Structure, Properties & Changes of State (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about structure, properties and changes of state.

The forces holding particles together in a lattice are the same forces that have to be overcome to change state, so the structure and bonding of a substance decides properties like its melting point and conductivity.

Changing state means overcoming the forces of attraction between particles, and those forces come from the bonding and the structure the substance has: ionic, covalent or metallic. The same idea therefore explains two things at once, the energy needed to melt or boil a substance, and the physical properties it has, such as its melting and boiling points, its electrical conductivity and its solubility.

We start with the energy changes that happen when a substance changes state. Then we take the three lattice types in turn: ionic, covalent and metallic. Finally we relate each structure back to the melting point and the conductivity it produces.

Changes of state are physical changes and they are reversible. They do not change the chemical properties or the chemical makeup of the substances involved.

Vaporisation covers both evaporation and boiling. Evaporation occurs only at the surface, and it takes place at temperatures below the boiling point. Boiling occurs at a specific temperature, when the vapour pressure reaches the external atmospheric pressure.

The relationship between temperature and energy can be shown on a graph. While a substance is heated within one state, the particles gain kinetic energy and the temperature rises. During melting and during boiling, all the energy goes into breaking bonds: there is no increase in kinetic energy and no increase in temperature. Those flat sections are the forces of attraction between the particles being overcome.

Most ionic, metallic and covalent solids are crystalline lattices, with the ions, atoms or molecules in a regular and repeating arrangement.

An ionic bond is an electrostatic force of attraction between a positively charged metal ion, the cation, and a negatively charged non-metal ion, the anion. The metal becomes positively charged as it transfers electrons to the non-metal.

When an ionic compound forms, the attraction between the ions happens in all directions, so the ions arrange into a giant ionic lattice with alternating positive and negative ions. The type of lattice depends on the sizes of the ions. Sodium chloride and magnesium oxide both form cubic lattices. Those attractions acting in every direction are the forces that a change of state has to overcome.

Covalent bonds are bonds between non-metals in which electrons are shared between the atoms.

Covalent compounds are arranged in one of two kinds of lattice. Simple molecular lattices include iodine, buckminsterfullerene and ice. Giant molecular lattices include silicon(IV) oxide, graphite and diamond. Graphite, diamond and buckminsterfullerene are all allotropes of carbon: different structural forms of the same element.

That split is what the properties follow from. A simple molecular lattice has weak intermolecular forces between its molecules, so only a little energy is required to break the lattice. A giant molecular lattice has a large number of strong covalent bonds linking the whole structure, so a lot of energy is required.

Metals form giant metallic lattices in which the metal ions are surrounded by a sea of delocalised electrons. The ions are often packed in hexagonal layers or in a cubic arrangement, and this layered structure with the delocalised electrons gives a metal its key properties.

Metals are malleable. When a force is applied the layers slide over each other, and because the attractive forces between the metal ions and the electrons act in all directions, the metallic bonds re-form. The lattice is not broken; it has changed shape.

Metals conduct electricity in the solid and the liquid state, because in both there are mobile electrons which can move freely. When a potential difference is applied, the delocalised electrons move towards the positive terminal.

The strong electrostatic forces of attraction between the cations and the delocalised electrons is what has to be overcome to melt a metal, which is why metals have high melting and boiling points.

Ionic compounds have high melting and boiling points, because the strong electrostatic forces of attraction between the ions act in all directions. They are brittle, and they conduct electricity only when molten or in solution, because only then can the ions move freely.

Metallic lattices are strong, malleable, have high melting and boiling points, and they conduct in both the solid and the liquid state.

Simple covalent lattices have low melting and boiling points, because the intermolecular forces between the molecules are weak. They do not conduct in the solid or liquid state, as there are no charged particles.

Giant covalent lattices have very high melting and boiling points, because a large number of covalent bonds link the whole structure. Most do not conduct electricity, but graphite does: it has delocalised electrons between its carbon layers which can move when a voltage is applied. Diamond does not conduct electricity, because all four outer electrons on every carbon atom are involved in a covalent bond.

To break bonds, energy is always needed to overcome the forces of attraction between the particles, so be careful to match the bond breaking and bond making processes to the flow of energy during state changes.

You also need to be able to state and fully explain the different properties which arise from the structure and bonding in a substance.

Changing state means putting in enough energy to overcome the forces of attraction between the particles, and while that is happening the temperature does not rise.

Those forces come from the structure: a giant ionic lattice of alternating ions, a covalent lattice that is either simple molecular or giant molecular, or a metallic lattice of ions in a sea of delocalised electrons.

Melting point and conductivity come from which structure a substance has. Therefore, the structure decides the properties.