Question

Ethylene glycol \(\left[\mathrm{CH}_{2}(\mathrm{OH}) \mathrm{CH}_{2}(\mathrm{OH})\right]\) is the major component of antifreeze. It is a slightly viscous liquid, not very volatile at room temperature, with a boiling point of \(198^{\circ} \mathrm{C}\). Pentane \(\left(\mathrm{C}_{5} \mathrm{H}_{12}\right)\), which has about the same molecular weight, is a nonviscous liquid that is highly volatile at room temperature and whose boiling point is \(36.1{ }^{\circ} \mathrm{C}\). Explain the differences in the physical properties of the two substances.

Short answer

The differences in physical properties between ethylene glycol and pentane can be attributed to the types of intermolecular forces present in each substance. Ethylene glycol exhibits strong hydrogen bonds due to its hydroxyl groups \((-\mathrm{OH})\), resulting in a higher boiling point (\(198^{\circ} \mathrm{C}\)), lower volatility, and higher viscosity. In contrast, pentane has weaker Van der Waals forces, leading to a lower boiling point (\(36.1{ }^{\circ} \mathrm{C}\)), higher volatility, and lower viscosity.

Step-by-step solution

1. Identifying Molecular Structures

: First, let's examine the molecular structures of both compounds. Ethylene glycol: \(\mathrm{CH}_{2}(\mathrm{OH})\mathrm{CH}_{2}(\mathrm{OH})\) Pentane: \(\mathrm{C}_{5}\mathrm{H}_{12}\)

2. Analyzing Intermolecular Forces

: Now we need to determine what intermolecular forces are present in each substance. Ethylene glycol has two hydroxyl groups \((-\mathrm{OH})\) within its structure. These groups can form hydrogen bonds with neighboring molecules, which are strong intermolecular forces. Pentane, on the other hand, consists of only carbon and hydrogen atoms and exhibits weak Van der Waals (London dispersion) forces, which are weaker intermolecular forces compared to hydrogen bonds.

3. Comparing Boiling Points

: The strength of intermolecular forces has a significant effect on boiling points. Substances with stronger intermolecular forces require more energy to break them apart and transition from liquid to gas phase. Ethylene glycol has a boiling point of \(198^{\circ} \mathrm{C}\) because of its strong hydrogen bonding between molecules. Pentane has a boiling point of \(36.1{ }^{\circ} \mathrm{C}\), much lower than ethylene glycol, due to its weak Van der Waals forces between molecules.

4. Comparing Volatility

: Volatility refers to a substance's tendency to vaporize. Since substances with weaker intermolecular forces require less energy to transition from liquid to gas phase, they have a higher volatility. As a result, pentane, with its weak Van der Waals forces, is highly volatile at room temperature, whereas ethylene glycol, with strong hydrogen bonds, is not very volatile.

5. Comparing Viscosity

: Viscosity refers to a liquid's resistance to flow. Substances with stronger intermolecular forces generally exhibit higher viscosities due to the increased attraction between molecules. Therefore, ethylene glycol, with its strong hydrogen bonding between molecules, is a slightly viscous liquid, while pentane, with weak Van der Waals forces, is a nonviscous liquid.

6. Conclusion

: The differences in physical properties between ethylene glycol and pentane can be attributed to the types of intermolecular forces present in each substance. Ethylene glycol exhibits strong hydrogen bonds, which results in a higher boiling point, lower volatility, and higher viscosity compared to pentane, which has weaker Van der Waals forces.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a significant type of intermolecular force that strongly influences a substance's physical properties. This specific bond occurs when hydrogen atoms are covalently bonded to electronegative atoms, such as oxygen, nitrogen, or fluorine. In ethylene glycol, hydrogen bonds play a crucial role due to the two hydroxyl groups (-OH), each capable of forming these connections with neighboring molecules.

This capacity for hydrogen bonding leads to higher cohesion and structural integrity, which are evident in the physical properties of ethylene glycol. Compared to weaker forces, like Van der Waals forces found in pentane, hydrogen bonds are much stronger, significantly affecting how molecules interact with one another.

Boiling Point

The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. This phase change requires breaking intermolecular forces to allow molecules to move freely as a gas. As a result, the stronger the intermolecular forces, the higher the boiling point becomes.

Ethylene glycol has a boiling point of 198°C due to its extensive hydrogen bonding network. These strong bonds require more energy to break, thus raising the boiling point. In contrast, pentane, with weaker Van der Waals forces, has a much lower boiling point of 36.1°C, as less energy is needed to overcome these forces.

Volatility

Volatility refers to how readily a substance can vaporize. Substances with weaker intermolecular forces tend to be more volatile since less energy is required for these molecules to enter the gaseous phase.

Pentane is highly volatile at room temperature due to its weak Van der Waals forces. These forces are easily overcome, allowing pentane molecules to evaporate quickly and readily. On the other hand, ethylene glycol is not very volatile because the robust hydrogen bonding restricts the molecules' ability to break free and convert into gas, demanding more energy input for this phase change.

Viscosity

Viscosity is a measure of a fluid's resistance to flow. This characteristic arises from the internal friction generated as molecules slide past one another. Intermolecular forces directly influence viscosity – stronger forces increase resistance, resulting in higher viscosity.

Ethylene glycol's slight viscosity can be attributed to the strong hydrogen bonding among its molecules. These connections make it more challenging for the liquid to flow smoothly, as the molecules tend to "stick" together. Conversely, pentane's nonviscous nature stems from its weak Van der Waals forces, which allow its molecules to move past each other easily, making it a free-flowing liquid.