Hydrogen Bonds and Dipole Forces

Description

Models containing magnets are constructed from ordinary materials. The magnetic interactions cause different models to become "sticky," and thereby permit the illustration of hydrogen bonding and dipoles forces.

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• Assume that the spheres represent either hydrogen or fluorine atoms, for the moment. F₂ and H₂ have vapor pressures greater than that of HF. F₂ is nearly twice as heavy as HF, showing that mass is not an important criterion.
• Electrical (chemical) interactions are much more important in determining intermolecular forces than are gravitational forces.
• The forces that cause HF molecules to be attracted to one another are due to uneven distribution of electron density in the HF molecule. The F atom, with its high electron density (or high electronegativity), is negatively charged when compared to the H atom.
• The HF molecule has a negative fluorine end and a positive hydrogen end. That is why it is called polar: it has two poles.
• A hydrogen atom tends to make a bridge, called a hydrogen bond, between two fluorine atoms.
• When HF molecules cluster together, the negative end of one molecule tends to be attracted to the positive end of another molecule. This intermolecular attraction may be interpreted in terms of dipole forces or hydrogen bonds. In either case, the interactions between HF molecules reduces the vapor pressure of the polar molecules compared to their non-polar counterparts.
• F₂ and H₂ molecules do not exhibit this tendency because the electron density distribution is uniform. These molecules do not exhibit dipoles; they are non-polar.

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1. Show the polar "attractive forces" between HF molecules by holding 2 HF molecules together and using one molecule to move or hold another molecule.
2. Note that only the white and blue HF molecule moves when another "HF" molecule is nearby.
3. Count the models, and note the distribution. Put the models into a large container such as a 3 gallon paper ice cream container. Shake the container vigorously so that some models spill out. After about half of the models spill out, dump the remaining contents onto a table.
4. Count the models and determine the distribution.

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Name _____________________________ Class _______

Teacher______________________________

DoChem 041 Hydrogen Bonds and Dipole Forces

1. The initial numbers of H₂, F₂, and HF were equal. Describe what happens when the molecules are shaken.
2. Relate this experiment to the boiling points of H₂, F₂, and HF. Which of these molecules has the higher boiling point? Why?

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Purpose

To illustrate intermolecular forces of attraction.

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• 24 styrofoam balls, 1" diameter
• 24 styrofoam balls, 2" diameter
• 8 pairs of button magnets
• magnet of known polarity
• small piece of woven fabric
• white glue and 24 toothpicks, or two-component glue
• sharp knife, razor blade, or Exacto₅ knife
• paint (suitable spray paint for styrofoam)
• large container (3 gallon ice cream carton; nonmetallic trash pail)
• liquid paper (correction fluid)

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• Instructions for Assembly of Models
  1. Orient the 16 magnets using a magnet of known polarity. Mark the poles using typewriter correction fluid.
  2. Slice off a small section (1/8th of the sphere) from 8 spheres of equal size. Do this for each size of sphere used.
  3. Select one magnet orientation, say south pole, facing out in the small sphere.
  4. When using two component glue, put one component on the flat part of the small sphere, the other component on the north pole of a magnet, and press the two together using finger pressure
  5. Repeat for the large sphere reversing the magnet polarity (south pole faces in).
  6. When using white glue and fabric material, select the polarity orientation, and press the magnet into the sphere using finger pressure. Cover with white glue, and then with a small piece of fabric material.
  7. Paint the spheres using a spray paint suitable for styrofoam.
  8. Glue spheres together to model diatomic molecules. For two component glue, spread one component on each sphere, and press together. For white glue, using a toothpick adds strength.
  9. Allow the glues to set.
• Models for nonpolar molecules are assembled without magnets.

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Preparation of the styrofoam "molecules" requires several hours. Once made, the models may be used for many years when stored with proper care.

Presentation: 15-20 minutes.

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Glues may cause allergic sensitivity. The spray paint may be flammable. The cuttings tools may cause injury.

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• Care should be taken when using the razor blade or Exacto knife.
• Consult and follow the manufacturers directions for the glue and the spray paint used.

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Store for future use.

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• The models for F₂ and H₂ tend to spill out first. The F₂ models are not actually heavier than HF because the magnets increase the weight of the HF models.
• Ask the students how they can design models that will provide a more fair test. (Using models containing nonmagnetic masses about the same as that of the magnets will work.)

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1. The models for F₂ and H₂ tend to spill out first. The magnetic interaction between the HF models retain them.
2. Intermolecular interactions (hydrogen bonding) similar to the magnetic interactions in the models hold HF molecules together. The boiling point is higher for molecules with higher molecular interaction. HF has the highest boiling point because of the large intermolecular interaction.

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• intermolecular attraction
• hydrogen bonding
• dipole forces
• polar
• non-polar

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