Question

Ethylene glycol [CH \(\left._{2}(\mathrm{OH}) \mathrm{CH}_{2}(\mathrm{OH})\right]\) 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

In summary, the differences in physical properties of ethylene glycol and pentane can be attributed to the differences in their intermolecular forces. Ethylene glycol experiences both hydrogen bonding and dispersion forces, resulting in stronger overall intermolecular forces, which leads to a higher viscosity, lower volatility, and higher boiling point. In contrast, pentane only experiences weaker dispersion forces, causing it to have a lower viscosity, higher volatility, and a lower boiling point.

Step-by-step solution

Determine the molecular structure of the two substances

Ethylene glycol has the molecular formula CH2(OH)CH2(OH). In this substance, the oxygen atom in each hydroxyl group (OH) forms a covalent bond with the carbon atom. Pentane has the molecular formula C5H12, and its structure is linear.

Identify the types of intermolecular forces in each substance

Ethylene glycol has two hydroxyl groups, which means that there are hydrogen bonds present between the molecules. In addition to hydrogen bonding, both substances exhibit dispersion forces (also known as London dispersion forces or van der Waals forces).

Compare the strength of intermolecular forces

Hydrogen bonding is stronger than dispersion forces. Ethylene glycol molecules are held together by both hydrogen bonding and dispersion forces, while pentane molecules are only held together by dispersion forces. Because hydrogen bonding is stronger, the intermolecular forces in ethylene glycol are stronger overall than those in pentane.

Relate intermolecular forces to physical properties

Viscosity and volatility are both influenced by intermolecular forces. A substance with stronger intermolecular forces generally has a higher viscosity and lower volatility. This is because stronger intermolecular forces make it more difficult for molecules to flow past each other (which leads to higher viscosity) and more difficult for molecules to escape from the liquid into the gas phase (which leads to lower volatility).

Explain the differences in physical properties

Ethylene glycol has a higher viscosity and lower volatility than pentane because its intermolecular forces are stronger due to the presence of hydrogen bonding, in addition to dispersion forces. Pentane only has dispersion forces, which are weaker, leading to a lower viscosity and higher volatility. Additionally, the stronger intermolecular forces present in ethylene glycol result in a higher boiling point compared to pentane.

Key concepts

Hydrogen Bonding

Hydrogen bonding is one of the strongest intermolecular forces and plays a crucial role in determining a substance's physical properties. It occurs when a hydrogen atom covalently bonded to a highly electronegative atom, such as oxygen, fluorine, or nitrogen, has an attractive interaction with another electronegative atom.

In the case of ethylene glycol (CH₂(OH)CH₂(OH)), each hydroxyl group (OH) forms a hydrogen bond due to the presence of a polar O-H bond. The oxygen atom, being electronegative, pulls the electron density towards itself, creating a partial negative charge, while the hydrogen becomes partially positive. These polarities allow hydrogen atoms to interact with oxygen atoms in neighboring molecules, resulting in a network of hydrogen bonds.

This bonding leads to a significant increase in boiling point and viscosity as the molecules are held tightly together, requiring more energy to separate them, thus explaining the high boiling point and low volatility of ethylene glycol compared to pentane, which lacks this strong intermolecular force.

Viscosity and Volatility

Viscosity is essentially a measure of a liquid's resistance to flow, while volatility describes how readily a substance will vaporize, or convert into a gas. These properties are heavily influenced by the nature and strength of intermolecular forces within the liquid.

Substances with strong intermolecular forces, like hydrogen bonds found in ethylene glycol, exhibit high viscosity. This is because the molecules are held together tightly and cannot easily slide past one another. Conversely, pentane, whose molecules are bound by weaker dispersion forces, has low viscosity which allows it to flow more freely.

For volatility, a substance with weak intermolecular forces allows molecules to escape more easily to the gas phase, resulting in high volatility. Thus, pentane is more volatile than ethylene glycol. The high boiling point of ethylene glycol, compared to the low boiling point of pentane, is a direct result of this difference in volatility, influenced by the presence or absence of hydrogen bonding.

Molecular Structure

Molecular structure plays a significant role in determining the type and strength of intermolecular forces. For instance, a linear structure, as seen in pentane (C₅H₁₂), features molecules that can easily align and slide past each other, which facilitates lower viscosity and higher volatility.

Conversely, the molecular structure of ethylene glycol includes two hydroxyl groups capable of forming hydrogen bonds, which adds rigidity to the liquid's structure and increases resistance to flow and evaporation. This structural difference explains why ethylene glycol is more viscous and less volatile than pentane.

The understanding of molecular structure is essential when predicting and explaining the physical properties of a substance, as well as its behavior in various environments, such as the low-temperature conditions ethylene glycol must withstand as an antifreeze.