Problem 14 - Chemistry

Question

The nonpolar hydrocarbon \(\mathrm{C}_{25} \mathrm{H}_{52}\) is a solid at room temperature. Its boiling point is greater than \(400^{\circ} \mathrm{C} .\) Which has the stronger intermolecular forces, \(\mathrm{C}_{25} \mathrm{H}_{52}\) or \(\mathrm{H}_{2} \mathrm{O}\) ? Explain.

Step-by-Step Solution

Answer

\(\mathrm{C}_{25}\mathrm{H}_{52}\) has stronger intermolecular forces than \(H_2O\). Although hydrogen bonding in \(H_2O\) is generally stronger than London dispersion forces, the large size of \(\mathrm{C}_{25}\mathrm{H}_{52}\) results in stronger London dispersion forces, leading to a higher boiling point compared to water.

Step 1: Identify the types of intermolecular forces present in each substance

First, we need to identify the types of intermolecular forces that exist in both substances. In \(\mathrm{C}_{25}\mathrm{H}_{52}\), a nonpolar hydrocarbon, the intermolecular force present is London dispersion forces (also known as van der Waals forces). In water, \(H_2O\), there are two types of intermolecular forces: hydrogen bonding (a strong type of dipole-dipole interaction) and London dispersion forces.

Step 2: Compare the boiling points of the substances

Given that the boiling point of \(\mathrm{C}_{25}\mathrm{H}_{52}\) is greater than \(400^{\circ} \mathrm{C}\) and the boiling point of \(H_2O\) is \(100^{\circ} \mathrm{C}\), it seems that \(\mathrm{C}_{25}\mathrm{H}_{52}\) has stronger intermolecular forces. However, we must also consider that \(\mathrm{C}_{25}\mathrm{H}_{52}\) has a much larger molecular weight compared to \(H_2O\). Thus, it also has more electrons which leads to stronger London dispersion forces.

Step 3: Analyze the strength of the intermolecular forces

We know that hydrogen bonding in \(H_2O\) is a stronger intermolecular force than London dispersion forces. However, due to the large size of \(\mathrm{C}_{25}\mathrm{H}_{52}\), the London dispersion forces experienced by the hydrocarbon are strong enough to result in a higher boiling point compared to water.

Step 4: Conclusion

Even though hydrogen bonding is generally stronger than London dispersion forces, the large size of \(\mathrm{C}_{25}\mathrm{H}_{52}\) results in stronger London dispersion forces than those experienced by \(H_2O\). Hence, \(\mathrm{C}_{25}\mathrm{H}_{52}\) has stronger intermolecular forces compared to \(H_2O\).

Key Concepts

London Dispersion Forces

London dispersion forces, also known as van der Waals forces, are the weakest type of intermolecular force. These forces occur due to temporary fluctuations in electron density around molecules.
This causes transient dipoles that induce similar dipoles in neighboring molecules, leading to an attractive force between them.
What's interesting is that these forces are present in all molecules, whether they are polar or nonpolar. However, they are the only type of intermolecular force present in nonpolar molecules, like hydrocarbons such as \(C_{25}H_{52}\).

• They are generally weaker than other intermolecular forces.
• Become stronger with increasing molecular size and weight.
• Essential in determining physical properties like boiling and melting points.

In the case of large hydrocarbons like \(C_{25}H_{52}\), the significant number of electrons and larger molecular size heighten the London dispersion forces, leading to higher boiling points.

Hydrogen Bonding

Hydrogen bonding is a specific type of dipole-dipole interaction considered one of the strongest types of intermolecular forces. This force occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, experiences attraction to a lone pair of electrons on another electronegative atom.
This is an essential force in water (\(H_2O\)), contributing to its unique properties.

• Hydrogen bonds are much stronger than London dispersion forces.
• They significantly affect boiling and melting points.
• Responsible for the high boiling point of water compared to other similar-sized molecules.

Despite its strength, hydrogen bonding can be surpassed by very strong London dispersion forces in large nonpolar molecules, as seen in \(C_{25}H_{52}\)'s high boiling point compared to water.

Boiling Points

Boiling points are a key physical property that reflect the strength of intermolecular forces in a substance. A higher boiling point indicates stronger intermolecular forces that need to be overcome to transition the substance from a liquid to a gas state.
It's fascinating how various intermolecular forces influence the boiling points of substances.

• Substances with strong intermolecular forces require more energy to break these forces, resulting in higher boiling points.
• Hydrogen bonding typically results in higher boiling points, as seen with water's boiling point of \(100^{\circ}C\).
• In large nonpolar molecules, strong London dispersion forces due to many electrons can also lead to high boiling points, evident in \(C_{25}H_{52}\)'s boiling point greater than \(400^{\circ}C\).

Molecular Weight

Molecular weight, or molar mass, measures the total mass of all the atoms in a molecule. It plays a crucial role in determining the strength of intermolecular forces, particularly London dispersion forces.
As molecular weight increases, so do the number of electrons, which results in stronger van der Waals forces.

• Bigger molecules generally have higher molecular weights, with more electrons prompting stronger dispersion forces.
• This is an essential reason behind \(C_{25}H_{52}\)'s solid state at room temperature and its high boiling point.
• Unlike smaller molecules like \(H_2O\), larger molecules need significantly more energy to transition from liquid to gas.

The influence of molecular weight is evident in comparing hydrocarbons with different chain lengths, where longer chains exhibit higher boiling points due to increased van der Waals forces.