C5- Chemicals of the natural environment

Air contains a number of gases. The amount of water vapour in the air varies from place to place, and day to day. For this reason, the proportions of the gases in the air are usually given for dry air.

Some of the gases in the air are elements: nitrogen (N₂), oxygen (O₂) and argon (Ar). But carbon dioxide (CO₂) is a compound (as is water vapour, H₂O).

Most non-metal elements are molecular, and most of these consist of molecules with just two atoms joined together. Examples include nitrogen (N₂), oxygen (O₂), chlorine (Cl₂) and bromine (Br₂).

Most compounds between non-metal elements are also molecular. Some examples are: water (H₂O), carbon dioxide (CO₂), nitrogen dioxide (NO₂) and ammonia (NH₃).

All of these substances have very strong covalent bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds.

Because the weak intermolecular forces break down easily these substances have low melting and boiling points. This means simple molecular substances are gases, liquids or solids with low melting points, and low boiling points.

Simple molecular substances do not conduct electricity. This is because they do not have any free electrons, and the molecules do not have electric charges.

The atom share an electron/ electrons. This only occurs between non metals.

An atom loses an electron/ electrons and another gains it/ them. Resulting in the atom that lost electrons gaining a positive charge and the atom that gained electrons getting a negative charge. The charges are equal but opposite, so the elements are strongly electrostatically attracted to one another.

The easiest way to represent covalent bonds is by using straight lines, with each line representing a shared pair of electrons. These 2D molecular diagrams show which atoms are joined by covalent bonds, but do not show the shape of each molecule. It is possible to draw more complex 3D diagrams to show the true shape of each molecule.

Some molecules have a double covalent bond, meaning they have two shared pairs of electrons. Others have a triple covalent bond, meaning they have three shared pairs of electrons. A double covalent bond is shown by a double line, and a triple bond by a triple line.

Covalent bonds are strong, and a lot of energy is needed to break them.

The strong bonds between atoms that are joined by covalent bonds are the result of electrostatic attraction between the positive nuclei of the atoms and the pairs of negative electrons that are shared between them.

In some covalent molecules, the atoms each form more than one covalent bond.

A molecule of carbon dioxide

For example each carbon dioxide molecule has a carbon atom joined by four covalent bonds to two oxygen atoms, which have two covalent bonds each.

The Earth's hydrosphere is the oceans. They consist mainly of water, with some dissolved compounds. The ionic compounds that are dissolved in sea water are mainly salts, and make the water taste salty.

When metals react with non-metals, electrons are transferred from the metal atoms to the non-metal atoms, forming ions. The resulting compound is called an ionic compound.

Here are some examples:

sodium + chlorine → sodium chloride

magnesium + oxygen → magnesium oxide

calcium + chlorine → calcium chloride

In each of these reactions, the metal atoms give electrons to the non-metal atoms, so that the metal atoms become positive ions and the non-metal atoms become negative ions.

There is a strong electrostatic force of attraction between these oppositely charged ions, called an ionic bond.

The ions in a compound such as sodium chloride are arranged in a lattice structure. This regular arrangement results in the formation of a crystal.

This pattern is repeated in all directions, giving a giant three-dimensional lattice structure in sodium chloride crystals.

Because of the strong electrostatic forces between them, it takes a great deal of energy to separate the positive and negative ions in a crystal lattice. This means that ionic compounds have high melting points and boiling points.

Solid ionic compounds do not conduct electricity, because the ions are held firmly in place. They cannot move to conduct the electric current. But when an ionic compound melts, the charged ions are free to move. Molten ionic compounds do conduct electricity.

When a crystal of an ionic compound dissolves in water, the ions separate. Again, the ions are free to move, so a solution of an ionic compound in water also conducts electricity

Sea water contains a number of dissolved salts. When these salts dissolve in water, the ions separate. Sea water therefore contains a mixture of ions. The most common ions in sea water are shown in the table:

It is possible to deduce from the table which salts dissolved to form the mixture of ions:

• When sodium chloride dissolves it forms sodium ions and chloride ions.

• When magnesium chloride dissolves it forms magnesium and chloride ions.

• When magnesium sulfate dissolves if forms magnesium ions and sulfate ions.

• When potassium chloride dissolves it forms potassium ions and chloride ions.

• When potassium bromide dissolves it forms potassium ions and bromide ions.

You can use the charge on the ions shown in the table to work out the formulae of the ionic compounds. Charges must cancel out to form a neutral compound. Sodium ions each have a single positive charge. Chloride ions each have a single negative charge. For the charges to cancel out in the neutral salt sodium chloride, they must be in a ration of 1:1. So the formula of sodium chloride is NaCl. Magnesium ions each have two positive charges. Chloride ions each have a single negative charge. For the charges to cancel out in the neutral salt magnesium chloride, they must be in a ration of 1:2. So the formula of magnesium chloride is MgCl₂.

You need to be able to construct the balanced symbol equations for the precipitation reactions described in this section.

Examples

Cu²⁺ + 2OH^- → Cu(OH)₂

Fe²⁺ + 2OH^- → Fe(OH)₂

Fe³⁺ + 3OH^- → Fe(OH)₃

Transition metal hydroxides are insoluble in water.

If a solution of any soluble transition metal compound is mixed with sodium hydroxide solution then there is displacement reaction. The sodium is the more reactive metal, and displaces the transition metal from its compound. The transition metal hydroxide is produced as a result. As this is insoluble in water it appears as a solid in the liquid. A solid produced in a liquid in this way is called a precipitate. The colour of the precipitate indicates which transition metal ion was in the initial solution.

Precipitate reactions can be used to detect non-metal ions.

Chloride (Cl^-), bromide (Br^-) and iodide (I^-) ions make insoluble precipitates with silver ions (Ag⁺). Each of these precipitates has a characteristic colour.

The ionic equation for the chloride ion test is:

Ag⁺_(aq) + Cl^-_(aq) → AgCl_(s)

The ionic equation for the sulfate ion test is:

Ba²⁺_(aq) + SO₄^2-_(aq) → BaSO_4(s)