C5- Chemicals of the natural environment part 2

The Earth's lithosphere is the rigid outer layer, which contains many useful chemical compounds. The Earth's lithosphere is the rigid outer layer that is made up of the crust and the part of the mantle just below it.

The Earth is almost a sphere. These are its main layers, starting with the outermost:

1. Crust - relatively thin and rocky
2. Mantle - has the properties of a solid, but can flow very slowly
3. Outer core - made from liquid nickel and iron
4. Inner core - made from solid nickel and iron

The lithosphere is made up of a mixture of minerals

The chart shows the relative abundance of some of the elements in the lithosphere. Oxygen, silicon and aluminium are the most common elements.

The rocks in the lithosphere are made up mainly of silicates. These are compounds that contain silicon and oxygen, together with smaller amounts of other elements. 

Much of the silicon and oxygen in the Earth's crust is present as the compound silicon dioxide also known as silica. Silicon dioxide has a giant covalent structure. Part of this structure is shown in the diagram - oxygen atoms are shown as red, silicon atoms shown as brown:

Each silicon atom is covalently bonded to four oxygen atoms. Each oxygen atom is covalently bonded to two silicon atoms. This means that, overall, the ratio is two oxygen atoms to each silicon atom, giving the formula SiO₂.

 Silicon dioxide is very hard. It has a very high melting point (1,610 °C) and boiling point (2,230 °C), is insoluble in water, and does not conduct electricity. These properties result from the very strong covalent bonds that hold the silicon and oxygen atoms in the giant covalent structure.

Silicon dioxide is found as quartz in granite, and is the major compound in sandstone. The sand on a beach is made mostly of silicon dioxide.

Pure quartz crystals are transparent, but the mineral is often found with traces of other minerals present. These minerals are very hard.

Because of their attractive appearance when polished, and their resistance to wear and corrosion, they are used as gemstones in jewellery.

Amethyst is a form of quartz that contains manganese and iron oxides. These give the gemstone its violet colour.

Many other giant covalent compounds are used in jewellery.

The most famous is diamond.Many other compounds have giant covalent structures. They generally have the similar properties of hardness, high melting and boiling points, and low electrical conductivity

A good example is diamond. This is made of carbon atoms, with each joined to four others by strong covalent bonds.

Diamond is the hardest naturally occurring substance. It is used as a gemstone, but also on the cutting edges of drills and saws.

Graphite is another giant covalent structure made of carbon atoms. In graphite, each carbon atom is joined to three others, forming layers:

The bonds between the layers are much weaker than covalent bonds. This enables the layers to slide across each other, making graphite soft. Graphite can also conduct electricity, between the layers of carbon atoms.

Graphite is used as pencil 'lead'. As the pencil moves across the paper, layers of graphite rub off. Graphite is also used as a lubricant, and as an electrode in electrolysis - for example, in the manufacture of aluminium.

Metals are extracted from ores. An ore is a rock that contains enough of a mineral (metal compound) for the metal to be extracted from it. Most metals are extracted from an ore by reduction with carbon or by electrolysis. The way metal atoms are arranged to make a crystal lattice gives metals particular properties. The uses made of metals depend on these properties.

Metals such as zinc, iron and copper are present in ores as their oxides. Each of these oxides is heated with carbon to obtain the metal.

The metal oxide loses oxygen, and is therefore reduced. The carbon gains oxygen, and is therefore oxidised.

Using iron as an example:

Iron oxide + carbon → iron + carbon dioxide

2Fe₂O₃(s) + 3C(s) → 4Fe(l) + 3CO₂(g)

The source of carbon for this reduction is coke, obtained by heating coal in the absence of oxygen. Note that the iron is liquid when it is formed, due to the very high temperature at which the reaction takes place.

Some metals, such as aluminium, are so reactive that their oxides cannot be reduced by carbon.

Ionic compounds contain charged particles called ions. For example, copper(II) chloride contains positively charged copper ions and negatively charged chloride ions. Ionic substances can be broken down into the elements they are made from by electricity, in a process called electrolysis.

For electrolysis to work, the ions must be free to move. When an ionic compound is dissolved in water, or melts, the ions break free from the ionic lattice. These ions are then free to move.

For example, if electricity is passed through copper(II) chloride solution, the copper(II) chloride is broken down to form copper metal and chlorine gas.

There is a similar result if electricity is passed through molten copper(II) chloride.

The solution or molten ionic compound is called an electrolyte. The negative electrode is called the cathode, while the positive electrode is called the anode.

This is what happens during electrolysis:

• Positively charged ions move to the negative electrode. Metal ions are positively charged, so metals are produced at the negative electrode (cathode)

• Negatively charged ions move to the positive electrode. Non-metal ions, such as oxide ions and chloride ions, are negatively charged, so gases such as oxygen or chlorine are produced at the positive electrode (anode).

