C5: Chemicals of the Natural Environment

• Atmosphere - a layer of gases surrounding the Earth. It is made up of the elements nitrogen (N), oxygen (O), traces of argon (Ar) and some compounds, e.g. carbon dioxide and water vapour.
• Hydrosphere - all the water on the Earth, including oceans, seas, rivers, lakes and underground reserves. The water contains dissolved compounds.
• Lithosphere - the rigid outer layer of the Earth made up of the crust and the part of the mantle just below it. It is a mixture of minerals, such as silicon dioxide. Abundant elements in the lithosphere include silicon, oxygen and aluminium.

• Oxygen (21%), nitrogen (78%), carbon dioxide (approx 0.035%), water vapour (varies from 0-4%) and argon (0.093%).
• The chemicals that make up the atmosphere consist of non-metal elements and molecular compounds made up from non-metal elements.
• The molecules (with the exception of water) that make up the atmosphere are gases at 20 degrees celsius because they have very low boiling points, i.e. they boil below 20 degrees. This can be explained by looking at the structure of the molecules. Molecular compounds have strong covalent bonds between the atoms that make up the compound but only weak forces of attraction between the small molecules.
• For example, gases consist of small molecules with weak forces of attraction between the molecules. Only small amounts of energy are needed to break these forces, which allows the molecules to move freely through the air.
• The atoms within molecules (e.g. hydrogen) are connected by strong covalent bonds.
• In covalent bonds, electrons are shared between the nuclei of two atoms. This causes a strong, electrostatic attraction between the nuclei and shared electrons.
• Unlike ionic compounds, pure molecular compounds do not conduct electricity because their molecules are not charged.

• Seawater in the hydrosphere is 'salty' because it contains dissolved ionic compounds. Examples of dissolved ionic compounds are: sodium chloride, magnesium chloride, magnesium sulfate, sodium sulfate, potassium chloride and potassium bromide.
• Water has some unexpected properties. For example, the boiling point of water is 100 degrees. This is a much higher boiling point than for the other molecules. Water is also a good solvent for salts. The properties can be explained by its structure.
• The water molecule is bent, because the electrons in the covalent bond are nearer to the oxygen atom than the hydrogen atoms. The result is a polar molecule.
• The small charges on the atoms mean that the forces between the molecules are slightly stronger than in other covalent molecules. Therefore, more energy is needed to separate them.
• The small charges also help water to dissolve ionic compounds as the water molecules attract the charges on the ions. The ions can then move freely through the liquid.

• Ions in ionic compounds can be detected and identified because they have distinct properties and they form chemicals with distinct properties.
• For example, an insoluble compound may precipitate on mixing two solutions of ionic compounds. This technique is often used to identify metal ions.
• In the oceans, dissolved calcium ions and carbonate ions combine to form a precipitate of calcium carbonate (limestone), which falls to the ocean floor.
• In order to identify a negative ion, a range of different tests can be carried out. These involve adding a reagent to the unknown sample, which reacts with the ions to form an insoluble salt.
• To identify the presence of a sulfate ion, add barium chloride solution and dilute hydrochloric acid to the suspected sulfate solution. A white precipitate of barium sulfate will be produced if a sulfate is present.
• To identify the presence of a chloride, bromide or iodide ion, add silver nitrate solution and nitric acid to the suspected halide solution. A white precipitate will form if silver chloride is present, a cream precipitate for silver bromide and a yellow precipitate for silver iodide.
• In general, most ionic compounds are soluble in water, but there are some exceptions. 

Testing with Acids

• Carbonates react with dilute acids to form carbon dioxide gas (and a salt and water). 
• For example, if we add calcium carbonate to dilute hydrochloric acid then the carbonate will 'fizz' as it reacts with the acid, giving off carbon dioxide.

Thermal Decomposition

• When copper carbonate and zinc carbonate are heated, a thermal decomposition reaction takes place.
• This results in a distinctive colour change, which enables the two compounds to be identified.

• The three most abundant elements in the Earth's crust are oxygen, silicon and aluminium. Much of the silicon and oxygen is present as the compound silicon dioxide.
• Silicon dioxide - forms a giant covalent structure, where each silicon atom is covalently bonded to four oxygen atoms.
• Each oxygen atom is bonded to two silicon atoms. The result is a very strong, rigid 3D structure, which is very difficult to break down. 
• Silicon dioxide does not conduct electricity because there are no ions or free electrons in the structure.  It does not dissolve in water because there are no charges to attract the water molecules. It also has high melting and boiling points.
• Silicon dioxide exists in different forms such as quartz in granite and it is the main constituent of sandstone.
• Amethyst is a form of quartz that is used as a gemstone. It is cut, polished and used in jewellary. The violet colour comes from traces of manganese oxide and iron oxides found in the quartz. Some gemstones are very valuable  because of their rarity, hardness and shiny appearance.

Carbon

• Carbon is another example of a mineral that forms a giant covalent structure. Two forms of carbon are diamond and graphite.
• Diamond - has a large number of covalent bonds, which means it has very high melting and boiling points. 
• Each carbon atom is covalently bonded to four other carbon atoms, resulting in a very strong, rigid, 3D structure that is difficult to break down. 
• Diamond is insoluble because there are no charges to attract water molecules. It does not conduct electricity because there are no ions or free electrons in the structure.
• Graphite - form of carbon that has a giant covalent structure. Each carbon atom is covalently bonded to three other carbon atoms in a layered structure. The layers can slide past each other, making it soft and slippery.
• Graphite is also insoluble and has high melting and boiling points as the carbon atoms in each layer are held together by strong covalent bonds. Graphite can conduct electricity because the electrons forming the weak bonds between the layers are free to move throughout the whole structure.

