University:
Eastern Kentucky University
Course:
CHE 111 | General Chemistry
Academic year:

2020
Views:

435

Pages:

2
Author:

butaforasv51

Intermolecular Force Worksheet Key 1. Identify the strongest intermolecular force present in pure samples of the following substances: SO2 H2O CH2Cl2 dipole-dipole forces hydrogen bonds dipole-dipole forces SCO PCl3 SO3 dipole-dipole forces dipole-dipole forces London dispersion forces 2. Identify the strongest intermolecular force operating in the condensed phases of the following substances. Fully explain how you determined this. a. Cl2 London dispersion forces The Cl-Cl bond is nonpolar so the molecule is nonpolar. Non polar molecules have only London dispersion forces operating in the substance. b. CO Dipole-dipole forces The C-O bond is polar so the molecule is polar. Polar molecules have dipole-dipole forces. They also have London dispersion forces, but dipole-dipole forces are stronger. c. SO2 Dipole-dipole forces SO2 is a bent, polar molecule. The strongest intermolecular force in a polar molecule is the dipole-dipole force d. CH2Cl2 Dipole-dipole forces The strongest intermolecular force in a polar molecule that cannot form hydrogen bonds is the dipole-dipole force e. HF Hydrogen bonding forces Molecules that have hydrogen attached to an O, N, or F can form hydrogen bonds. These are the strongest of the intermolecular forces. g. CH3-O-CH3 Dipole-dipole forces The hydrogen atoms are not bonded to the oxygen, so this molecule cannot form hydrogen bonds. It is polar, so it will have dipole-dipole forces. 3. Based on the intermolecular forces present, predict the relative boiling points of each of the substances below. Arrange each series of substances in order of increasing boiling point. State your reasons for the order you use (identify the forces and explain how they affect the boiling point). a. dimethyl ether (CH3OCH3), ethanol (CH3CH2OH), and propane (CH3CH2CH3) lowest bp: propane (CH3CH2CH3) < dimethyl ether (CH3OCH3)< ethanol (CH3CH2OH) highest bp Dimethyl ether cannot form hydrogen bonds (no O-H bond), but is polar and has dipole-dipole forces. Ethanol can form hydrogen bonds. propane is nonpolar, so it has only London dispersion forces. The boiling point increases as the strength of the intermolecular forces increase: London dispersion < dipole-dipole forces < hydrogen bonds [All have similar molar masses: 46.07g/mol, 46.07g/mol and 44.09g/mol respectively.] b. Br2, Cl2, I2 lowest bp: Cl2 < Br2 < I2 highest bp All are nonpolar molecules so only London dispersion forces are present. London dispersion forces get stronger as molar mass increases. 4. For each pair of substance identify the substance that is likely to have the higher vapor pressure. Explain your reasoning. a. CO2 or SO2 CO2 will have the higher vapor pressure. Vapor pressure tends to decrease as the strength of the intermolecular forces increase. Carbon dioxide is non-polar (dispersion forces only). Sulfur dioxide is polar (dipole-dipole forces are present). b. CH3OH or CH3-O-CH3 CH3OCH3 will have the higher vapor pressure. Vapor pressure tends to decrease as the strength of the intermolecular forces increase. CH3OH can hydrogen bond. CH3OCH3 is polar (bent shape around the oxygen), so dipole-dipole forces are the strongest forces in this compound.