Van der Waals Forces

1. In each of the following pairs, which would you expect to have the higher boiling point? C7H16 or C10H22; C3H8 or (CH3)2O; CH3CH2SH or CH3CH2OH.

2. Arrange the following in order of increasing melting point: KI, Ne, K2SO4, C3H8, CH3CH2OH, MgO, CH2OHCHOHCH2OH.

3. Which member of the following pair has the stronger intermolecular dispersion forces: CH3CH2CH2CH2SH or CH3CH2CH2CH2CH2SH; Br2 or O2; CH3CH2CH2Cl or (CH3)2CHCl?

4. Identify the types of intermolecular forces present in each of the following substances, and select the substance in each pair that has the higher boiling point: C6H14 or C8H18; C3H8 or CH3OCH3; HOOH or HSSH; NH2NH2 or CH3CH3.

Summary of Van der Waals Forces

• Dispersion (London) forces exists between all molecules. They increase with increasing molecular mass. The forces also depend on the molecular shape.

• Forces associated with permanent dipoles involve displacement of electron pairs in bonds rather than in molecules as a whole. These forces are found in molecules with resultant dipole moments (polar molecules). This existence adds to the effect of dispersion forces also present.

• When comparing substances of roughly comparable molecular masses, dipole forces can produce significant differences in properties such as melting point, boiling point and enthalpy of vaporization.

• When comparing substances of widely different molecular masses, dispersion forces are usually more significant than dipole forces.

Solid State Problems

1. Categorize each substance based on the type of crystalline solids: Si; SiCl4 (molecular); CaCl2; Ag; SO2.

2. Iridium crystallizes in a face-centered cubic unit cell that has an edge length of 3.833Å. The atom in the center of the phase is in contact with the corner atoms. Calculate the atomic radius of an iridium atom. Calculate the density of Iridium.

;

the unit cell contains 4 Ir atoms since it is face-centered cubic. Therefore, the amu is multplied by 4.

3. Indicate the type of crystal (molecular, metallic, covalent network, or ionic) each of the following would form upon solidification: CaCO3, Pt, ZrO2 (MP=2677C; ionic), Kr, benzene and I2.

4. For each of the following pair, determine which will have the higher melting point: B (2092C; covalent) and BF3(-127C; molecular); Na(97.8C; metallic) and NaCl(~775C; ionic); TiO2(~1830C; ionic) and TiCl4(-24C; molecular); LiF and MgF2.

Solution to Problem 11.98

7. Empirical formula of Spinel: Mg2.24Al4.48O8.96 = MgAl2O4

Phase Diagrams

1. Explain why the heats of fusion is generally lower than heats of vaporization.

2. Name the phase transitions in each of the following situations:– Bromine vapor turns to bromine liquid upon cooling.– Crystals of iodine disappears from an evaporating dish.– Rubbing alcohol disappears in an open container .– Molten lava turns to rock

3. Refer to Figure 11.27b in your book. Describe the phase changes (and the temperatures at which they occur) when CO2 is heated from -80C to -20C at a) constant P of 3 atm and b) a constant pressure of 6atm.

4. Given the same phase diagram on the problem above, will solid CO2 float on liquid CO2 (Hint: the CO2 diagram shows a positive slope at the solid to liquid

interphase)? As it is heated, will solid CO2 sublime or melt under a pressure of

1atm?