Question

The boiling points, surface tensions, and viscosities of water and several alchohols are as follows: $$ \begin{array}{lrcc} & \begin{array}{l} \text { Boiling } \\ \text { Point }\left({ }^{\circ} \mathbf{C}\right) \end{array} & \begin{array}{l} \text { Surface } \\ \text { Tension }\left(\mathbf{J} / \mathbf{m}^{2}\right) \end{array} & \begin{array}{l} \text { Viscosity } \\ (\mathbf{k g} / \mathbf{m}-\mathbf{s}) \end{array} \\ \hline \text { Water, } \mathrm{H}_{2} \mathrm{O} & 100 & 7.3 \times 10^{-2} & 0.9 \times 10^{-3} \\ \text {Ethanol, } \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} & 78 & 2.3 \times 10^{-2} & 1.1 \times 10^{-3} \\ \text {Propanol, } \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} & 97 & 2.4 \times 10^{-2} & 2.2 \times 10^{-3} \\ n \text { -Butanol, } \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} & 117 & 2.6 \times 10^{-2} & 2.6 \times 10^{-3} \\\ \text {Ethylene glycol, } \mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH} & 197 & 4.8 \times 10^{-2} & 26 \times 10^{-3} \end{array} $$ (a) For ethanol, propanol, and \(n\) -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 \(62 \mathrm{amu}),\) 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 for ethanol, propanol, and n-butanol is due to their molecular structures, which have increasing numbers of carbon and hydrogen atoms in the carbon chain. This allows for stronger van der Waals forces as the molecules grow in size. (b) Ethylene glycol has a much larger viscosity than propanol despite similar molecular weights because of its stronger intermolecular hydrogen bonding due to its two hydroxyl groups, which makes its molecules more resistant to flow. (c) Water has the highest surface tension but the lowest viscosity due to its strong hydrogen bonding and small molecular size. The strong hydrogen bonding creates a tight-knit structure at the surface, resulting in high surface tension, while the small size allows for easier flow, resulting in low viscosity.

Step-by-step solution

(a) Increasing boiling points, surface tensions, and viscosities

For ethanol, propanol, and n-butanol, the boiling points, surface tensions, and viscosities all increase due to their molecular structures. All three are alcohols with an increasing number of carbon and hydrogen atoms in the carbon chain, which allows for more possibilities of van der Waals forces (London Dispersion Forces) between the molecules. As the molecules grow in size, the strength of van der Waals forces also increases, leading to a rise in boiling points, surface tensions, and viscosities.

(b) Viscosity difference between ethylene glycol and propanol

Ethylene glycol and propanol have similar molecular weights (62 amu vs. 60 amu), yet their viscosities are significantly different. This difference can be explained by their molecular structures and interactions. Ethylene glycol has a different structure, featuring two hydroxyl groups instead of one, which allows it to form stronger intermolecular hydrogen bonding compared to propanol. This results in a stronger molecular interaction that makes ethylene glycol's molecules more resistant to flow, leading to much higher viscosity.

(c) Water's highest surface tension and lowest viscosity

Water has the highest surface tension but the lowest viscosity among the substances listed. The high surface tension of water can be attributed to its strong hydrogen bonding between the water molecules, which creates a tight-knit structure at the surface. As for viscosity, since the water molecule is much smaller compared to the other substances, it can flow more easily past other water molecules, resulting in a lower viscosity. The balance between strong hydrogen bonding and water's small size results in its relatively unique properties of high surface tension and low viscosity.

Key concepts

Boiling Point

When studying various substances, a critical thermal property to consider is the boiling point. It represents the temperature at which a substance's vapor pressure equals the atmospheric pressure, compelling it to change from liquid to gas.

For alcohols like ethanol, propanol, and butanol, increasing the length of the carbon chain results in a higher boiling point. This is due to greater van der Waals forces, which require more energy to overcome. Comparatively, water's boiling point is exceptionally high for its molecular weight due to hydrogen bonding, which is remarkably strong amongst water molecules.

Surface Tension

The surface tension of a liquid describes the elastic tendency of its surface, making it acquire the least surface area possible. This force is primarily due to the molecules at the surface experiencing a net inward force, as they're not surrounded by similar molecules on all sides.

In water, hydrogen bonding accounts for its high surface tension, pulling molecules closer together and creating a sort of 'skin' on the water's surface. For alcohols, as the molecule size increases, so does the surface tension, albeit to a lesser extent than in water. Surface tension is vital for various natural and technological processes, including the way some organisms can walk on water or the way liquids are administered drop-wise in medicine.

Viscosity

Another important physical property is viscosity, which is the measure of a fluid's resistance to flow. Thicker fluids with higher viscosities move slower under the same force compared to thinner fluids with lower viscosities.

Alcohols exhibit an increase in viscosity with a longer carbon chain due to increased intermolecular interactions. Ethylene glycol, with its two hydroxyl groups, forms strong hydrogen bonds resulting in a significant increase in its viscosity compared to alcohols with similar molecular weights. In contrast, water's low viscosity despite its high surface tension is unique, as its small molecular size and specific hydrogen bond network allow for easier movement between molecules.

Van der Waals Forces

Delving into the realm of molecular interactions, van der Waals forces are a group of weak intermolecular forces that include attractions and repulsions between atoms, molecules, and surfaces. They encompass forces such as London Dispersion Forces, dipole-dipole interactions, and hydrogen bonding.

These forces are responsible for the properties of many substances when it comes to boiling points, surface tension, and viscosity. In the case of our alcohols, the lengthening of the carbon chain increases the London Dispersion Forces, resulting in higher boiling points and greater surface tensions. Understanding these forces is essential for grasping why different substances behave the way they do under various conditions.

Hydrogen Bonding

One of the strongest van der Waals forces is hydrogen bonding. This occurs when a hydrogen atom covalently bonded to a highly electronegative atom, like oxygen or nitrogen, experiences an electrostatic attraction to another electronegative atom.

Water is a classic example, with each molecule forming up to four hydrogen bonds, leading to its high boiling point and surface tension. Ethylene glycol also benefits from hydrogen bonding due to its two hydroxyl groups, drastically increasing its viscosity. An understanding of hydrogen bonds is crucial for many areas of chemistry and biology, including the folding of proteins and the structure of DNA.