Intermolecular Forces
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Table of Contents |
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Gateway Page |
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Introduction
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Phase Changes
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Dipole-Dipole Forces
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London Forces
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| Summary |
How strong are intermolecular forces?
While intermolecular forces are strong enough to keep molecules together in the solid and liquid state, they are not nearly as strong as covalent bonds. The table below compares the strength of various intermoelcular forces.
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* For comparison, the approximate magnitude of covalent bonds is 100-400 kJ/mol.
As you can see in the table, the strength of each type of intermolecular force covers a range of values. More importantly, the ranges for the various intermolecular forces overlap. This means a statement such as "Dipole-dipole forces are stronger than London forces" can not be made.
With these overlapping ranges, how can the intermolecular forces of two different substances be compared? The summary below will answer this question.
Summary
When considering a substance, following the steps below will help you determine what type(s) of intermolecular forces exist in the substance.
Click on each number to see steps to follow.
| 1. | London forces exist in ALL substances. London forces will be strongest in large molecules (or ions, or atoms) and weakest in small molecules. When comparing different molecules, if they have similar molecular weights, the strengths of the London forces will be similar. |
| 2. | If the molecule is polar, dipole-dipole forces will also exist. The stronger the dipole moment, the stronger the dipole-dipole forces. Dipole-dipole forces are stronger than London forces in small molecules. In larger molecules, London forces tend to be stronger than dipole-dipole forces (even stronger than hydrogen bonds). |
| 3. | If the molecule contains a hydrogen atom bonded to a highly electronegative atom AND contains a fluorine, oxygen, or nitrogen atom that has a lone pair of electrons, hydrogen bonding (a special class of dipole-dipole forces) will exist in the substance. Hydrogen bonds are typically stronger than other dipole-dipole forces. |
Clicking on the right will take you to the debriefing for this module. |
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Intermolecular Forces |
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