Question

Although methane, \(\mathrm{CH}_{4}\), and ammonia, \(\mathrm{NH}_{3}\), differ in molar mass by only one unit, the boiling point of ammonia is over \(100^{\circ} \mathrm{C}\) higher than that of methane (a nonpolar molecule). Explain.

Short answer

Ammonia has a higher boiling point than methane due to the presence of stronger intermolecular forces, specifically hydrogen bonding, in ammonia. Methane consists of weaker London dispersion forces, requiring less energy to overcome and leading to a lower boiling point.

Step-by-step solution

Identify the types of intermolecular forces present

In order to understand the difference in boiling points, we first need to identify the types of intermolecular forces present in both methane and ammonia. Methane is a nonpolar molecule with nonpolar covalent bonds between carbon and hydrogen atoms, which means that the only intermolecular forces present are London dispersion forces. On the other hand, ammonia is a polar molecule due to the presence of hydrogen bonding. The nitrogen atom in NH3 has a partial negative charge, while the hydrogen atoms have partial positive charges, which result in dipole-dipole interactions. Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs between hydrogen atoms bonded to highly electronegative atoms (in this case, nitrogen).

Compare the strength of intermolecular forces

Hydrogen bonding is a stronger intermolecular force compared to London dispersion forces. As a result, ammonia molecules are more strongly attracted to each other than methane molecules are to each other.

Relate intermolecular forces to boiling points

The factor that influences the boiling point of a substance is the strength of its intermolecular forces. The stronger the forces between molecules, the more energy is required to separate them and cause the substance to transition from the liquid phase to the gas phase (i.e. boiling).

Explain the difference in boiling points

Ammonia has a boiling point over 100°C higher than methane because its molecules experience stronger hydrogen bonding, which requires more energy to overcome than the London dispersion forces present in methane. The stronger intermolecular forces in ammonia lead to a higher boiling point when compared to methane.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces that exist between molecules and influence many physical properties like boiling points and melting points. These forces can be seen as the glue that holds molecules together.

They play a critical role in determining how substances behave in different states of matter. There are several types of intermolecular forces:

• London Dispersion Forces
• Dipole-Dipole Interactions
• Hydrogen Bonding

Each type of force varies in strength, with hydrogen bonding being one of the strongest types, while London dispersion forces are generally weaker. Understanding these forces helps explain why different substances boil at different temperatures.

Hydrogen Bonding

Hydrogen bonding is a specific type of dipole-dipole interaction that occurs in molecules where hydrogen is bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine.

These bonds form due to the significant difference in electronegativity between hydrogen and the other atom, creating a strong polar bond. This strong interaction plays a major role in the high boiling points of substances that exhibit hydrogen bonding.

For instance, water ( H_2O ) and ammonia ( NH_3 ) both have notably high boiling points due to hydrogen bonding.

• Hydrogen bonds are stronger than regular dipole-dipole interactions.
• They are significantly stronger than London dispersion forces.

As a result, substances with hydrogen bonds require more energy to overcome these forces during boiling.

London Dispersion Forces

London dispersion forces are the weakest type of intermolecular force and are present in all molecules, whether polar or nonpolar. These forces are caused by temporary fluctuations in the electron distribution within molecules, leading to temporary dipoles that attract neighboring molecules.

Although these forces are weak, they can add up to have significant effects in larger, heavier molecules.

• They're the only type of intermolecular force present in nonpolar molecules like CH_4 (methane).
• They influence the boiling and melting points of substances.

Despite their relative weakness, London dispersion forces play an essential role in the physical properties of nonpolar substances.

Polar Molecules

Polar molecules have a permanent dipole moment due to the unequal sharing of electrons between atoms with different electronegativities. This creates a molecule with partially positive and negative charges on opposite ends.

Ammonia ( NH_3 ) is a classic example of a polar molecule due to the nitrogen atom's significant electronegativity compared to hydrogen. This polarity gives rise to strong intermolecular interactions like dipole-dipole interactions and hydrogen bonding.

• Polar molecules tend to have higher boiling points than nonpolar molecules.
• This is because they can engage in stronger intermolecular forces.

These characteristics result in substantial effects on the physical properties of polar substances, making them markedly different from their nonpolar counterparts.

Nonpolar Molecules

Nonpolar molecules have an equal distribution of electrons, resulting in no permanent dipole moment. This usually occurs in molecules where atoms have similar or identical electronegativities.

Methane ( CH_4 ) is an example of a nonpolar molecule with its tetrahedral geometry causing even distribution of electron density. Consequently, nonpolar molecules primarily experience London dispersion forces.

• These forces are the weakest among intermolecular forces.
• Nonpolar molecules generally have lower boiling points.

Their boiling and melting points are typically lower compared to polar molecules due to the weaker intermolecular forces acting between them.