Question

Hexane, which is a liquid at room temperature, is more volatile than water. (a) What does this mean? (b) Which liquid has the higher vapor pressure at a given temperature? (c) Which liquid most likely has the higher boiling point? (d) Why is hexane more volatile than water?

Short answer

(a) Volatility refers to a substance's tendency to evaporate easily at room temperature. (b) Hexane has a higher vapor pressure at a given temperature, as it is more volatile than water. (c) Water most likely has the higher boiling point, as substances with higher vapor pressures generally have lower boiling points. (d) Hexane is more volatile than water due to weaker intermolecular forces (London dispersion forces) between its molecules compared to the strong hydrogen bonds in water.

Step-by-step solution

(a) Defining Volatility

When a substance is described as being "volatile," it means that the substance evaporates easily at room temperature. The higher the volatility of a substance, the more readily it evaporates.

(b) Comparing Vapor Pressures

Vapor pressure is a measure of the tendency of a substance to evaporate. A higher vapor pressure implies a higher tendency to evaporate. Since hexane is more volatile than water, it has a higher vapor pressure at a given temperature.

(c) Identifying the Liquid with Higher Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the atmospheric pressure. Substances with higher vapor pressures will generally have lower boiling points. Therefore, since hexane has a higher vapor pressure than water, it most likely has a lower boiling point than water.

(d) Explaining Hexane's Higher Volatility

Hexane is more volatile than water because of the weaker intermolecular forces between its molecules compared to those in water. Water molecules form strong hydrogen bonds with each other, which require more energy to break and cause evaporation. Hexane, on the other hand, has weaker London dispersion forces (or van der Waals forces) acting between its molecules. As a result, hexane molecules can more easily escape from the liquid phase into the gas phase, making hexane more volatile than water.

Key concepts

Vapor Pressure

Vapor pressure is an essential concept in understanding the volatility of substances. It represents the pressure exerted by the vapor of a liquid (or solid) when it is in equilibrium with its condensed phases at a certain temperature. Think of it as the measure of a liquid's tendency to turn into a gas. When a liquid has a high vapor pressure at a specific temperature, it indicates that the molecules have a high escaping tendency and can easily transition to the vapor phase.

For example, in the case of hexane, because it is more volatile than water, its vapor pressure is higher. This means that, given the same conditions, hexane’s molecules are more likely to be found in the gaseous state compared to water's molecules.

Boiling Point

The boiling point is the temperature at which a liquid's vapor pressure equals the external pressure, commonly the atmospheric pressure. This is the point at which the liquid becomes a gas. A substance with a high vapor pressure needs less additional energy to reach the atmospheric pressure; thus, it will have a lower boiling point. It's like having a runner who's already close to the finish line at the start of a race – they don't have as far to go to reach the end goal.

Since hexane has a higher vapor pressure than water, it is able to reach that equilibrium point at a lower temperature. Therefore, hexane has a lower boiling point as compared to water, which needs more heat to overcome the atmospheric pressure due to its lower vapor pressure.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles: atoms, molecules, or ions. They are one of the primary factors that determine the physical properties of a substance, including boiling point, melting point, and volatility. There are several types of intermolecular forces, but they vary greatly in strength. For instance, hydrogen bonds are much stronger than London dispersion forces.

These forces keep molecules together and need to be overcome for a molecule to transition into the gas phase. The strength of these forces correlates with a substance's volatility. Weaker forces mean higher volatility because it's easier for molecules to part ways and escape into the air.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of intermolecular force. It occurs when a hydrogen atom covalently bound to a highly electronegative atom, like oxygen or nitrogen, experiences the electrostatic field of another electronegative atom nearby. In water, for example, oxygen's strong pull on the hydrogen's electrons allows it to also partially attract neighboring hydrogen atoms of other water molecules, creating a network of connections.

This network gives water some of its unique properties, such as a high boiling point and surface tension. Because these bonds need more energy to break, substances with hydrogen bonds, such as water, are less volatile and have higher boiling points compared to those without such bonds, like hexane.

London Dispersion Forces

London dispersion forces, also known as van der Waals forces, are the weakest intermolecular force and occur between all molecules, whether polar or nonpolar. These forces are the result of the temporary fluctuations in the electron distribution within molecules, which create temporary dipoles that attract other molecules. The strength of these forces increases with the size of the molecules and the number of electrons.

In hexane, these London dispersion forces are the primary intermolecular force at play. Because they are relatively weak, especially when compared to hydrogen bonding in water, hexane molecules don't require as much energy to escape into the gaseous state, making hexane more volatile.