Aluminium is the most abundant metal on Earth. Despite this, it is expensive, largely because of the amount of electricity used up in the extraction process.

Aluminium ore is called bauxite. The bauxite is purified to yield a white powder, aluminium oxide, from which aluminium can be extracted.

The extraction is done by electrolysis. But first the aluminium oxide must be made molten so that electricity can pass through it. Aluminium oxide has a very high melting point (over 2,000ºC), so it would be expensive to melt it. Instead, it is dissolved in an aluminium compound with a lower melting point than aluminium oxide. This reduces some of the energy costs involved in extracting aluminium.

The diagram shows an aluminium oxide electrolysis tank. Both the negative electrode (cathode) and positive electrode (anode) are made of carbon.

Aluminium metal forms at the negative electrode and sinks to the bottom of the tank, where it is tapped off.

Oxygen forms at the positive electrodes. This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide, and they gradually burn away. Consequently, the positive electrodes have to be replaced frequently, which adds to the cost of the process.

Most metals are very strong. They have high melting points and they have high heat and electrical conductivity. They are also malleable, which means they can be beaten or pressed into thin sheets.

Compare the melting and boiling points of the metals and non-metals in this table:

The uses we make of metals are related to their properties:

• Car bodies are made from steel, which is mostly iron, because it is a very strong material that is easy to press into the required shape.

• Electrical wiring is made from copper because it is a very good conductor of electricity.

• The filament of a light bulb is made from tungsten because this metal does not melt at the very high temperature needed to make it white hot.

Metals have their characteristic properties because of their giant structure. In a metal crystal, the atoms are in a regular arrangement and strongly bonded together. Strong metallic bonding makes metals hard, but allows layers of atoms to slide so that the metal is malleable. The layers of atoms also allow an electric current to pass through.

The extensive use of metals is having an effect on our environment:

• ***** mining of metal ores creates large areas of barren and lifeless land.

• Waste material from metal extraction is left in spoil tips that scar the landscape.

• While some used metals are recycled, many metal articles are simply dumped.

• Toxic metal compounds leach out of waste material to pollute the environment, killing wildlife

We can explain the properties of metals by taking a more detailed look at their structure. Metal crystals are made up of positive metal ions surrounded by a sea of negative electrons.

The strong electrostatic attraction between positive ions and negative electrons means that a lot of energy is needed to separate these particles from the crystal lattice. This means that metals are strong and have high melting and boiling points.

Much less energy is needed to slide one layer of positive metal ions over another layer. This explains why some metals are malleable, meaning that they are easy to beat or press into shape.

The sea of electrons in a metal crystal is mobile. If a potential difference is applied across a piece of metal, the electrons will move, carrying an electrical current. This means that metals are good conductors of electricity.

What mass of iron metal can be made from 5 tonnes of iron(III) oxide?

1. The equation is: 2Fe₂O₃(s) + 3C(s) = 4Fe(l) + 3CO₂(g)

2. Using the Periodic Table, the relative atomic mass of each element can be found. Relative atomic mass (A_r) of each element is: C = 12, Fe = 56, O = 16

3. These relative atomic masses can be used to find the formula masses of the metal compound. Fe₂O₃ has the formula mass = (2 x 56) + (3 x 16) = 160

4. From the equation, 2Fe₂O₃ produces 4Fe. Working this out in tonnes gives: 2Fe₂O₃ = (2 x 160) = 320 tonnes produces 4Fe = (4 x 56) = 224 tonnes of iron metal.

5. Therefore: 5 tonnes of iron(III) oxide produces 5 x 224/320 = 3.5 tonnes of iron.

It is also possible to calculate the maximum mass of a metal that can be obtained from a specific mass of ore by comparing their formulae.

1. Fe₂O₃ contains 2Fe

2. So 160 tonnes of Fe₂O₃ contains 2 x 56 = 112 tonnes of Fe

3. Therefore:

   5 tonnes of iron(III) oxide produces 5 x 112/160 = 3.5 tonnes of iron

During electrolysis, metal ions, which are positive, gain electrons from the negative electrode (cathode) to form neutral metal atoms.

In the extraction of aluminium, this is the equation for the reaction at the negative electrode:

Al³⁺ + 3e⁻ → Al

At the positive electrode (anode), non-metal ions, which are negative, lose electrons to form neutral atoms. These atoms join to make molecules of the non-metal element, such as molecules of oxygen gas.

In the extraction of aluminium, this is the equation for the reaction at the positive electrode:

2O²⁻ → O₂ + 4e⁻

The removal of electrons from the cathode and addition of electrons to the anode means that an electrical current is passing through the electrolyte.