12.2.3 Vapor Pressure and Boiling Point

Question 1

Vapor Pressure and Boiling Point

Answer 1

• A liquid is in equilibrium with its vapor when the rate of molecules leaving a liquid equals the rate of molecules returning to the liquid in a closed system.
• The vapor pressure curve is a plot that relates vapor pressure and temperature.
• Although evaporation is a phase change, boiling is an equilibrium process that occurs when the vapor pressure has exceeded the atmospheric pressure.

Question 2

note

Answer 2

• Vapor pressure (P vap ) is the pressure exerted by a gas in equilibrium with a liquid (or solid) at a given temperature.
• A liquid is in equilibrium with its vapor when the rate of molecules leaving a liquid equals the rate of molecules returning to the liquid in a closed system.
• Vapor pressure of a liquid at a given temperature is an intensive property (a characteristic of that liquid) which
  depends only on the temperature and the composition of the liquid.
• At a high enough temperature, all of the liquid will move into the gas phase.
• The vapor pressure curve is a plot that relates vapor pressure and temperature. Vapor pressure depends only on the temperature and the composition of the liquid.
• A liquid’s vapor pressure curve is useful in predicting the temperature at which a liquid will boil.
• The normal boiling point is the temperature at which a liquid’s vapor pressure equals one atmosphere.
• The stronger the intermolecular forces, the higher the boiling point of the substance. When water reaches a boil, bubbles form and rise to the surface.
• The water boils when the vapor pressure overcomes the atmospheric pressure.
• Evaporation is liquid converting into gas at room
  temperature. At a high enough temperature, all of the water will move into the gas phase.
• Although evaporation is a phase change, boiling is an equilibrium process that occurs when the vapor pressure has exceeded the atmospheric pressure.

Question 3

When is the boiling point of a liquid reached?

Answer 3

When vapor pressure equals atmospheric pressure

Question 4

A pressure cooker allows foods to be cooked at a higher pressure, above atmospheric pressure. What effect does this have on the boiling point of the liquid in the pressure cooker?

Answer 4

The pressure cooker raises the boiling point of the liquid.

Question 5

Certain factors can alter at what temperature a liquid boils. To what does the normal boiling point of a liquid refer?

Answer 5

The boiling point at atmospheric pressure

Question 6

Which of the following is not a difference between boiling and evaporation?

Answer 6

Evaporation involves a change in phase, but boiling does not.

Question 7

What do you need to add into a liquid system that has a higher intermolecular attraction to cause it to boil?

Answer 7

Energy

Question 8

For a given liquid, on what does vapor pressure depend?

Answer 8

Temperature

Question 9

At equilibrium, how does the number of molecules leaving the surface of the liquid compare to the number of molecules entering the surface of the liquid?

Answer 9

The number of molecules leaving is equal to the number entering

Question 10

Evaporation can occur at any temperature. Which of the following is the best explanation of this phenomenon?

Answer 10

Statistically, at any temperature, there will always be some molecules with enough energy to escape

Question 11

At a certain temperature, a liquid in a container is at equilibrium with its vapor. The volume of the container is suddenly increased and the system is allowed to come to a new equilibrium at the same temperature. At the new equilibrium, both liquid and vapor phases are still present but the volume of the liquid is less. Which of the following statements best describes what happens to the vapor pressure?

Answer 11

Since the vapor pressure depends only on temperature, the new vapor pressure is the same.

Question 12

What happens to vapor pressure when the surface area is increased?

Answer 12

Surface area has no effect on vapor pressure.