2 1.2 Intermolecular Forces
Chapter 2 Learning Objectives
- Be able to state and define the four types of intermolecular forces.
- Be able to arrange the intermolecular forces based on their strength.
- Be able to predict physical properties such a smelting point, boiling point and solubility of various types of organic compounds based on the intermolecular forces present.
1.2.1 Introduction
The physical properties of a compound are determined by the forces between the individual molecules known as intermolecular forces. Understanding how intermolecular forces operate allows us to compare compounds to each other in a relative way. By applying predictable trends trends, we can, for example, identify which compound will boil at a higher temperature. A practical example of this is the process of fractional distillation. Crude oil is an important natural resource. Different parts (fractions) of crude oil are used to make fuels, plastics, medicines, fabrics, and dyes. Crude oil is separated by fractional distillation. In this process, the crude oil is heated up until different fractions of it boils. The fractions are cooled and collected. This process is effective because each fraction has a different boiling point. Smaller molecules with weaker intermolecular forces are easier to separate and boil at lower temperatures. Larger molecules with very strong intermolecular forces require significantly more energy to separate and have higher boiling points. This same concept also extends to other properties such as melting point, volatility, viscosity, surface tension and solubility in polar and nonpolar solvents.
To understand intermolecular forces between molecules, one must have a good knowledge of chemical bonding. If you need to review important aspects of chemical bonding, visit this site. Intermolecular forces are forces between molecules. It is important to note that they are different, but dependent upon intramolecular forces which are forces within molecules.
1.2.2 Types of Intermolecular Forces
We will examine four types of intermolecular forces:
- Ion-dipole/ionic
- Hydrogen bonding
- Dipole-dipole
- London dispersion forces
These intermolecular forces are listed from strongest (1) to weakest (4). Visit this site for a detailed explanation of each of these forces, worked examples and additional practice problems.
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A notable instance of hydrogen bond can be found in the DNA backbone. See how the base pairs hydrogen bond and hold the two strands together (figure 1.2.1).
Figure 1.2.1: Hydrogen Bonding in DNA
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1.2.3 Effect of Intermolecular Forces on Boiling Points and Melting Points
We will now examine how intermolecular forces affect boiling point. As a rule of thumb, the stronger the intermolecular forces between molecules, the higher the boiling point. This video gives an overview of intermolecular forces on boiling points. This site examines how intermolecular forces can be used to predict melting and boiling point trends.
Key Takeaways: Influence of IMF on Melting and Boiling Points
- Stronger the IMF, higher are the boiling & melting points of organic molecules.
- Greater the number of IMF, higher are the boiling & melting points.
- Symmetrical structures have a higher melting point than less symmetrical structures.
1.2.4 Effect of Intermolecular Forces on Solubility
Intermolecular forces can also be used to predict solubility. The stronger the intermolecular forces between solute molecules and solvent molecules, the greater the solubility of the solute in the solvent. Do you remember the rule, “like dissolves like”?
Key Takeaways: Influence of IMF on Solubility
- Ionic compounds and polar molecules are soluble in polar solvents.
- Nonpolar molecules are soluble in nonpolar solvents.
- Stronger the IMF, higher is the solubility of organic molecules.
- Greater the number of IMF, higher is the solubility of polar molecules in polar solvents.
Additional Resources:
Fundamentals of Intermolecular Forces
Full Lecture on Intermolecular Forces
Additional Intermolecular Forces Review
Van der Waals Forces Explained
Intermolecular Forces and Solubility
Boiling Points of Organic Compounds
Exercise: Test your knowledge in IMF
Exercises: Watch this interactive video to test your knowledge in IMF.
Key Takeaways: IMF
- Intermolecular forces occur between molecules. They play a vital role in the physical properties of the compound such as boiling point, melting point and solubility.
- There are three main types of IMF: dispersion forces, dipole forces and hydrogen bonding.
- Strength of IMF: Dispersion forces < Dipole forces < Hydrogen bonding
- Greater the number of intermolecular forces, greater is the melting and boiling point.
- Compounds that are capable of hydrogen bonding are soluble in water.
