Problem 20 - Chemistry and Chemical Reactivity

Question

Answer each of the following questions with increases, decreases, or does not change. (a) If the intermolecular forces in a liquid increase, the normal boiling point of the liquid ____. (b) If the intermolecular forces in a liquid decrease, the vapor pressure of the liquid ____. (c) If the surface area of a liquid decreases, the vapor pressure ____. (d) If the temperature of a liquid increases, the equilibrium vapor pressure ____.

Step-by-Step Solution

Answer

The boiling point increases, vapor pressure increases, vapor pressure does not change, and equilibrium vapor pressure increases.

Step 1: Determine the Effect of Stronger Intermolecular Forces on Boiling Point

When intermolecular forces in a liquid increase, molecules hold onto each other more tightly. Thus, more heat (higher temperature) is required for the molecules to escape into the gas phase, thereby increasing the boiling point.

Step 2: Determine the Effect of Weaker Intermolecular Forces on Vapor Pressure

When intermolecular forces decrease, it becomes easier for molecules to escape the liquid and enter the gas phase. This results in an increase in vapor pressure as more molecules contribute to the gaseous phase.

Step 3: Consider the Impact of Surface Area on Vapor Pressure

The vapor pressure of a liquid is determined by the volatility of the substance, not the surface area. Therefore, changes in surface area do not affect the vapor pressure.

Step 4: Analyze the Relationship Between Temperature and Equilibrium Vapor Pressure

As temperature increases, the kinetic energy of the liquid molecules also increases, making it easier for them to escape into the gas phase. Hence, the equilibrium vapor pressure rises with an increase in temperature.

Key Concepts

Boiling Point

The boiling point of a liquid is the temperature at which the liquid's vapor pressure equals the external atmospheric pressure. This is influenced significantly by the intermolecular forces present. When intermolecular forces strengthen, molecules are more attracted to each other, making it more difficult for them to break free and transition into the gas phase.
As a result, more energy (in the form of heat) is needed to overcome these forces, leading to an increase in the boiling point.

• Stronger intermolecular forces imply a higher boiling point.
• Weaker intermolecular forces imply a lower boiling point.

Understanding this helps predict how a liquid behaves under different conditions, which is crucial in various scientific and chemical applications.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase. It is a measure of a liquid's tendency to evaporate. When intermolecular forces in a liquid decrease, molecules can escape from the liquid surface more easily and enter the vapor phase; thus, increasing the vapor pressure.

• Strong intermolecular forces result in low vapor pressure.
• Weak intermolecular forces result in high vapor pressure.

This concept is essential in understanding how volatile a substance is, which is vital for applications like predicting climatic conditions or processing industrial chemicals.

Temperature Effects

Temperature directly impacts the kinetic energy of a liquid's molecules. As the temperature rises, molecules move faster, gaining enough energy to overcome intermolecular attractions and evaporate. Consequently, the vapor pressure increases with temperature.

• Higher temperature increases vapor pressure.
• Lower temperature decreases vapor pressure.

This relationship is used in practical scenarios like food preservation using evaporative cooling or designing pressure containers, making it crucial for various scientific and industrial settings.

Surface Area Impact

Contrary to intuition, the surface area of a liquid does not influence its vapor pressure. Although it might seem that more surface allows more molecules to escape, vapor pressure is determined by the substance's intrinsic properties and temperature, not the surface size.

• Surface area does not affect vapor pressure.
• Vapor pressure is mainly influenced by intermolecular forces and temperature.

This understanding is key when addressing misconceptions concerning evaporation in large bodies of water versus small ones, ensuring clear predictions in environmental and chemical contexts.