Question

Complete the following statements about the effect of intermolecular forces on the physical properties of a substance. (a) The higher the boiling point of a liquid, the (stronger, weaker) are its intermolecular forces. (b) Substances with strong intermolecular forces have (high, low) vapor pressures. (c) Substances with strong intermolecular forces typically have (high, low) surface tensions. (d) The higher the vapor pressure of a liquid, the (stronger, weaker) are its intermolecular forces. (c) Because nitrogen, \(\mathrm{N}_{2}\), has (strong, weak) intermolecular forces, it has a (high, low) critical temperature. (f) Substances with high vapor pressures have correspondingly (high, low) boiling points. (g) Because water has a high boiling point, it must have (strong, weak) intermolecular forces and a correspondingly (high, low) enthalpy of vaporization.

Short answer

(a) stronger, (b) low, (c) high, (d) weaker, (e) weak, low, (f) low, (g) strong, high

Step-by-step solution

Relating Boiling Point and Intermolecular Forces

The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure. A higher boiling point indicates that more energy is required to overcome the intermolecular forces and convert the liquid into a gas. Therefore, the stronger the intermolecular forces, the higher the boiling point.

Intermolecular Forces and Vapor Pressure

Vapor pressure is a measure of a substance's tendency to evaporate. It is inversely related to the strength of intermolecular forces. Stronger intermolecular forces make it harder for molecules to escape into the vapor phase, resulting in lower vapor pressures.

Intermolecular Forces and Surface Tension

Surface tension is a physical property that describes the elastic tendency of a fluid surface. It is directly related to intermolecular forces. Stronger intermolecular forces result in higher surface tensions because the molecules at the surface are more tightly held together.

Vapor Pressure and Intermolecular Forces

As discussed in Step 2, vapor pressure is inversely related to the strength of intermolecular forces. Therefore, a higher vapor pressure indicates weaker intermolecular forces, as it is easier for the molecules to escape the liquid phase.

Nitrogen's Intermolecular Forces and Critical Temperature

The critical temperature of a substance is the temperature above which it cannot be liquefied, regardless of pressure. As nitrogen has weak van der Waals forces (dispersion forces) as its primary intermolecular forces, it has a relatively low critical temperature.

Vapor Pressure and Boiling Points

If a substance has a high vapor pressure, it means that it evaporates easily and has weaker intermolecular forces. Such a substance would have a low boiling point because it takes less energy to convert it into a gas.

Water's Intermolecular Forces and Enthalpy of Vaporization

Water has a high boiling point, which indicates it has strong hydrogen bonds as its intermolecular forces. These strong forces also mean that there is a correspondingly high enthalpy of vaporization, which is the amount of energy required to convert one mole of liquid into gas at constant temperature and pressure.

Key concepts

Boiling Point

The boiling point of a liquid is a telltale sign of the strength of its intermolecular forces. In simple terms, it's the temperature at which a liquid turns to gas. This process happens when the vapor pressure of the liquid matches the pressure of the surrounding environment. Strong intermolecular forces mean that the molecules in the liquid hold on to each other tightly, making it harder for them to move into the gas phase. As a result, a higher temperature (thus more energy) is needed to boil the liquid. That's why substances with strong intermolecular forces, like water, have a higher boiling point.

For students puzzling over why a substance boils at a certain temperature, think of it as a tug-of-war between the molecules and heat. When the forces among the molecules are very strong, heat has to work harder (i.e., you need more of it) to get those molecules to let go and vaporize.

Vapor Pressure

Vapor pressure might sound complex, but it's essentially a measure of a substance's eagerness to transform from liquid to vapor. When the intermolecular forces within a liquid are weaker, molecules can escape more easily, leading to higher vapor pressure. On the flip side, stronger forces mean fewer molecules can break away, resulting in lower vapor pressure.

If you imagine the surface of a liquid as a tightly packed party, a high vapor pressure means it's very easy for guests (molecules) to leave the party (the liquid), whereas a low vapor pressure indicates that the partygoers are having too much fun together to leave easily.

Surface Tension

Picture trying to break the surface of a still pond with your hand; the slight resistance you feel is due to surface tension. It's a liquid's ability to behave like an elastic sheet. This occurs due to strong intermolecular forces acting between molecules at the liquid's surface. They're a bit like a team of acrobats holding hands to form a strong net. The stronger the intermolecular forces, the higher the surface tension, making it harder to penetrate the surface.

For example, water beads on a waxy car hood because water's strong hydrogen bonds create high surface tension, preventing the liquid from spreading out.

Critical Temperature

At the critical temperature, no matter how much you squeeze a gas (increase the pressure), you can't make it a liquid; it's the point of no return. Substances with weak intermolecular forces like nitrogen have lower critical temperatures. That's because their molecules can easily wander away from each other, and at a certain temperature, no amount of pushing or compressing can bring them back together into a liquid.

Thus, the critical temperature gives us a clue about the strength of a substance's intermolecular forces: low critical temperature equals weak forces, and vice versa.

Enthalpy of Vaporization

The enthalpy of vaporization is a fancy term for the heat energy required to turn a liquid into a gas at a given pressure. It's closely tied to the boiling point and intermolecular forces. A high enthalpy of vaporization means it takes a lot of energy to vaporize the liquid, which in turn means the liquid must have strong intermolecular forces holding it together.

To understand this, you can think of it as the energy needed to get all the molecules to 'un-stick' from each other. Water, with its strong hydrogen bonds, has a high enthalpy of vaporization compared to many other liquids because those bonds take a lot of energy to break.