Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 123

The bonds that hold atoms together in a molecule are known generally as intramolecular forces because they are inside a molecule. They are very strong and not easily broken. For example, in the molecule water, H₂O, the covalent O—H bonds are the intramolecular forces. It is very difficult to split water into its component atoms, oxygen and hydrogen. In fact, it takes roughly 51.5 kJ energy to split 1 g of water into its elements, oxygen and hydrogen. If we could easily separate water molecules into gaseous hydrogen and oxygen, we would have a low‐cost way of producing hydrogen as a fuel stock.

However, simple covalent molecules such as water don't exist in isolation; they are surrounded by many millions of other similar molecules with much weaker forces between them. The forces that exist between molecules are called intermolecular forces. They are much weaker than the forces holding atoms together but are very important. The intermolecular forces holding water molecules together in liquid water are around 1.3 kJ per gram, which is about one‐twentieth the strength of an O—H bond. If it weren't for these intermolecular forces, we wouldn't have any liquid water to drink – water would exist as a gas on the earth's surface.

Intermolecular forces determine the properties of a covalently bonded compound, such as its melting and boiling point. It is important to understand how they arise in order to have an idea of the strength of these forces.

Figure 2.28 shows the strong intramolecular forces within the methane, CH₄, molecule and the weaker intermolecular forces between the methane molecules.

Figure 2.28 Comparison of inter‐ and intramolecular forces.

It is important that you understand the difference between inter‐ and intramolecular forces, as it is subtle but very important. Intermolecular forces are between two different molecules (like international flights are between two different countries); intramolecular forces are between the atoms that form a bond and are within a molecule

There are three main types of intermolecular force between molecules:

 Instantaneous dipole–induced dipole or London dispersion forces

 Permanent dipole–permanent dipole

 Hydrogen bonding

The first two types of intermolecular forces that involve dipole‐to‐dipole interactions (both permanent and instantaneous) are called van der Waals forces. Van der Waals forces are attractive forces between slightly positively and slightly negatively charged areas of a molecule. The term van der Waals is reasonably general and does not take into account the type of dipoles that are interacting. The term London dispersion forces is more specific, and this name is used for instantaneous dipole to induced dipole interactions. The third type of intermolecular force, hydrogen bonding, is a special type of dipole–dipole interaction.

The origins of these interactions will be discussed in the following sections.