C5- Chemicals of the Natural Environment

a brief summary of the OCR 21st century chemistry module C5- Chemicals of the Natural Environment

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  • Created by: R_Hall
  • Created on: 22-06-11 14:23

Chemicals in Four Spheres

  • The lithosphere is broken into tectonic plates which move as they slide on top of the mantle. The crust and upper mantle make up the lithosphere.
  • The rocks of the lithosphere are mainly made of silicates (compounds of silicon and oxygen). Carbon and hydrogen are in the crude oil.
  • Oceans and rivers make up the hydrosphere.Its almost entirely the compound water.
  • The biosphere includes all the living organisms on Earth. Most of the chemicals that make up living things are compounds of carbon with oxygen and hydrogen.
  • The atmosphere is a layer of air that keeps the Earth warms. Carbon is present in carbon dioxide.
  • Chemicals constantly move between the spheres.
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Chemicals of the Atmosphere

  • Earth is just the right size and distance from the Sun so that gravity can hold the gases and it is the correct temp for liquid water to exist.
  • Composition of atmosphere- 78% N2, 21% O2, 1% Ar, 0.03% CO2.
  • All the chemicals in the atmosphere have low boiling and melting points because they are gases at room temperature.
  • All the chemicals in the atmosphere are either non-metallic elements (O2,N2 and Ar) or are compounds made from non-metals (CO2).
  • Most of the chemicals in the atmosphere are made up of small atoms.
  • All molecules have a tendency to stick together; the attractive forces are weak.The forces inside molecules that hold atoms together are strong.
  • Molecular models show the arrangement of atoms in molecules, and the bonds between them.
  • A molecule of H2 is held together by electrostatic attraction between the 2 nuclei and the shared pair of electrons.
  • This type of strong bonding is covalent bonding.
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Chemicals of the Hydrosphere

  • Molecules of water have a tendency to stick together (greater than a gas).
  • As water cools (below 0c), it expands and becomes less dense- floats.
  • Water is a good solvent for salts. Most common solvents do not dissolve ions, but water does.
  • Pure water does not conduct electricity- it does not contain charged particles that are free to move. Impure water does conduct electricity.
  • The 3 atoms in a water molecule are not arranged in a straight line- an angle.
  • In the covalent bonds between the atoms, the electrons are not evenly shared. The oxygen atoms have a -ive charge, H has +ive.
  • The small charges on opposite sides of the molecules cause slightly stronger attractive forces- help to dissolve ionic compounds.
  • The attractions between water molecules an the angular shape mean in ice they line up to create a very open structure (ice less dense than liquid).
  • Most of the compounds that are dissolved in sea water are made up of +ively charged metal ions and -ively charged non-metal ions- salts.
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Chemicals of the Lithosphere

  • Rocks are made from 1 or more minerals- naturally occurring elements (elements or compounds).
  • The 2 most abundant chemicals in the lithosphere (O2 47%, Si 28%) form the major types of minerals. SiO2 (silicon dioxide) can take various crystalline forms (quartz).
  • When water evaporates, ionic compounds crystallize. Minerals formed this way are called evaporites.
  • The ions of NaCl are held together very strongly by the attraction between their opposite charges- ionic bonding, structure is giant ionic structure.
  • Because of the strong attractive forces, it takes a lot of energy to break down the regular arrangement of ions.
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Silica and Silicates

  • The mineral silica consists of SiO2- the commonest crystalline form in quartz.
  • When sand is compressed, it forms sandstone (made of SiO2).
  • Each Si atom forms a covalent bond to 4 O atoms; each O atom forms a covalent bond to 2 Si atoms.
  • Instead of forming small molecules, Si and O atoms form a giant covalent structure.
  • The Si-O covalent bond is very strong- the giant structure is strong and rigid.
  • Over 95% of rocks in the Earth's continental crust are formed by silica and the silicate materials.
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Chemicals of the Biosphere

