Question

Ethylene glycol \(\left(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)\) and pentane \(\left(\mathrm{C}_{5} \mathrm{H}_{12}\right)\) are both liquids at room temperature and room pressure, and have about the same molecular weight. (a) One of these liquids is much more viscous than the other. Which one do you predict is more viscous? (b) One of these liquids has a much lower normal boiling point \(\left(36.1^{\circ} \mathrm{C}\right)\) compared to the other one \(\left(198^{\circ} \mathrm{C}\right) .\) Which liquid has the lower normal boiling point? (c) One of these liquids is the major component in antifreeze in automobile engines. Which liquid would you expect to be used as antifreeze? (d) One of these liquids is used as a "blowing agent" in the manufacture of polystyrene foam because it is so volatile. Which liquid would you expect to be used as a blowing agent?

Short answer

(a) Ethylene glycol is more viscous due to the presence of hydrogen bonding.

Step-by-step solution

1. Identifying the more viscous liquid#

When looking for viscosity, we should consider the intermolecular forces working between the molecules. In the case of ethylene glycol, hydrogen bonding is present due to the hydroxyl group (OH), which causes stronger intermolecular forces compared to pentane, which only has van der Waals forces (London dispersion forces) acting between the molecules. Given that stronger intermolecular forces lead to higher viscosity, we predict that ethylene glycol is more viscous than pentane.

Key concepts

Viscosity

Viscosity is a measure of a fluid's resistance to flow. Imagine honey and water. Honey flows much more slowly than water, making it more viscous. The concept is quite important in understanding how different fluids behave. Viscosity is influenced by the type and strength of intermolecular forces present in a liquid.

Stronger intermolecular forces make a liquid more resistant to flow, hence more viscous. Ethylene glycol, for example, forms strong hydrogen bonds which make it more viscous compared to substances with weaker intermolecular forces like van der Waals forces. On the other hand, pentane experiences only van der Waals forces, which are weaker than hydrogen bonds. This results in pentane having lower viscosity than ethylene glycol.

When things like temperature change, so too does viscosity. Higher temperatures typically decrease viscosity as thermal motion overcomes some of the intermolecular forces. However, the fundamental nature of the forces in each liquid doesn't change.

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that occurs when hydrogen is bonded to oxygen, nitrogen, or fluorine — the three most electronegative elements. This bond is stronger than regular van der Waals forces but weaker than covalent or ionic bonds.

In ethylene glycol \[\left( \mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH} \right)\]the presence of two hydroxyl groups (OH) facilitates the formation of hydrogen bonds. These bonds are significant because they increase the attraction between molecules. This leads to higher boiling points and greater viscosity.

Hydrogen bonds are also highly directional, meaning the molecules align in a specific manner due to their polar nature, further increasing boiling point and viscosity. This is why ethylene glycol not only has a much higher boiling point than pentane, but also makes it more suitable for uses like antifreeze. The ability of hydrogen bonding to stabilize complex structures is leveraged in various practical applications.

Van der Waals Forces

Van der Waals forces are weak interactions that occur between molecules. They include attractions and repulsions between atoms, molecules, and surfaces, and unlike other forces, they don't require a permanent dipole. The most common types are London dispersion forces, which are the weakest.

These forces are temporary and occur due to fluctuations in electron distribution within molecules, leading to temporary dipoles. Pentane \[\left(\mathrm{C}_{5} \mathrm{H}_{12}\right)\]is an example of a molecule where van der Waals forces are the primary type of intermolecular interaction. Because of these weak interactions, pentane has a low boiling point and low viscosity, making it volatile and useful as a blowing agent.

While not as strong as hydrogen bonds, van der Waals forces are crucial in explaining why nonpolar molecules, like pentane, have properties similar to liquids with stronger intermolecular forces when at different conditions. Despite their weakness, they are involved in many critical biological and chemical processes where slight attractions are sufficient to maintain interaction without forming strong, permanent bonds.