Question

As the intermolecular attractive forces between molecules increase in magnitude, do you expect each of the following to increase or decrease in magnitude? (a) Vapor pressure, (b) heat of vaporization, (c) boiling point, (d) freezing point, (e) viscosity, (f) surface tension, ( g) critical temperature.

Short answer

As the intermolecular attractive forces between molecules increase in magnitude, we expect: (a) Vapor pressure to decrease, (b) Heat of vaporization to increase, (c) Boiling point to increase, (d) Freezing point to increase, (e) Viscosity to increase, (f) Surface tension to increase, and (g) Critical temperature to increase.

Step-by-step solution

(a) Vapor pressure

As intermolecular attractive forces increase, the ability of molecules to escape the liquid phase and enter the gas phase decreases due to stronger interactions between molecules. This leads to a lower equilibrium vapor pressure above the liquid, because fewer molecules have the energy to escape the liquid phase. So, as intermolecular attractive forces increase, we expect the vapor pressure to decrease in magnitude.

(b) Heat of vaporization

Heat of vaporization is the amount of energy required to convert a liquid into a vapor at constant temperature and pressure. When intermolecular attractive forces are stronger, more energy is needed to overcome these forces and separate the molecules from each other as they transition from the liquid to the gas phase. Thus, we expect the heat of vaporization to increase in magnitude as intermolecular attractive forces increase.

(c) Boiling point

Boiling point is the temperature at which a substance's vapor pressure equals the external atmospheric pressure, and the liquid starts turning into a gas. If the vapor pressure of a substance decreases with stronger intermolecular attractive forces (as we determined in part (a)), then the temperature must be raised to reach the atmospheric pressure, meaning the boiling point will also increase. So, we expect the boiling point to increase in magnitude as intermolecular attractive forces increase.

(d) Freezing point

As the intermolecular attractive forces increase, molecules tend to stick together more, which makes it easier for them to form a solid structure. This means that freezing may occur at higher temperatures when intermolecular forces are stronger because less energy needs to be removed from the system to form a solid. Therefore, we expect the freezing point to increase in magnitude as intermolecular attractive forces increase.

(e) Viscosity

Viscosity is the resistance of a liquid to flow. When intermolecular attractive forces are stronger, molecules stick together more, making it harder for them to slide past each other. This results in a higher resistance to flow or an increased viscosity. So, we expect the viscosity to increase in magnitude as intermolecular attractive forces increase.

(f) Surface tension

Surface tension is the energy required to increase the surface area of a liquid. When intermolecular attractive forces are stronger, the molecules at the surface of the liquid are held together more tightly, meaning they are harder to separate and require more energy to increase the surface area. Therefore, we expect the surface tension to increase in magnitude as intermolecular attractive forces increase.

(g) Critical temperature

Critical temperature is the temperature above which a gas cannot be liquefied regardless of how much pressure is applied. When intermolecular attractive forces are stronger, it becomes harder for the gas molecules to be separated from each other, allowing liquefaction at higher temperatures. Thus, we expect the critical temperature to increase in magnitude as intermolecular attractive forces increase.

Key concepts

Vapor Pressure

Vapor pressure refers to the tendency of molecules within a liquid to escape into the gas phase. When a liquid is placed in a closed container, some molecules will naturally evaporate and form a vapor above the liquid. This process continues until equilibrium is reached, where the rate of evaporation equals the rate of condensation. The pressure exerted by this vapor is called vapor pressure.

Intermolecular forces play a key role in vapor pressure. When these forces are strong, they hold molecules tightly in the liquid phase, making it difficult for them to escape. As a result, the vapor pressure is lower. On the other hand, weaker intermolecular forces allow more molecules to vaporize, leading to a higher vapor pressure. Thus, substances with strong intermolecular forces generally have low vapor pressures.

Heat of Vaporization

The heat of vaporization is the amount of energy required to convert a given quantity of a liquid into its vapor at constant temperature and pressure. This process involves breaking the intermolecular forces that hold the liquid molecules together so they can transition to the gaseous state.

When intermolecular forces are strong, more energy is needed to overcome them, resulting in a higher heat of vaporization. This extra energy helps separate tightly-bound liquid molecules during evaporation. Example factors influencing this include hydrogen bonding and van der Waals forces. In contrast, weaker intermolecular forces mean less energy is needed, leading to a lower heat of vaporization. Understanding this concept helps explain why different substances require varying amounts of energy to boil.

Boiling Point

The boiling point of a liquid is the temperature at which it changes to a gas because its vapor pressure equals the surrounding atmospheric pressure. At this temperature, bubbles of vapor form within the liquid and rise to the surface.

The strength of intermolecular forces directly affects the boiling point. When these forces are strong, they prevent molecules from easily entering the gas phase, requiring a higher temperature to match the atmospheric pressure. Hence, a higher boiling point. Conversely, if intermolecular forces are weak, a lower temperature is sufficient for boiling, leading to a lower boiling point. This is why water (with hydrogen bonds) has a higher boiling point than many other small molecules.

Freezing Point

The freezing point of a substance is the temperature at which its liquid turns into a solid as its molecules arrange into a structured crystal lattice. When intermolecular forces are strong, they promote adherence between molecules even at higher temperatures.

This makes forming a solid structure easier, often resulting in a higher freezing point because less energy needs to be extracted from the liquid to freeze it. This means less heat is needed to reduce molecular motion to the extent that a solid can form. For substances with weaker forces, the freezing point will occur at a lower temperature, as more energy must be removed to solidify the liquid.

Viscosity

Viscosity is a measure of a liquid's resistance to flow. It essentially describes how "thick" or "sticky" the liquid is. If you've ever poured honey, you'd notice it flows slowly—demonstrating high viscosity—compared to water, which flows quickly because of its low viscosity.

Strong intermolecular forces increase a liquid's viscosity, making it more resistant to flow because molecules are attracted to each other more tightly. They struggle to slide past one another, hindering the fluidity. Therefore, liquids with strong hydrogen bonds, for instance, tend to be more viscous. In contrast, liquids with weaker intermolecular attractions flow easily due to lower viscosity.

Surface Tension

Surface tension is the measure of the energy required to increase the surface area of a liquid due to intermolecular forces. It's what allows small insects to walk on water or droplets to form on leaves.

Strong intermolecular forces create a stronger "skin" on the surface of the liquid, which needs more energy to be penetrated or stretched, resulting in high surface tension. Molecules at the surface experience a net downward force, pulling them closer together and thus reducing the surface area. Conversely, in liquids with weak intermolecular attractions, surface tension is lower, making the liquid surface more easily distortable.

Critical Temperature

Critical temperature is the highest temperature at which it is possible to liquefy a gas by applying pressure. Beyond this temperature, the kinetic energy of the molecules is too high for any amount of pressure to bring them together into a liquid form.

Strong intermolecular forces allow gases to remain in a condensed state (as a liquid) at higher temperatures, leading to a higher critical temperature. This is because these forces allow molecules to resist the thermal motion that normally separates them. Thus, substances that have strong intermolecular forces will have a higher critical temperature, meaning they can be liquefied at relatively high temperatures with adequate pressure.