_{3}\).

Key concepts

London Dispersion Forces

London dispersion forces (LDF) are the weakest intermolecular forces and exist in all molecules, no matter if they are polar or nonpolar. They result from temporary dipoles that occur when electron distribution within a molecule fluctuates. These temporary dipoles induce dipoles in neighboring molecules, causing an attraction. Despite being weak, these forces can become significant in larger molecules with more electrons.
London's dispersion forces are particularly important in nonpolar molecules, like \(\text{CH}_{3}\text{CH}_{3}\), which lack other stronger intermolecular forces. The more electrons a molecule has, the stronger its London dispersion forces can be. This concept is essential, especially when comparing different types of molecular interactions.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, which have permanent dipoles due to the unequal sharing of electrons in a bond. For example, in \(\text{CH}_{3}\text{Cl}\), the chlorine atom is more electronegative than the carbon atom, causing a partial negative charge on the chlorine and a partial positive charge on the carbon. This creates a permanent dipole.
When these molecules are near each other, the positive end of one molecule is attracted to the negative end of another molecule. These forces are stronger than London dispersion forces but weaker than hydrogen bonds. Understanding dipole-dipole interactions helps explain the physical properties of molecular substances, like higher boiling and melting points compared to nonpolar molecules of similar size.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction but is significantly stronger. It occurs only in molecules where hydrogen is bonded to highly electronegative atoms, specifically nitrogen (N), oxygen (O), or fluorine (F). A classic example is \(\text{NH}_{3}\) (ammonia), where hydrogen is bonded to nitrogen. The large difference in electronegativity creates a strong permanent dipole.
The hydrogen bonds form between the partially positively charged hydrogen atom of one molecule and the lone pair of electrons on the electronegative atom (N, O, or F) of another molecule. These extensive hydrogen bonds result in unique properties, like higher boiling points and solubility in water. Recognizing the presence of hydrogen bonding can explain many of the unusual behaviors of substances that participate in these interactions.