Q: A student is asked to calculate the amount of heat required to vaporize $2.00\ \mathrm{mol}$ of water at its boiling point. The heat of vaporization of water is $40.7\ \mathrm{kJ/mol}$. What is the correct calculation?

$q = n \times \Delta H_{\mathrm{vap}} = 2.00\ \mathrm{mol} \times 40.7\ \mathrm{kJ/mol} = 81.4\ \mathrm{kJ}$

Question 1

Which of the following best explains why ice floats on liquid water?

Accepted Answer

Ice has a lower density than liquid water due to its open crystalline structure formed by hydrogen bonding.

Question 2

Which of the following substances would you expect to have the highest boiling point, and why?

Accepted Answer

HF, because it can form hydrogen bonds, which are stronger than dipole–dipole and dispersion forces.

Question 3

Which of the following best describes the process of vaporization?

Accepted Answer

It is an endothermic process in which molecules at the surface of a liquid gain enough energy to overcome intermolecular forces and become gas.

Question 4

A sample of water is heated from $0^\circ \mathrm{C}$ to $100^\circ \mathrm{C}$ and then boiled. Which segment of the heating curve corresponds to the greatest energy input for a single mole of water?

Accepted Answer

The segment where water is boiling (liquid to gas), because the heat of vaporization is much larger than the heat required to raise the temperature or melt the solid.

Question 5

Which of the following statements about intermolecular forces is correct?

Accepted Answer

Hydrogen bonds are stronger than dipole–dipole forces but weaker than ion–dipole forces.

Question 6

A liquid rises in a thin glass tube, forming a concave meniscus. Which statement best explains this observation?

Accepted Answer

The adhesive forces between the liquid and glass are stronger than the cohesive forces within the liquid.

Question 7

Which of the following changes will increase the vapor pressure of a liquid in a closed container?

Accepted Answer

Increasing the temperature of the liquid.

Question 8

Which of the following best describes why pentane and water are immiscible?

Accepted Answer

Water molecules are held together by strong hydrogen bonds, which pentane molecules cannot disrupt due to their nonpolarity.

Question 9

A student is asked to calculate the amount of heat required to vaporize $2.00\ \mathrm{mol}$ of water at its boiling point. The heat of vaporization of water is $40.7\ \mathrm{kJ/mol}$. What is the correct calculation?

Accepted Answer

$q = n 	imes \Delta H_{\mathrm{vap}} = 2.00\ \mathrm{mol} 	imes 40.7\ \mathrm{kJ/mol} = 81.4\ \mathrm{kJ}$

Question 10

Which of the following best describes the effect of increasing atmospheric pressure on the boiling point of water?

Accepted Answer

The boiling point increases because a higher temperature is required for the vapor pressure to equal the external pressure.

Question 11

Which of the following molecules would have only dispersion forces as its intermolecular force?

Accepted Answer

$\mathrm{CCl}_4$

Question 12

A sample of carbon dioxide is at $-80^\circ \mathrm{C}$ and $0.50\ \mathrm{atm}$. It is warmed to $-40^\circ \mathrm{C}$ at constant pressure. According to the phase diagram, what phase transitions occur?

Accepted Answer

Solid to gas (sublimation)

Question 13

Which of the following statements about the Clausius–Clapeyron equation is correct?

Accepted Answer

It relates the natural logarithm of vapor pressure to the inverse of temperature and can be used to determine the heat of vaporization.

Question 14

Which of the following best explains why the viscosity of a liquid decreases with increasing temperature?

Accepted Answer

Increased temperature increases the average kinetic energy, making it easier for molecules to overcome intermolecular forces and flow.

Question 15

Which of the following is true about dynamic equilibrium in a closed container containing a liquid and its vapor?

Accepted Answer

The rates of vaporization and condensation are equal, so the amount of liquid and vapor remains constant.

Question 16

Which of the following best describes the difference between crystalline and amorphous solids?

Accepted Answer

Crystalline solids have a regular, ordered structure, while amorphous solids lack long-range order.

Question 17

Which of the following is a real-life application of hydrogen bonding?

Accepted Answer

The high boiling point of water compared to other group 6A hydrides.

Question 18

Which of the following best explains why straight-chain alkanes have higher boiling points than their branched isomers?

Accepted Answer

Straight-chain alkanes have more surface-to-surface contact, leading to stronger dispersion forces.

Liquids, Solids, and Intermolecular Forces: Structure and Properties

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