Question

Explain the effect of strong intermolecular forces on each of these parameters: (a) critical temperature; (b) boiling point; (c) vapor pressure; (d) heat of vaporization.

Short answer

Strong intermolecular forces increase critical temperature and boiling point, but decrease vapor pressure and increase heat of vaporization.

Step-by-step solution

Understanding Critical Temperature

Critical temperature is the temperature above which a gas cannot be liquefied, no matter how much pressure is applied. Strong intermolecular forces increase the critical temperature because stronger attractions between molecules require more energy (higher temperature) to overcome and turn a liquid into gas.

Examining Boiling Point

Boiling point is the temperature at which a liquid turns into vapor. Strong intermolecular forces cause a higher boiling point since more energy (higher temperature) is needed to break these forces for the liquid to become a gas.

Analyzing Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature. Strong intermolecular forces result in lower vapor pressure because fewer molecules have enough energy to escape the liquid phase and enter the vapor phase.

Investigating Heat of Vaporization

Heat of vaporization is the amount of heat required to convert a liquid into a gas at constant temperature and pressure. Strong intermolecular forces lead to a higher heat of vaporization since more energy is needed to break these forces and change the state from liquid to gas.

Key concepts

Critical Temperature

Critical temperature is a crucial concept in thermodynamics. It represents the temperature above which a gas cannot be turned into a liquid, regardless of the pressure applied. This happens because above the critical temperature, the kinetic energy of the molecules is too high for intermolecular forces to pull them together into a liquid state.
Strong intermolecular forces increase the critical temperature. Why? Because more energy (and thus a higher temperature) is needed to overcome these forces. Think of it as trying to pull apart two magnets that are stuck together tightly. The stronger the magnetic force, the more effort you need to separate them. Similarly, with stronger intermolecular forces, you need more energy to transform a liquid into a gas, pushing the critical temperature higher.

Boiling Point

The boiling point is the temperature at which a liquid becomes a gas. At this point, the vapor pressure of the liquid equals the external pressure surrounding the liquid. When intermolecular forces are strong, the boiling point rises. This is because stronger forces between molecules mean more energy is needed to separate these molecules and allow them to escape into the gas phase.
Imagine water versus oil. Water has strong hydrogen bonds, while oil has weaker van der Waals forces. As a result, water has a higher boiling point than oil. Hence, substances with strong intermolecular forces, like water, need a higher temperature (boiling point) to transition from liquid to gas.

Vapor Pressure

Vapor pressure is the pressure created by a vapor in equilibrium with its liquid or solid phase at a specific temperature. It indicates how readily molecules escape from the liquid phase to the vapor phase. Strong intermolecular forces result in lower vapor pressure. This occurs because fewer molecules have enough energy to break free from the liquid phase and enter the vapor phase.
Let's take water again as an example. The strong hydrogen bonds in water restrict many molecules from escaping into the vapor phase, resulting in lower vapor pressure compared to liquids with weaker intermolecular forces. Therefore, the stronger the intermolecular force, the lower the vapor pressure.

Heat of Vaporization

Heat of vaporization is the amount of heat required to turn a liquid into a gas at constant temperature and pressure. It's a measure of the energy needed to overcome intermolecular forces and convert a liquid into its gaseous state. Strong intermolecular forces lead to a higher heat of vaporization because more energy is required to break these forces.
Considers water and alcohol. Water, with its strong hydrogen bonds, has a higher heat of vaporization compared to alcohol, which has weaker forces holding its molecules together. Breaking the strong bonds in water requires more energy, hence a higher heat of vaporization. In summary, stronger intermolecular forces equate to a higher heat of vaporization.