Question 1

Which of the following statements best explains why the vapor pressure of a liquid increases with temperature?

Accepted Answer

As temperature increases, a greater fraction of molecules have enough kinetic energy to overcome intermolecular forces and escape into the vapor phase.

Question 2

Which of the following liquids would you expect to have the highest normal boiling point based on the data below?

Water: $$\Delta H_{vap} = 40.7 kJ/$$mol
Isopropyl alcohol: $$\Delta H_{vap} = 39.9 kJ/$$mol
Acetone: $$\Delta H_{vap} = 29.1 kJ/$$mol
Diethyl ether: $$\Delta H_{vap} = 26.5 kJ/$$mol

Accepted Answer

Water

Question 3

A sealed flask contains liquid water and water vapor at equilibrium. What will happen to the vapor pressure if the temperature is increased?

Accepted Answer

The vapor pressure will increase because more molecules have enough energy to escape into the vapor phase.

Question 4

Which of the following best describes the process occurring at the flat (horizontal) portion of a heating curve for water at 100°C?

Accepted Answer

Liquid water is being converted to water vapor at constant temperature.

Question 5

Which of the following phase changes is exothermic?

Accepted Answer

Condensation

Question 6

The Clausius-Clapeyron equation relates vapor pressure and temperature as follows:

$$\ln P_{vap} = -\frac{\Delta H_{vap}}{R} \frac{1}{T} + \ln \beta$$

What is the physical meaning of the slope in a plot of $$\ln P_{vap}$$ versus $$1/T$$?

Accepted Answer

The slope is equal to $$-\Delta H_{vap}/R.$$

Question 7

Which of the following best explains why water has a higher boiling point than ethanol, even though their molar masses are similar?

Accepted Answer

Water has more extensive hydrogen bonding than ethanol.

Question 8

A sample of methanol ($$\Delta H_{vap} = 35.2 kJ/$$mol) has a vapor pressure of 760 torr at 64.6°C. What is the vapor pressure at 15.0°C? (Use $$R = 8.314 J/$$mol·K)

Accepted Answer

88 torr

Question 9

Which of the following statements about phase diagrams is correct?

Accepted Answer

The critical point is the highest temperature and pressure at which a liquid and vapor can coexist as distinct phases.

Question 10

Which of the following best describes a supercritical fluid?

Accepted Answer

A state above the critical temperature and pressure where liquid and gas phases are indistinguishable.

Question 11

Which of the following would increase the rate of vaporization of a liquid?

Accepted Answer

Increasing the surface area of the liquid

Question 12

Which of the following correctly describes the relationship between intermolecular forces (IMFs), vapor pressure (VP), and boiling point (BP)?

Accepted Answer

Stronger IMFs lead to lower VP and higher BP.

Question 13

Which of the following phase changes is endothermic?

Accepted Answer

Sublimation

Question 14

Which property would you expect to be highest for ethylene glycol (HOCH$$_2CH$_2OH) $$compared to propanol (CH$$_3CH$_2CH$_2OH$$)?

Accepted Answer

Boiling point

Question 15

Which of the following best explains why the solid/liquid equilibrium line in the phase diagram of water slopes to the left?

Accepted Answer

The liquid phase of water is denser than the solid phase.

Question 16

Which of the following is the correct expression for the two-point form of the Clausius-Clapeyron equation?

Accepted Answer

$$\ln \frac{P_2$}{P_1$} = -\frac{\Delta H_{vap}}{R} \left( \frac{1}{T_2$} - \frac{1}{T_1$} \right)$$

Question 17

Which of the following is true about the normal boiling point of a liquid?

Accepted Answer

It is the temperature at which the vapor pressure equals 1 atm.

Question 18

Which of the following best describes what happens to the boiling point of water as external pressure decreases (e.g., at high altitude)?

Accepted Answer

The boiling point decreases.

Question 19

Which of the following is the correct relationship for the enthalpy of sublimation ($$\Delta H_{sub}$$)?

Accepted Answer

$$\Delta H_{sub} = \Delta H_{vap} + \Delta H_{fus}$$

Solids, Liquids, and Intermolecular Forces: Vapor Pressure, Boiling Point, and Phase Changes

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