WebLecture: Intermolecular Forces

Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 11: Sections 4-6.

• 11.4 When molecules don't have a permanent charge or dipole, they can be coaxed through electrical induction to form local net charged areas, or induced dipoles. These molecules can then form much weaker intermolecular bonds, including
  • dipole-induced dipole : oxygen gas O₂ normally does not have a dipole, but when it is near a water molecule, the negative charge around water's oxygen molecule can repel the electrons on one of the oxygen atoms in the gas molecule, temporarily creating a dipole in the gas molecule, and then attracting the "positive" end it has created.
  • induced dipole-induced dipole : (also known as London dispersion forces), electrical forces arise when charge is concentrated as the electrons in one molecule respond to the proximity of electrons in another molecule.
• 11.5 Summing up van der Walls forces, in order of strength the intermolecular forces are:
  1. ion-dipole
  2. dipole-dipole
  3. hydrogen bonds
  4. dipole/induced dipole
  5. induced dipole/induced dipole (London dispersion forces)
• 11.6 Liquid states have two boundaries: where they evaporate to form gases, and where they condense to form solids. For a given substance, the heat of vaporization ΔH^°_vap depends on the type of intermolecular bonds involved. Small, diatomic elements whose only intermolecular forces are London forces have very low vaporization tempertures (boiling points) and small heats of vaporization. Molecules with permanent dipoles will have high vaporization temperatures and heats of vaporization.

  Pressure of the gas above a liquid influences the temperature at which a liquid boils. The Clausius-Clapeyron equation can be used to determin the heat of vaporization and temperature for changing pressures. As pressure increases, the temperature required to bring a liquid to a boil also increases (which is why water boils at a lower temperature in Denver than on the seacoast). At some a high enough combination of pressure and temperature, the interface between the liquid and gas states of the substance disappears and the liquid becomes supercritical, with new solvent properties.

  Liquids also exhibit attraction to each other (cohesion) and to other substances (adhesion), resulting in the ability to climb a surface (capillary action).

Videos for Chapter 11: Intermolecular Forces

Review the Videos at Thinkwell Video Lessons.

• Under "Condensed Phases: Liquids and Solids"
  • Physical Properties of Liquids
    • Properties of Liquids
    • Vapor Pressure and Boiling Point
    • Molecular Structure and Boiling Point
    • Phase Diagrams

Homework problems: See your Moodle assignment!

AP #9 GUIDED INQUIRY — Measuring the dependence of reaction rates on temperature, surface area, concentration, and catalysts -- Phase I

Design an experiment to vary temperature, surface area, concentration, and introduce catalysts into a series of reactions, and determine how changing each factor affects the reaction rate. Use one or more of the reactions suggested in the references below as the basis of your inquiry. Identify the materials and equipment you will require, outline your procedure, and determine what measurements you will take.

References:

• AP2009 12 Determination of the Rate of a reaction and its order
• APGIE Investigation 10 Kinetics: Rate of Reaction: How Long Will that Marble Statue Last?
• IGHCE Lab 12.1-4 Determination of the effect of Temperature, Surface Area, and Catalysts on Reaction Rates
• IGHCE Lab 12.2 Determination of the effect of Surface Area on Reaction Rate

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