C5 - Chemicals of the Natural Environment

Atmosphere - COVALENT (sharing)
1. the chemicals consist of non-metal elements and molecular compounds from non-metal elements
  1. gases consist of small molecules with weak forces of attraction between the molecules. so, a small amount of energy is needed to break these forces, allowing molecules to move freely through air.
2. molecules (exception of water) are gases at room temperature and have low boiling points - molecular structures. MOLECULAR COMPOUNDS HAVE STRONG COVALENT BONDS BETWEEN THE ATOMS but WEAK FORCES OF ATTRCATION BETWEEN THE SMALL MOLECULES
  1. In covalent binds, the electrons are shared between the nuclei of two atoms. this causes a strong electrostatic attraction between the nuclei and shared electrons
Lithosphere
1. a mixture of minerals, such as silicon dioxide (SiO2) Abundant elements include: Silicon (Si), Oxygen (O2) and Aluminium (Al). much of the silicone and oxygen are present as silicon dioxide
  1. silicon dioxide forms a GIANT COVALENT STRUCTURE. here, each silicon atom is covalently bonded to 4 oxygen atoms
    1. each oxygen atom is bonded to 2 silicon atoms. the result is a very strong rigid three structure
      1. silicon dioxide doesn't does not conduct electricity as there are no ions or free electrons in the structure. it doesn't dissolve in water because there are no charges to attract 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 in jewelry. the violet colour comes from traces of magnesium oxide and iron oxides found in the quartz. some gemstones are very valuable because of their rarity, hardness and shiny appearance
  2. Carbon is another example of a mineral that forms a giant covalent structure. 2 forms of carbon are:
    1. Diamond
      1. large number of covalent bonds - high melting and boiling points. each carbon atom is covalently bonded to 4 other carbon atoms - results 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 doesn't conduct electricity because there are no ions or free electrons in the structure
    2. Graphite
      1. a 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 over each other making it soft and slippery
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Hydrosphere - IONIC (+ve,-ve)
1. dissolved ionic compounds (making water taste salty) are:
  1. Sodium Chloride - NaCl
    1. Magnesium Chloride - MgCl2
      1. magnesium sulphate - MgSO4
        sodium sulphate - Na2SO4
        potassium chloride - KCl
        potassium bromide - KBr
  2. water has some unexpected properties - boiling point is 100 - much higher than the other molecules. WATER IS ALSO A GOOD SOLVENT FOR SALTS - PROPERTIES EXPLAINED BY ITS STRUCTURE
    1. the water molecule is bent - the electrons in the the covalent bond are nearer to the oxygen atom than the hydrogen atoms. this results in a polar molecule.
      1. the top is slightly negative and the bottom is slightly positive. the small charges on the atoms mean that the forces between the molecules are SLIGHTLY STRONGER THAN IN OTHER COVALENT MOLECULES. - more energy is needed to seperate them.
        smaller charges also help water to dissolve ionic compounds as the water molecules attract the charges on the ions - ions can move freely through the liquid.
2. ions in ionic compounds can be detected and identified as they have distinct properties and form chemicals with distinct properties.
  1. e.g. an insoluble compound may precipitate on mixing 2 solutions of ionic compounds
    1. this technique is often used to identify metal ions.
      1. in this example, a white precipitate of calcium hydroxide is formed as well as sodium chloride solution. Ca2+ + 20H- -> Ca(OH)2
3. in the oceans, dissolved calcium ions and carbonate ions combine to form a precipitate of calcium carbonate - limestone which falls to the sea floor.
4. in order to identify a negative ion, a range of different tests may be carried out, involving adding a reagent to an unknown sample - reacts with the ions to form an insoluble salt.
  1. to identify the presence of a sulfate ion, add barium chloride solution and dilute HCl to the suspected sulfate solution. - A WHITE PRECIPITATE OF BARIUM SULFATE WILL BE PRODUCED IF A SULFATE IS PRESENT,
  2. Chloride, Bromide and Iodide Ions - to identify the presence of these, add silver nitrate solution and nitric acid to the suspected halide solution - A WHITE PRECIPITATE WILL FORM IF SILVER CHLORIDE IS PRESENT; CREAM PRECIPITATE FOR SILVER BROMIDE AND A YELLOW PRECIPITATE FOR SILVER IODIDE.
5. In general, most ionic compounds are soluble in water but there are some exceptions.
extracting useful metals from minerals
structure and properties of metals
testing for carbonates
1. testing with acids
  1. carbonates react with dilute acids to form carbon dioxide gas and a salt and water. e.g. if we add calcium carbonate to dilute hydrochloric acid, the carbonate will FIZZ, giving off carbon dioxide. so, the dilute acid and carbonate will turn lime water milky.
  2. thermal decomposition
    1. when copper carbonate and zinc carbonate are heated, thermal decomposition takes place - resulting in a distinctive color change, enabling the 2 compounds to be identified.
      1. strongly heating copper carbonate in a combustion tube causes it to go black as copper oxide is formed.
      2. strongly heating zinc carbonate in a combustion tube causes it to go yellow as zinc oxide is formed. the yellow colour is only seen at high temperatures as it is of a different cystalline form. when it cools, the zinc carbonate turns white

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C5 - Chemicals of the Natural Environment

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