Problem 73 - Chemistry The Central Science In Si Units

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, \((\mathrm{g})\) critical temperature.

Step-by-Step Solution

Answer

(a) Decrease, (b) Increase, (c) Increase, (d) Increase, (e) Increase, (f) Increase, (g) Increase.

Step 1: Understanding Vapor Pressure

Vapor pressure refers to the pressure exerted by a vapor in equilibrium with its liquid phase. When intermolecular forces increase, fewer molecules have the energy to escape the liquid and become vapor. As a result, we expect the vapor pressure to decrease.

Step 2: Examining Heat of Vaporization

The heat of vaporization is the energy required to convert a liquid to a gas. Stronger intermolecular forces mean more energy is needed to overcome these forces to vaporize. Therefore, as intermolecular forces increase, the heat of vaporization also increases.

Step 3: Analyzing Boiling Point

The boiling point is the temperature at which a liquid's vapor pressure equals the external pressure. With stronger intermolecular forces, a higher temperature is needed to reach this pressure, leading to an increase in boiling point.

Step 4: Considering Freezing Point

The freezing point is the temperature at which a liquid becomes solid. Stronger intermolecular forces generally make a substance more structurally stable, thus increasing the temperature needed to freeze it. Therefore, the freezing point increases with stronger intermolecular forces.

Step 5: Assessing Viscosity

Viscosity is a measure of a liquid's resistance to flow. Stronger intermolecular forces mean that molecules are more tightly held together, which increases resistance to flow and leads to an increase in viscosity.

Step 6: Observing Surface Tension

Surface tension arises from molecules at a liquid's surface being attracted inwards. With increased intermolecular forces, this inward attraction is stronger, thus increasing surface tension.

Step 7: Understanding Critical Temperature

The critical temperature is the highest temperature at which a substance can exist as a liquid. With stronger intermolecular forces, a substance can remain a liquid even at higher temperatures, thus raising the critical temperature.

Key Concepts

Vapor Pressure

Vapor pressure is an essential concept in chemistry, referring to the pressure exerted by vapor in equilibrium with its liquid form. When the intermolecular forces between molecules are strong, fewer molecules have the energy to escape from the liquid state to the vapor phase. This results in a lower vapor pressure.
Understanding vapor pressure helps us predict how liquids will behave under different conditions. For example, liquids with weak intermolecular forces, like ether, have high vapor pressures. They evaporate quickly and may even boil at room temperature.

• If you know the vapor pressure, you can determine how volatile a liquid is.
• Low vapor pressure indicates strong intermolecular forces.
• Strong forces mean that molecules tend to stay in the liquid phase.

Heat of Vaporization

The heat of vaporization is the energy required to transform a liquid into a gas at its boiling point. It showcases how much energy is needed to overcome intermolecular forces in the liquid. Stronger intermolecular forces mean a higher heat of vaporization is needed.
In practical terms, this implies that liquids like water, which have higher heats of vaporization, require more energy to change into gases compared to substances like alcohol.

• An increased heat of vaporization indicates stronger cohesive forces.
• This results in a greater amount of energy to separate the molecules.
• Substances with high heat of vaporization evaporate slower.

Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure surrounding it. For substances with stronger intermolecular forces, the boiling point is higher. This is because more energy (in the form of heat) is needed to overcome these forces to convert the liquid to vapor.
This phenomenon explains why water, with strong hydrogen bonds, has a much higher boiling point compared to a liquid like propane, which is held together by weaker van der Waals forces.

• High boiling points signify strong intermolecular bonds.
• The need for higher temperatures to break these bonds leads to an increase in boiling points.

Freezing Point

The freezing point is the temperature at which a liquid turns into a solid. With stronger intermolecular forces, the energy required to move molecules into a rigid, organized solid structure decreases, leading to an increase in the freezing point.
This is why substances like water, which have hydrogen bonding, have relatively high freezing points compared to substances with weaker intermolecular forces.

• Stronger forces create stable molecular structures.
• This stability increases the freezing point.
• A higher freezing point means a lower tendency to remain a liquid at lower temperatures.

Viscosity

Viscosity measures how quickly a liquid flows. It reflects the internal friction within a liquid that resists movement. Stronger intermolecular forces hold molecules tightly together, reducing flow, and increasing viscosity.
Consider the difference between honey and water. Honey, with its high viscosity, flows much more slowly than water, indicating stronger intermolecular attractions within the liquid.

• High viscosity means slow flow.
• Stronger intermolecular forces lead to higher viscosity.
• Temperature influences viscosity, with most liquids becoming less viscous as they warm up.

Surface Tension

Surface tension describes the elastic tendency of a fluid’s surface, allowing it to resist external force. This occurs because molecules on the surface are pulled inward by strong cohesive forces. With stronger intermolecular forces, surface tension is higher, meaning the liquid's surface is more resistant to breaking.
This principle is observable when you see small insects, like water striders, walking on water. Their legs do not break through the water's surface due to its strong surface tension.

• High surface tension indicates strong cohesive forces.
• It can be observed in water droplets forming on a surface.
• Increased tension makes surfaces more pulled together and stable.

Critical Temperature

The critical temperature of a substance is the maximum temperature at which it can exist as a liquid. Strong intermolecular forces mean that a substance can remain in the liquid state even at higher temperatures, thus increasing its critical temperature.
For instance, substances with strong molecular attractions, like water, have higher critical temperatures, allowing them to resist vaporization at higher thermal energy compared to substances with weaker forces.

• Critical temperature is key for supercritical fluid formation.
• Helps in determining the limits of a substance's phase.
• Supports the understanding of phase diagrams.