  • Most biochemicals are based on C, H and O (sometimes S, N and P).
  • Carbon is the element life is based on. Special things-
  • 1. Carbon atoms can form chains by joining themselves.
  • 2. Carbon forms 4 strong covalent, so other atoms can join onto the chain.
  • These properties mean that carbon can make a variety of compounds. This is why life is varied. Most biochemicals are polymers.
  • Hair, skin, enzymes and muscle are made from proteins.
  • Photosynthesis in the leaves of plants turns CO2 and H2O into glucose, a sugar in the carbohydrate family.
  • There are 3 elements in carbohydrates- O, H and C.
  • Glucose is a very soluble sugar. Plants convert it to starch, an insoluble polymer made up of long chains of glucose.
  • Cellulose is also made by plants from glucose; makes up cell walls.
  • DNA and RNA are nucleic acids that carry the genetic code.
  • The backbone of DNA is a polymer with alternating sugar and phosphate groups. A, T, G and C are attached to it.
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Human Impacts on the Environment

  • As living things grow, die and decay, elements move between the biosphere, hydrosphere, atmosphere and lithosphere.
  • Some human activities make a difference to the natural cycles. An important example is the effect of fossil fuels on the carbon cycle.
  • The flow of CO2 in the air from burning fuels is small compared with the natural flows of photosynthesis and respiration.
  • Nitrogen is essential for making biochemicals (proteins).
  • N2 has small molecules with weak bonds between them.
  • NO3- and NH4+ are ions that are attracted to H2O molecules- dissolve.
  • Growing crops take the N they need from the soil in the form of nitrate ions. Fertiliser replace nitrate removed from the soil when crops are grown.
  • Only a few bacteria and algae convert N2 gas to nitrates- Fixing nitrogen.
  • 3 main ways of fixing nitrogen-
  • 1. Action of microorganisms.
  • 2. A chemical reaction in the air during lightening flashes.
  • 3. The Haber process to manufacture fertilisers.
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Metals from the Lithosphere

  • All metals are from the lithosphere, but some are too reactive to exist on their own- compounds.
  • Ores are rocks containing useful mineral, minerals are often the oxides of sulfides or metals.
  • More reactive metals like iron were not used by humans until extraction methods were developed.
  • Rich deposits of ore have built up but they are not pure (dirt and rocks)- need to concentrate the ore.
  • Factors to think about when extracting a metal-
  • 1. How can the ore be reduced?
  • 2. Is there a good supply of ore?
  • 3. What are the energy costs?
  • 4. What is the impact on the environment?
  • To convert ZnO to Zn, the oxygen is removed through reduction.The process needs a reducing agent that is oxidised to remove O.
  • To find out how much metal can be extracted from ore, relative formula masses and relative atomic masses are used.
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Extraction Through Electrolysis

  • Some reactive metals (Al) hold onto C so strongly that cannot be extracted using carbon- electrolysis.
  • Al is the most abundant metal in the lithosphere; the main ore is bauxite.
  • The Al is put in steel tanks lined with carbon (the negative electrode). The electrolyte is molten Al2O3 (containing Al3+ and O2- ions).
  • Aluminium forms at the negative electrode. Oxygen forms at positive.
  • Positive metal ions gain electrons from the negative electrode and turn into atoms. Negative ions give up electrons to positive electrode and turn back.
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Structure and Bonding in Metals

  • The 4 main properties of metals-
  • 1. Strong.
  • 2. Can be bent or pressed into shape.
  • 3. High melting points.
  • 4. Conduct electricity.
  • The model for the structure of metals. Metal atoms are-
  • -Tiny spheres.
  • - Arranged in a regular pattern.
  • - Packed close together in a crystal as a giant structure.
  • Metallic bonds are strong and flexible; they allow the atoms to move to a new position.
  • In a metal, the atoms lose e- from outer shell and become positive ions. The e- drift between metal atoms+. The attraction holds the metal together.
  • The moving e- conduct electricity; when an electric current flows, the e- move from 1 end of wire to other.
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The Life Cycle of Metals

  • The 5 main stages of metal production, use and disposal can have serious impact on the environment-
  • Mining- Produces large volumes of waste rock and can leave holes in the ground. Explosives are used to blast the rocks- noisy and dusty.
  • Processing ores- When mining copper, nearly 99.6% of the rock dug up is waste. Waste tips are needed (traces or toxic mercury and lead).
  • Metal extraction- All stages of metal fabrication and extraction need energy, use lots of water and emit air pollutants. However, social and economic pressure favours less harmful techniques and equipment.
  • Metals in use- Choice of metals can reduce environmental impact of our life style (lighter vehicles- less fuels).
  • Recycling- Scrap metal can be melted down and reshaped. Cuts down on water usage, coal usage and use of raw materials.
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