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 \(62 \mathrm{u}\) ), 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 boiling points, surface tensions, and viscosities of ethanol, propanol, and n-butanol increase due to the increasing size and surface area of their molecules, resulting in greater van der Waals forces between molecules as the carbon chain length increases. (b) Despite having similar molecular weights, ethylene glycol has higher viscosity than propanol because of its molecular structure with two hydroxyl (-OH) groups, allowing it to form stronger intermolecular forces through more hydrogen bonding compared to propanol. (c) Water has the highest surface tension due to its extensive hydrogen bonding between molecules, but the lowest viscosity because its smaller molecular size leads to lesser internal friction and van der Waals forces compared to alcohols.

Step-by-step solution

Understand molecular structures and intermolecular forces

Before analyzing the properties of these compounds, it is essential to understand their molecular structures and intermolecular forces. Intermolecular forces play a significant role in determining boiling points, surface tension, and viscosities. (a)

Analyzing alphanumeric sequence

Ethanol (C2H5OH), propanol (C3H7OH), and n-butanol (C4H9OH) are alcohols with an increasing number of carbon atoms. As the carbon chain length increases, the size and surface area of the molecule increases. The increase in molecular size results in more significant van der Waals forces between the molecules.

Explaining the increased boiling points

Boiling points increase with an increase in van der Waals forces. As the size of alcohols increases from ethanol to propanol to n-butanol, van der Waals forces also increase. Therefore, the boiling points increase along the series.

Explaining the increased surface tension

The increase in van der Waals forces also causes an increase in surface tension along the series of alcohols. Surface tension is directly proportional to the forces maintaining the molecule's cohesion on the surface. With greater van der Waals forces, the molecules exhibit stronger cohesion, resulting in higher surface tension.

Explaining the increased viscosity

The size and surface area of the molecules in the series of alcohols play a role in determining the viscosity. Viscosity is the measure of a fluid's resistance to flow due to internal friction. In larger molecules, van der Waals forces and internal friction increase, leading to an increase in viscosity. (b)

Comparing molecular structures of propanol and ethylene glycol

Both propanol (C3H7OH) and ethylene glycol (C2H6O2) have similar molecular weights but different molecular structures. The structure of ethylene glycol has two hydroxyl (-OH) groups attached to its carbons, which gives it the ability to form more hydrogen bonds.

Explaining the difference in viscosity

The viscosity of a compound is influenced by the extent of intermolecular forces present within the fluid. In ethylene glycol, the presence of multiple hydroxyl (-OH) groups increases the intermolecular forces through hydrogen bonding which causes higher viscosity compared to propanol. (c)

Water molecular structure and hydrogen bonding

Water (H2O) has a unique molecular structure that allows it to form strong hydrogen bonds between its molecules. These hydrogen bonds are strong intermolecular forces, which contribute to the high surface tension of water.

Explaining the highest surface tension and lowest viscosity

The high surface tension in water is because of the extensive hydrogen bonding between its molecules. However, the low viscosity of water is due to its relatively small molecular size compared to the alcohols. The smaller molecular size leads to a lesser internal friction and van der Waals forces compared to the alcohols, thus resulting in lower viscosity.

Key concepts

Boiling Points

The boiling point of a substance is the temperature at which it changes from a liquid to a gas. Behind the boiling process, the main player is the intermolecular forces that hold the liquid molecules together. A stronger intermolecular force requires a higher temperature to overcome and separate the molecules, resulting in a higher boiling point.

In a series of alcohols, like ethanol, propanol, and n-butanol, as the chain length of carbon atoms increases, so does the surface area of the molecules. This leads to an increase in van der Waals forces, which are the weak intermolecular forces acting between molecules. More carbon atoms mean a larger area for these forces to act, hence stronger intermolecular forces overall.

• Stronger van der Waals forces = higher boiling point
• Larger molecules need more energy (higher temperature) to change phase from liquid to gas

Thus, moving from ethanol to propanol to n-butanol, we observe an increase in boiling points due to more robust intermolecular attractions.

Surface Tension

Surface tension is the energy required to stretch or increase the surface of a liquid by a unit area. It's essentially how much the liquid wants to "stick together" at the surface. This property is also heavily influenced by intermolecular forces.

In the series of alcohols from ethanol to n-butanol, as van der Waals forces strengthen with the increase in molecular size, surface tension increases too. This is because the molecules at the surface hold onto each other more tightly, resisting external forces trying to break the surface.

• Higher intermolecular forces result in stronger surface tension
• Liquid molecules "stick" more, causing greater cohesion at the surface

Water, on the other hand, exhibits very high surface tension due to its ability to form hydrogen bonds. These are even stronger than van der Waals forces, giving water exceptionally high surface tension among common liquids, despite water's small molecular size.

Viscosity

Viscosity refers to a fluid's resistance to flow. In simple terms, it's how "thick" a liquid is. Imagine honey versus water. Honey is much thicker and flows slowly, demonstrating higher viscosity.

The viscosity of a liquid is directly linked to intermolecular forces and molecular structure. Larger molecules, like n-butanol compared to ethanol, have higher viscosity because the increased surface area leads to stronger van der Waals forces. Therefore, they are "stickier" and flow less easily.

• Stronger intermolecular forces lead to higher viscosity
• Larger molecules increase flow resistance, yielding higher viscosity

Additionally, particular molecular structures, like that of ethylene glycol, which includes multiple hydroxyl (-OH) groups, lead to extensive hydrogen bonding. These bonds further increase viscosity significantly compared to a molecule like propanol, which has fewer hydroxyl groups.