Question

The boiling points, surface tensions, and viscosities of water and several alcohols are as shown below:(a) From ethanol to propanol to \(n\) -butanol the boiling points, surface tensions, and viscosities all increase. What is the reason for this increase? (b) How do you explain the fact that propanol and ethylene glycol have similar molecular weights (60 versus 62 amu), yet the viscosity of ethylene glycol is more than 10 times larger than propanol? (c) How do you explain the fact that water has the highest surface tension but the lowest viscosity?

Short answer

(a) The increase in boiling points, surface tensions, and viscosities from ethanol to propanol to n-butanol is due to the increase in molecular size and mass, resulting in stronger van der Waals forces between the molecules. (b) The higher viscosity of ethylene glycol compared to propanol, despite similar molecular weights, is due to the presence of two hydroxyl groups in ethylene glycol, leading to stronger hydrogen bonding networks. (c) Water has the highest surface tension due to its strong hydrogen bonding, but the lowest viscosity because of its smaller molecular weight, less extensive hydrogen bonding networks than ethylene glycol, and higher molecular mobility.

Step-by-step solution

Question (a) Explanation

For ethanol, propanol, and n-butanol, as we move from one alcohol to another, the number of carbon and hydrogen atoms increases, making the molecules larger and heavier. Due to this increase in molecular size and mass, the van der Waals forces between the molecules also increase. These forces result in higher boiling points, surface tensions, and viscosities for these alcohols.

Question (b) Explanation

Although propanol and ethylene glycol have similar molecular weights (60 amu for propanol and 62 amu for ethylene glycol), the viscosity of ethylene glycol is much higher. This is because ethylene glycol has two hydroxyl groups that can form hydrogen bonds with other molecules. The strong hydrogen bonding networks lead to a much higher degree of intermolecular cohesion in ethylene glycol compared to propanol, which has only one hydroxyl group. This results in a significantly higher viscosity for ethylene glycol than for propanol.

Question (c) Explanation

Water has the highest surface tension among the given liquids because it can form strong hydrogen bonds between its molecules. The polar nature of water molecules allows them to form a strong intermolecular network at the surface, requiring a significant amount of energy to break these bonds, leading to the high surface tension. However, water has the lowest viscosity because it has a smaller molecular weight compared to the other liquids, and the hydrogen bonding networks in water are not as extensive as those in ethylene glycol (which has two hydrogen bond-forming hydroxyl groups per molecule). Furthermore, water molecules have a higher molecular mobility due to their smaller size. This combination of factors results in lower resistance to flow and thus a lower viscosity for water compared to the other liquids.

Key concepts

Boiling Points

Understanding the boiling points of substances involves analyzing their intermolecular forces. When we consider various alcohols like ethanol, propanol, and n-butanol, we notice that boiling points increase with the size of the molecule. This is because larger molecules have a greater area over which van der Waals forces, specifically London dispersion forces, can operate.

As the number of carbon atoms in a molecule's chain increases, so does its mass and surface area, leading to stronger intermolecular attractions. These attractions require more energy to overcome in the transition from liquid to gas, hence a higher boiling point.

This is fundamental to understanding why substances with similar molecular weights can have different boiling points. In the classroom, we can demonstrate this principle by comparing the boiling points of molecules with slight differences in structure or mass to elucidate the effect of intermolecular forces on the boiling point.

Surface Tension

Surface tension is a property resulting from the cohesive forces between liquid molecules. In the case of water, it has a remarkably high surface tension due to the strong hydrogen bonds between its molecules.

These hydrogen bonds act like little springs, pulling the molecules at the surface together and minimizing the surface area, leading to a phenomenon where the surface of the water behaves like an elastic sheet.

This fascinating observation can be demonstrated with simple experiments such as floating a paper clip on the surface of water or observing water droplets on a non-porous surface, showing the effects of surface tension in a tangible way.

Viscosity

Viscosity can be thought of as a fluid's resistance to flow. The higher the viscosity, the thicker the fluid and the more slowly it moves. Viscosity increases with stronger intermolecular forces and with molecular complexity.

For example, ethylene glycol has a much higher viscosity than propanol, despite their similar molecular weights. The reason lies in ethylene glycol's two hydroxyl groups, which enable more hydrogen bonding, dramatically increasing its intermolecular attractions and, consequently, its viscosity.

Viscosity has practical implications in industries such as lubrication and in everyday products like honey or motor oil, reinforcing the relevance of this property beyond the science classroom.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs between molecules that have a hydrogen atom bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine.

The strength of hydrogen bonds has significant consequences for the physical properties of substances. For instance, it explains why water, which is capable of extensive hydrogen bonding, has such high boiling points and surface tension relative to other substances of similar molecular weight.

Hydrogen bonding also influences solubility, heat capacity, and the structure of DNA, thus its understanding is crucial for many scientific disciplines from chemistry to biology.

Van der Waals Forces

Under the umbrella term 'van der Waals forces,' there are London dispersion forces, Keesom forces (dipole-dipole attractions), and Debye forces (induced dipole interactions). These are the weakest types of intermolecular forces, yet they are pervasive and can dictate the behavior of non-polar and polar molecules alike.

London dispersion forces increase with the size and shape of molecules, affecting boiling points and viscosity. These forces are often discussed in chemistry to explain why certain gases are liquefied at different temperatures and why some substances have higher vapor pressures than others.

Understanding van der Waals forces helps us grasp why noble gases, though inert, can be liquified or why geckos can climb walls (due to the dispersion forces acting between the tiny hairs on their feet and the surfaces they climb).