Phase Transitions - a review for Quiz 1.

Skills to be tested

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

     P1     Hvap    1     1
ln (---) = ----  (--- - ---)
     P2     R      T2    T1

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

• Which list of processes gives all endothermic processes?
  a. melting, freezing, deposition
  b. sublimation, condensation, vaporization
  c. sublimation, vaporization, melting
  d. freezing, condensation, deposition
  
  Hint . . . c.

  Skill -
  Identify endothermic phase changes.

• What is the vapour 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).

• What is the vapour pressure in kPa for water at 373 K?
  Hint . . . 101.325 kPa

  Skill -
  Convert units of pressure.

• The quantity of heat required to vaporize 1 mol of a liquid is called
  a. the molar heat of fusion
  b. the molar heat of melting
  c. the molar heat of boiling
  d. the molar heat of vaporization
  e. the molar heat of sublimation
  
  Hint . . . d.

  Skill -
  Explain molar heat of a process.

• 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 H₂O is 18.0 g/mol).
  Hint . . . 80 cal/g * 18.0 g/mol = ? cal/mol

  Skill -
  Perform unit conversion.

• 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 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 nit conversion

• In a pressure cooker, the vapour pressure is greater than 1 atm. Under such conditions the boiling temperature of water will be
  a. 373 K
  b. lower than 373 K
  c. higher than 373 K
  d. 237 K
  
  Hint . . . c. higher than 373 K

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

• At a pressure of 2 atm, the melting point of water is
  a. 273 K
  b. lower than 273 K
  c. higher than 273 K
  d. 273 C
  Hint . . . b. lower than 273 K

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

• The triple point for 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
  a. 216 K
  b. lower than 216 K
  c. higher than 216 K
  d. 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 CO₂ increases with pressure increases.

• Temporarily induced dipole interaction between molecules gives rise to a net attractive force, which is called
  a. van der Waals force
  b. dipole-dipole force
  c. attractive force
  d. London dispersion force
  
  Hint . . . d. London dispersion force

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

• Which of the following molecule has the highest melting point? O₂, F₂, Cl₂, Br₂, or 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.

• Among the intermolecular forces, dipole-dipole force, London dispersion force, ionic interaction, and hydrogen bond, which is present in all substance?
  
  Hint . . . London dispersion force

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

• Which one of the following has the highest boiling point? N₂, NO, or O₂.
  Hint . . . NO

  Discussion -
  The b.p. for N₂, NO, and O₂ are 77 K, 121 K, and 90 K respectively.

• Consider intermolecular forces, which compound would have the highest molar heat of vaporization?
  a. CH₃CH₂OH (ethanol)
  b. CH₃CH₂CH₃ (propane)
  c. CH₃CH₃ (ethane)
  
  Hint . . . a.

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

• Which four of the following substances would you expect to find strongest hydrogen bonding?
  a. HF
  b. HI
  c. C₂H₅OH
  d. C₂H₂SH
  e. CH₃COOH
  f. CH₂O
  g. H₂O
  
  Hint . . . a. c. e. g.

  Skill -
  Identify substances that have strong hydrogen bonding.
  C₂H₅OH, HF, H₂O and CH₃COOH have strong hydrogen bonding. C₂H₂SH and CH₂O have weak hydrogen bonding.