Question

Liquid hexane \(\left(b p=69^{\circ} \mathrm{C}\right)\) is placed in a closed container at room temperature. At first, the pressure of the vapor phase increases, but, after a short time, it stops changing. Why?

Short answer

Vapor pressure stops changing when dynamic equilibrium between evaporation and condensation is reached.

Step-by-step solution

Identify the Initial Conditions

Hexane is placed in a closed container at room temperature. Note that the boiling point of hexane is 69°C.

Understand Vapor Pressure

When a liquid is placed in a closed container, it starts to evaporate. Initially, the vapor pressure increases as more liquid molecules enter the vapor phase.

Concept of Dynamic Equilibrium

As the pressure of the vapor phase increases, condensation also starts to occur. Eventually, the rate of evaporation equals the rate of condensation, leading to a dynamic equilibrium.

Explain the Equilibrium Concept in This Context

Once the dynamic equilibrium is reached, the number of molecules evaporating is equal to the number that are condensing, so the vapor pressure remains constant.

Link the Given Temperature to Equilibrium

Since the process is occurring at room temperature, which is below the boiling point, the equilibrium vapor pressure will be less than the vapor pressure at boiling point.

Conclusion

Therefore, the pressure of the vapor phase initially increases but eventually stops changing because the system reaches a dynamic equilibrium.

Key concepts

vapor pressure

When a liquid is placed in a closed container, some of its molecules will enter the vapor phase, becoming gas.
This process creates a pressure known as vapor pressure. Initially, if there is no gas present, the vapor pressure is zero.
As more liquid molecules escape into the vapor phase, the vapor pressure increases.
However, this increase doesn't continue indefinitely. At some point, a balance is reached.
At this balance, the vapor pressure becomes constant because the rate of liquid molecules evaporating into gas is equal to the rate of gas molecules condensing back into liquid.

It is important to understand that the vapor pressure largely depends on the temperature and the specific liquid.
Different liquids have different vapor pressures at the same temperature. For example, hexane's vapor pressure at room temperature will be different compared to water or alcohol at the same temperature.

evaporation

Evaporation is the process where molecules transition from the liquid phase to the vapor phase.
This happens when molecules at the surface of the liquid gain enough energy to escape into the air.
Evaporation leads to an increase in vapor pressure inside a closed container.
Notably, evaporation occurs at temperatures that are below the boiling point of a liquid.
In an open system, as molecules leave the liquid, they dissipate into the surroundings. But, in a closed system, they accumulate and increase the vapor pressure until equilibrium is reached.

Key points to remember about evaporation:

• It primarily occurs at the surface of the liquid.
• It does not require the temperature to reach the boiling point.
• The rate of evaporation is higher at higher temperatures and lower at lower temperatures.

condensation

Condensation is the reverse process of evaporation, where gas molecules lose energy and transition back into the liquid phase.
In a closed system, as more liquid evaporates, vapor pressure increases, and molecules in the vapor phase will start to collide more frequently.
This collision causes them to lose energy, returning to the liquid form.
Condensation is crucial for dynamic equilibrium. As more molecules condense out of the vapor phase, the vapor pressure begins to stabilize.

Important aspects of condensation include:

• Occurs when gas molecules lose energy and transition to the liquid state.
• Helps in establishing dynamic equilibrium alongside evaporation.
• Influenced by temperature; lower temperatures favor condensation.

boiling point

The boiling point of a liquid is the temperature at which its vapor pressure equals the external atmospheric pressure.
At this point, the liquid boils and transitions rapidly to the vapor phase. For hexane, the boiling point is 69°C.
Remember that below the boiling point, such as at room temperature, evaporation happens slower because the vapor pressure is lower.

Noteworthy points about boiling point:

• The boiling point depends on the atmospheric pressure; at higher altitudes, liquids boil at lower temperatures.
• The boiling point is the specific temperature where the liquid phase changes to vapor throughout the substance, not just at the surface.
• At the boiling point, dynamic equilibrium between evaporation and condensation doesn't apply because the rapid vaporization dominates the process.