• Carbon - diamond, very hard, used for drill tips. A lot of energy is needed to break the strong covalent bonds between the atoms.
• Carbon - graphite, soft, used for pencils. Layers easily removed and stick to paper.
• Silicon dioxide, high melting point (1610 degrees celsius), used for furnace linings. A lot of energy is needed to break the strong covalent bonds between the atoms.
• Silica glass, does not conduct electricity, used as insulator in electrical devices. No free electrons or ions to carry electrical charge.

• Atoms are too small for their actual atomic mass to be of much use for us so we use the relative atomic mass (RAM). This is a number that compares the mass of one atom to the mass of other atoms.
• Each element in the periodic table has two numbers. The larger of the two (at the top of the symbol) is the mass number, which also doubles up as the relative atomic mass.

• The relative formula mass (RFM) of a compound is the relative atomic masses of all its elements added together. To calculate RFM we need to know the formula of the compound and the RAM of each of the atoms involved.

• The lithosphere contains many naturally occurring elements and compounds called minerals. Ores are rocks that contain varying amounts of minerals, from which metals can be extracted. Sometimes very large amounts of ores need to be mined in order to recover a small percentage of valuable minerals, e.g. copper. The method of extraction depends on the metal's position in the reactivity series.
• Most reactive - Sodium
• Calcium
• Magnesium    - These metals are very reactive so electrolysis is used to extract.
• Aluminium
• Carbon
• Zinc    -These metals are below carbon and are extracted by their ores by reduction w/ carbon.
• Iron
• Lead
• Hydrogen   -All metals below hydrogen can be extracted using hydrogen.
• Copper
• Gold   -These metals are unreactive and exist naturally so obtained by physical processes.
• Least reactive - Platinum

Making the material from natural raw materials 

• Mining - lots of rock wasted, leaves a scar on the landscape, air pollution, noise pollution.
• Processing - pollutants caused by transportation, energy usage.
• Extracting the metal - electrolysis uses more energy than reduction.

Making the product from the material

• Manufacturing products - energy usage in processing and transportation.

Use

• Transport - pollutants caused by transportation.
• Running product - energy usage.

Disposal

• Reuse - No impact.
• Recycle - Uses a lot less energy than the initial manufacturing.
• Throw away - landfill sites remove wildlife habitats and are unsightly.

• Electrolysis is the decomposition of an electrolyte (a solution that conducts electricity) using an electric current. The process is used in industry to extract reactive metals from their ores.
• Ionic compounds will only conduct electricity when their ions are free to move. This occurs when the compound is either molten or dissolved in solution.
• During melting of an ionic compound, the electrostatic forces between the charged ions are broken. The crystal lattice is broken down and the ions are free to move.
• Molten lead bromide is a liquid containing positive lead ions and negative bromide ions that are free to move throughout the liquid. 
• When a direct current is passed through the molten salt, the positively charged lead ions are attracted towards the negative electrode. 
• The negatively charged bromide ions are attracted towards the positive electrode.
• As a result, lead is formed at the negative electrode and bromine at the positive electrode.
• When the ions get to the oppositely charged electrode, they are discharged (they lose their charge) The bromine ion loses electrons to the positive electrode to form a bromine atom which then bonds with a second atom to form a bromine molecule. The lead ions gain electrons from the negative electrode to form a lead atom.
• This process completes the circuit as the electrons are exchanged at the electrodes.

Aluminium must be obtained from its ore by electrolysis because it is too reactive to be extracted by heating with carbon. (Look at its position in the reactivity series.) The steps in this process are:

• Aluminium ore (bauxite) is purified to leave aluminium oxide.
• Aluminium oxide is mixed with cryolite (a compound of aluminium) to lower its melting point.
• The mixture of aluminium oxide and cryolite is melted so that the ions can move.
• When a current passes through the molten mixture, positively charged aluminium ions move towards the negative electrode (the cathode) and aluminium is formed. Negatively charged oxide ions move towards the positive electrode (the anode) and oxygen is formed.
• This causes the positive electrodes to burn away quickly. They have to be replaced frequently.

The electrolysis of bauxite to obtain aluminium is quite an expensive process due to the cost of the large amounts of electrical energy needed to carry it out. The equation for this reaction is:

aluminium oxide --------// aluminium + oxygen

The reactions at the electrodes can be written as half equations - seperate equations for what is happening at each of the electrodes during electrolysis. At the negative electrode (cathode) -reduction. At the positive electrode (anode) - oxidation.

Generally, metals are strong and malleable, have high melting points and can conduct electricity. Their properties determine how each metal can be used:

• Titanium is very strong, lightweight and resistant to corrosion so it is used for replacement hip joints, bicycles and submarines.
• Aluminium is malleable, lightweight and resistant to corrosion so it is used for drinks cans, window frames, saucepans and aircraft.
• Iron has a high melting point and is strong so it is used for saucepans and cars.
• Copper conducts electricity and conducts heat so it is used for cables, e.g. kettle cable, electromagnets, electrical switches and saucepans.

In a metal crystalline structure, the positively charged metal ions are held closely together by a 'sea' of electrons that are free to move.

The properties of a metal can be explained by its structure. The force of attraction that keeps the structure together is known as the metallic bond. The metallic bond can be used to explain the properties of metals.

• Very strong - Metal ions are closely packed in a lattice structure.
• High melting point - A lot of energy is needed to break the strong force of attraction between the metal ions and sea of electrons.
• Malleable - External forces cause layers of metal ions to move by sliding over other layers.
• Conducts electricity - Electrons are free to move throughout the structure. When an electrical force is applied, the electrons move along the metal in one direction.