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 intermolecular attractive forces increase, the following properties are affected: (a) vapor pressure decreases, (b) heat of vaporization increases, (c) boiling point increases, (d) freezing point increases, (e) viscosity increases, (f) surface tension increases, and (g) critical temperature increases.

Step-by-step solution

(a) Vapor Pressure

When the intermolecular attractive forces between molecules increase, it becomes more difficult for the molecules to escape from the liquid phase and enter the gas phase. As a result, the vapor pressure will decrease with an increase in intermolecular attractive forces.

(b) Heat of Vaporization

The heat of vaporization is the amount of energy required to convert a substance from its liquid phase to its vapor phase. When the intermolecular attractive forces increase, it takes more energy to overcome these forces and cause the phase change. Therefore, the heat of vaporization will increase with an increase in intermolecular attractive forces.

(c) Boiling Point

The boiling point of a substance is the temperature at which the vapor pressure equals the atmospheric pressure. As the vapor pressure decreases with an increase in intermolecular attractive forces, the temperature at which this occurs will also increase. Thus, the boiling point will increase with an increase in intermolecular attractive forces.

(d) Freezing Point

The freezing point is the temperature at which a substance changes from a liquid to a solid. When intermolecular attractive forces increase, molecules are more strongly bound together, and it takes less energy to arrange the molecules in an ordered (solid) structure. This causes the freezing point to increase with an increase in intermolecular attractive forces.

(e) Viscosity

Viscosity is the resistance of a fluid to flow. When the intermolecular attractive forces increase, the fluid molecules are more strongly bound together, and it becomes more difficult for them to flow past each other. Thus, the viscosity increases with an increase in intermolecular attractive forces.

(f) Surface Tension

Surface tension is the force that holds the surface of a liquid together. When the intermolecular attractive forces increase, the force holding the surface together also increases. Therefore, surface tension increases with an increase in intermolecular attractive forces.

(g) Critical Temperature

The critical temperature is the highest temperature at which a substance can exist as a liquid. When the intermolecular attractive forces increase, it takes more energy to cause the phase change from liquid to vapor. This means that the critical temperature will also increase with an increase in intermolecular attractive forces.

Key concepts

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It is a crucial concept in understanding how substances behave when they are subjected to changes in temperature. When intermolecular forces are strong, molecules are held more tightly within the liquid phase. As a result, fewer molecules escape into the gas phase, which reduces vapor pressure. Key points to remember about vapor pressure:

• Vapor pressure decreases with stronger intermolecular forces.
• A lower vapor pressure indicates that fewer molecules are in the gas phase.
• Temperature can affect vapor pressure significantly.

An example of vapor pressure in action is how water evaporates at room temperature, demonstrating a balance between the liquid and vapor phases.

Heat of Vaporization

The heat of vaporization is the amount of energy necessary to change a liquid into a gas at its boiling point. It reflects the strength of intermolecular forces in a substance since more energy is required to overcome stronger attractions between molecules. When you heat a liquid, you're supplying energy to break these interactions, converting it into vapor. Factors affecting heat of vaporization include:

• Substances with strong intermolecular forces have a higher heat of vaporization.
• A higher heat of vaporization means more energy is needed for the phase change.
• Comparing substances, those with similar structures often have similar heats of vaporization.

This concept helps explain why some substances require much more energy to evaporate than others, such as the difference between water and alcohol.

Boiling Point

The boiling point is the temperature at which a liquid's vapor pressure equals the external pressure, allowing it to transition into a gas. It's a direct reflection of intermolecular attractions: stronger forces mean a higher boiling point. Understanding boiling points is essential in processes like distillation, where separating components of a mixture based on their boiling points is crucial. Important notes about boiling points:

• Substances with strong intermolecular forces have higher boiling points.
• Boiling point is pressure-dependent; it's lower at high altitudes.
• Increased boiling points indicate more energy is required to transition to the gas phase.

Boiling water on a mountain, which occurs at lower temperatures due to reduced atmospheric pressure, is a practical manifestation of this concept.

Freezing Point

Freezing point is the temperature at which a liquid becomes solid. It's influenced by the strength of intermolecular forces: stronger attractions mean molecules are more easily arranged in a solid structure, raising the freezing point. Knowing the freezing point is vital in applications such as food preservation and antifreeze technology. Essential insights into freezing point:

• Stronger intermolecular forces usually cause a higher freezing point.
• A higher freezing point indicates that less energy is needed to form a solid.
• Freezing point can change with impurities in the liquid (e.g., salt in water).

Understanding freezing points can help explain why substances like salt water freeze at lower temperatures than pure water.

Viscosity

Viscosity is a measure of a liquid's resistance to flow. It's heavily influenced by intermolecular forces, as stronger forces hinder the movement of molecules past each other, resulting in higher viscosity. This property is important in industries such as lubrication, where the flow of liquids determines performance. Crucial points about viscosity:

• Increased intermolecular forces lead to higher viscosity.
• Higher viscosity means a liquid flows more slowly.
• Viscosity is temperature dependent; it generally decreases as temperature increases.

Motor oil, which becomes less viscous (flows more easily) when heated, is an example of viscosity affected by temperature.

Surface Tension

Surface tension is the energy or force at the surface of a liquid, pulling molecules together and resisting external force. Stronger intermolecular forces lead to a higher surface tension because molecules at the surface are more tightly bound to each other. Surface tension affects phenomena such as the formation of droplets and the ability of insects to walk on water. Key aspects of surface tension:

• Higher surface tension is due to stronger intermolecular forces.
• It enables phenomena like "water beading" on surfaces.
• Surface tension decreases with increasing temperature.

Surface tension is the reason why some objects can float on water even if they are denser, owing to the liquid's strong molecular cohesion at its surface.

Critical Temperature

Critical temperature is the highest temperature at which a substance can exist as a liquid, regardless of pressure. It signifies the point at which the intermolecular forces are overcome completely, allowing the transition to a gas. The stronger the intermolecular forces, the higher the critical temperature, as more energy is required for molecular separation. Highlights of critical temperature include:

• Substances with strong intermolecular forces have higher critical temperatures.
• Above this temperature, a gas cannot be liquefied by pressure alone.
• It's a crucial parameter for the liquefaction of gases in industrial processes.

Understanding critical temperatures informs decisions in areas such as chemical engineering and the storage of gases under pressure.