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# Phase Transition - A Review

Skills to Be Tested

Quiz problems and practice questions in this module cover these subjects:

• Phase changes
• Phase diagram, triple point, critical point
• Intermolecular forces
• Ionic interaction
• dipole-dipole interaction
• London dispersion forces
• hydrogen bonding
• Chemical bonds
• Heating and cooling curves

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

All these subjects involve the key concept of intermolecular forces. Refer to previous modules for these concepts.

This quiz covers aspects related to phase transitions or phase changes. You will encounter calculations and short-answer questions involving the following terms and concepts:

• freezing point, melting point
• simulation curve, vaporization curve
• simulation, deposition
• boiling point, condensation point
• cooling curve, heating curve
• heat of vaporization, molar heat of vaporization
• heat of sublimation, molar heat of sublimation
• heat capacity
• phase transition
• packing of spheres
• calculations of density, unit cell edges, number of atoms,
• distances between atoms for simple cubic packing,
• f.c.c packing, and body-centered cubic structures
• hexagonal closest packing

## Confidence Building Questions

1. Which list of processes gives all endothermic processes?
1. melting, freezing, deposition
2. sublimation, condensation, vaporization
3. sublimation, vaporization, melting
4. freezing, condensation, deposition

Hint: c.

Skill -
Identify endothermic phase changes.

2. What is the vapor pressure (in torr) for water at 373.15 K (the normal boiling point)?

Hint: 760 torr (1.0 atm)

Skill -
Explain boiling point (760 torr = 101.325 kPa).

3. What is the vapor pressure in kPa for water at 373 K?

Hint: 101.325 kPa

Skill -
Convert units of pressure.

4. The quantity of heat required to vaporize 1 mol of a liquid is called
1. the molar heat of fusion
2. the molar heat of melting
3. the molar heat of boiling
4. the molar heat of vaporization
5. the molar heat of sublimation

Hint: d.

Skill -
Explain molar heat of a process.

5. The heat of fusion for water is 80 cal/g. What is the molar heat in cal/mol of fusion for water? (Molar mass for $$\ce{H2O}$$ is 18.0 g/mol).

Hint: 80 cal/g * 18.0 g/mol = ? cal/mol

Skill -
Perform unit conversion.

6. The heat of fusion for water is 80 cal/g. What is the molar heat in kJ/mol of fusion for water? (Molar mass for $$\ce{H2O}$$ is 18.0 g/mol; 1 cal = 4.18 J).

Hint: 80 cal/g * 18.0 g/mol * 4.18 = ? J/mol ==> kJ/mol

Skill -
Perform unit conversion.

7. In a pressure cooker, the vapor pressure is greater than 1 atm. Under such conditions the boiling temperature of water will be
1. 373 K
2. lower than 373 K
3. higher than 373 K
4. 237 K

Hint: c. higher than 373 K

Discussion -
Examine how a pressure cooker increases the pressure. A pressure cooker saves time, because it cooks at a higher temperature.

8. At a pressure of 2 atm, the melting point of water is
1. 273 K
2. lower than 273 K
3. higher than 273 K
4. 273 C

Hint: b. lower than 273 K

Discussion -
Water is special in this regard. Under pressure, its m.p. decreases.

9. The triple point for $$\ce{CO2}$$ is located at a pressure of 5.2 atm and a temperature of 216 K. The melting point for dry ice at a pressure of 20 atm is
1. 216 K
2. lower than 216 K
3. higher than 216 K
4. 216 F

Hint: c. higher than 216 K

Discussion -
Only the m.p. of water decreases when the pressure increases. Like other substances, m.p. of $$\ce{CO2}$$ increases when pressure increases.

10. Temporarily induced dipole interaction between molecules gives rise to a net attractive force, which is called
1. van der Waals force
2. dipole-dipole force
3. attractive force
4. London dispersion force

Hint: d. London dispersion force

Discussion -
This question gives the definition of London dispersion force.

11. Which of the following molecules has the highest melting point? $$\ce{O2}$$$$\ce{F2}$$$$\ce{Cl2}$$$$\ce{Br2}$$, or $$\ce{I2}$$

Hint: iodine

Skill -
Predict trend in melting point. The higher the molar mass, the higher the melting point. The molecules are arranged in order of increasing molecular weight.

12. Among the intermolecular forces, dipole-dipole force, London dispersion force, ionic interaction, and hydrogen bonding, which is present in all substances?

Hint: London dispersion force

Discussion -
Ionic interaction, dipole-dipole interaction and hydrogen bonding are present only in some substances.

13. Which one of the following has the highest boiling point? $$\ce{N2}$$$$\ce{NO}$$, or $$\ce{O2}$$.

Hint: $$\ce{NO}$$

Discussion -
The b.p. for $$\ce{N2}$$,  $$\ce{NO}$$, and $$\ce{O2}$$ are 77 K, 121 K, and 90 K respectively.

14. Considering intermolecular forces, which compound would have the highest molar heat of vaporization?
1. $$\ce{CH3CH2OH}$$ (ethanol)
2. $$\ce{CH3CH2CH3}$$ (propane)
3. $$\ce{CH3CH3}$$ (ethane)

Hint: a.

Skill -
Explain hydrogen bonds. Hydrogen bonding causes the molar heat of vaporization to be higher since energy is required to break up the hydrogen bonds.

15. In which four of the following substances would you expect to find the strongest hydrogen bonding?
1. $$\ce{HF}$$
2. $$\ce{HI}$$
3. $$\ce{C2H5OH}$$
4. $$\ce{C2H2SH}$$
5. $$\ce{CH3COOH}$$
6. $$\ce{CH2O}$$
7. $$\ce{H2O}$$

Hint: a. c. e. g.

Skill -
Identify substances that have strong hydrogen bonding.
$$\ce{C2H5OH}$$,  $$\ce{HF}$$,  $$\ce{H2O}$$ and $$\ce{CH3COOH}$$ have strong hydrogen bonding. $$\ce{C2H2SH}$$ and $$\ce{CH2O}$$ have weak hydrogen bonding.