Q: Which equation correctly relates the heat required to vaporize a given mass of a substance to its heat of vaporization ($$\Delta H_{\mathrm{vap}}$$)?

$$q = n \times \Delta H_{\mathrm{vap}}$$

Question 1

Which of the following best explains why water has a much higher boiling point than other molecules of similar molar mass, such as methane ($$\mathrm{CH_4$}$$)?

Accepted Answer

Water forms strong hydrogen bonds between its molecules, requiring more energy to break.

Question 2

A liquid is observed to have high surface tension and low volatility. Which type of intermolecular force is most likely dominant in this liquid?

Accepted Answer

Hydrogen bonding

Question 3

Which of the following statements about vapor pressure is correct?

Accepted Answer

Vapor pressure increases as intermolecular forces decrease.

Question 4

A heating curve for a substance shows two plateaus. What is occurring at these plateaus?

Accepted Answer

The substance is undergoing phase changes: melting at the first plateau and boiling at the second.

Question 5

Which equation correctly relates the heat required to vaporize a given mass of a substance to its heat of vaporization ($$\Delta H_{\mathrm{vap}}$$)?

Accepted Answer

$$q = n 	imes \Delta H_{\mathrm{vap}}$$

Question 6

Which of the following substances would you expect to have the highest heat of fusion ($$\Delta H_{\mathrm{fus}}$$)?

Accepted Answer

Sodium chloride ($$\mathrm{NaCl}$$), an ionic solid

Question 7

Which type of intermolecular force is present in all substances, regardless of their molecular structure?

Accepted Answer

Dispersion forces (London forces)

Question 8

A sample of ice is heated until it completely melts. Which process is occurring, and what is the sign of $$\Delta H$$ for this process?

Accepted Answer

Melting; $$\Delta H is $$positive

Question 9

Which of the following best describes why water expands when it freezes?

Accepted Answer

Hydrogen bonds arrange water molecules into an open hexagonal structure, increasing volume.

Question 10

Which of the following is a real-life example of sublimation?

Accepted Answer

Dry ice ($$\mathrm{CO_2$}) $$turning directly into gas at room temperature

Question 11

Which type of solid is diamond, and what is the primary force holding its structure together?

Accepted Answer

Covalent atomic solid; covalent bonds

Question 12

Which of the following statements about viscosity is correct?

Accepted Answer

Viscosity increases as intermolecular forces become stronger.

Question 13

Which of the following best describes the relationship between heat of fusion ($$\Delta H_{\mathrm{fus}}) $$and heat of vaporization ($$\Delta H_{\mathrm{vap}}) $$for most substances?

Accepted Answer

$$\Delta H_{\mathrm{vap}} is $$greater than $$\Delta H_{\mathrm{fus}}$$

Question 14

Which of the following molecules can exhibit hydrogen bonding?

Accepted Answer

Ammonia ($$\mathrm{NH_3$})$$

Question 15

Which property is most likely to increase when a liquid's intermolecular forces are weak?

Accepted Answer

Volatility

Question 16

Which of the following best describes the dynamic equilibrium in a closed container of liquid?

Accepted Answer

The rate of evaporation equals the rate of condensation.

Question 17

Which type of intermolecular force is responsible for the miscibility of ethanol and water?

Accepted Answer

Hydrogen bonding

Question 18

Which of the following solids is most likely to be amorphous?

Accepted Answer

Glass

Question 19

Which of the following best explains why ionic solids generally have high melting points?

Accepted Answer

Strong electrostatic forces between ions require large amounts of energy to break.

Question 20

A student wants to calculate the energy required to melt 50.0 g of ice at $$0^\circC. $$Which equation should they use, and what additional information is needed?

Accepted Answer

$$q = n 	imes \Delta H_{\mathrm{fus}}$$; they need the molar mass of water to convert grams to moles.

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