Chemistry » Organic Molecules » Physical Properties And Structure

Physical Properties and Intermolecular Forces

Physical properties and intermolecular forces

Have the learners research the safety data for various compounds, especially those being used in the experiments in this section, as a way of linking the properties of organic molecules with their molecular structure.

The types of intermolecular forces that occur in a substance will affect its physical properties, such as its phase, melting point and boiling point. You should remember from the kinetic theory of matter (see Chemistry 103), that the phase of a substance is determined by how strong the forces are between its particles. The weaker the forces, the more likely the substance is to exist as a gas.

This is because the particles are able to move far apart since they are not held together very strongly. If the forces are very strong, the particles are held closely together in a solid structure. Remember also that the temperature of a material affects the energy of its particles.

The more energy the particles have, the more likely they are to be able to overcome the forces that are holding them together. This can cause a change in phase.

The figure below shows the three phases of water. Note that we are showing two-dimensional figures when in reality these are three-dimensional.

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Submicroscopic diagrams of the three phases of water. Photos by Iaszlo-photo, Fields of View and Capt’ Gorgeous on Flickr.

The effects of intermolecular forces

The forces between molecules that bind them together are known as intermolecular forces. Intermolecular forces allow us to determine which substances are likely to dissolve in other substances, and what the melting and boiling points of substances are. Without intermolecular forces holding molecules together we would not exist.

Definition: Intermolecular forces

Intermolecular forces are forces that act between molecules.

Recall that a dipole molecule is a molecule that has its charge unevenly distributed. One end of the molecule is slightly positive and the other is slightly negative. An overview of the different types of intermolecular forces that are discussed in this tutorial are given below:

  1. Dipole-dipole forces

    When one dipole molecule comes into contact with another dipole molecule, the positive pole of the one molecule will be attracted to the negative pole of the other, and the molecules will be held together in this way.

    One special case of this is hydrogen bonding:

    Tip:

    Do not confuse hydrogen bonds with intramolecular covalent bonds. Hydrogen bonding is an example of a scientist naming something, believing it to be one thing when in fact it was another. In this case the strength of the hydrogen bonds misled scientists into thinking this was an intramolecular bond when it is really just a strong intermolecular force.

    • Hydrogen bonds

      As the name implies, this type of intermolecular bond involves a hydrogen atom. When a molecule contains a hydrogen atom covalently bonded to a small, highly electronegative atom (e.g. \(\text{O}\), \(\text{N}\) or \(\text{F}\)) this type of intermolecular force can occur. The highly electronegative atom on one molecule attracts the hydrogen atom on a nearby molecule (see figure below).

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      Hydrogen bonding between two molecules of ethanol.

  2. Van der Waals forces

    • Induced-dipole forces

      Fact:

      Dipole-induced-dipole intermolecular forces are also sometimes called London forces or dispersion forces.

      In non-polar molecules the electronic charge is usually evenly distributed but it is possible that at a particular moment in time, the electrons might not be evenly distributed (remember that the electrons are always moving in their orbitals). The molecule will have a temporary dipole. When this happens, molecules that are next to each other attract each other very weakly.

    • Dipole-induced-dipole forces

      These forces exist between dipoles and non-polar molecules. The dipole induces a dipole in the non-polar molecule leading to a weak, short lived force which holds the compounds together.

In this section, we will focus on the effects of van der Waals forces and hydrogen bonding on the physical properties of organic molecules.

 

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