2.1 Understanding intermolecular forces

Understanding intermolecular forces

The types of intermolecular forces that occur in a substance will affect its properties, such as its phase , melting point and boiling point . You should remember, if you think back to the kinetic theory of matter, 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 because the particles are far apart. 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.

Boiling point

The temperature at which a material will change from being a liquid to being a gas.

Melting point

The temperature at which a material will change from being a solid to being a liquid.

Now look at the data in [link] .

The melting point and boiling point of a substance, give us information about the phase of the substance at room temperature, and the strength of the intermolecular forces . The examples below will help to explain this.

Example 1: Fluorine (F ${}_2$ )

Phase at room temperature

Fluorine (F ${}_2$ ) has a melting point of -220 ${}^0$ C and a boiling point of -188 ${}^0$ C. This means that for any temperature that is greater than -188 ${}^0$ C, fluorine will be a gas. At temperatures below -220 ${}^0$ C, fluorine would be a solid, and at any temperature inbetween these two, fluorine will be a liquid. So, at room temperature, fluorine exists as a gas.

Strength of intermolecular forces

What does this information tell us about the intermolecular forces in fluorine? In fluorine, these forces must be very weak for it to exist as a gas at room temperature. Only at temperatures below -188 ${}^0$ C will the molecules have a low enough energy that they will come close enough to each other for forces of attraction to act between the molecules. The intermolecular forces in fluorine are very weak van der Waals forces because the molecules are non-polar .

Example 2: Hydrogen fluoride (HF)

Phase at room temperature

For temperatures below -83 ${}^0$ C, hydrogen fluoride is a solid. Between -83 ${}^0$ C and 20 ${}^0$ C, it exists as a liquid, and if the temperature is increased above 20 ${}^0$ C, it will become a gas.

Strength of intermolecular forces

What does this tell us about the intermolecular forces in hydrogen fluoride? The forces are stronger than those in fluorine, because more energy is needed for fluorine to change into the gaseous phase. In other words, more energy is needed for the intermolecular forces to be overcome so that the molecules can move further apart. Intermolecular forces will exist between the hydrogen atom of one molecule and the fluorine atom of another. These are hydrogen bonds , which are stronger than van der Waals forces.

What do you notice about water? Luckily for us, water behaves quite differently from the rest of the halides. Imagine if water were like ammonia (NH ${}_3$ ), which is a gas above a temperature of -33 ${}^0$ C! There would be no liquid water on the planet, and that would mean that no life would be able to survive here. The hydrogen bonds in water are particularly strong and this gives water unique qualities when compared to other molecules with hydrogen bonds. This will be discussed more in chapter [link] . You should also note that the strength of the intermolecular forces increases with an increase in formula mass. This can be seen by the increasing melting and boiling points of substances as formula mass increases.

Applying your knowledge of intermolecular forces

Refer to the data in [link] and then use your knowledge of different types of intermolecular forces to explain the following statements:

• The boiling point of F ${}_2$ is much lower than the boiling point of NH ${}_3$
• At room temperature, many elements exist naturally as gases
• The boiling point of HF is higher than the boiling point of Cl ${}_2$
• The boiling point of water is much higher than HF, even though they both contain hydrogen bonds