Core Chemistry - Topic 4 Obtaining and Using Metals

• Metals found as elements
  • Gold, platinum, silver (very few)
  • Very unreactive
  • Have never reacted with oxygen
• Metals found in compounds
  • If metal is reactive, it easily forms compounds with other substances
  • Most metals are obtained from their compounds by chemical reactions
  • Most compounds are oxides
  • Found in rocks that are called ores if metal can be profitably extracted
• Examples of ores
  • Bauxite – Aluminium
  • Cinnabar – Mercury
  • Galena – Lead
  • Haemaitte – Iron
  • Malachite – Copper

• Extraction – chemical reaction process of getting metal from a compound in a rock
  • Some extracted by heating their compounds with carbon
  • Others extracted through electrolysis of a molten compound
  • Electrolysis decomposes some compounds into their elements
• Method of metal extraction depends on its reactivity
  • More reactive – harder to extract 
• Electrolysis of a molten compound – Most reactive
  • Potassium, sodium, calcium, magnesium, aluminium
• Heat an ore with carbon – Medium reactive
  • Zinc, iron, tin, lead, copper
• Found as the uncombined element – Least reactive
  • Silver, gold, platinum
• Electrolysis is more expensive than heating with carbon (cost of electricity)
  • Reactive metals are harder and more expensive to extract

• Metal extraction is reduction
  • To obtain a metal from its compound (usually oxide), oxygen must be removed
  • When removed, its reduced and called reduction
• Corrosion of metals is oxidation
  • Most metals corrode – metal becoming oxidised
  • Surface of the is metal changed by reacting with oxygen and sometimes water
  • Oxidation is the addition of oxygen to a substance
• High reactivity – more rapid corrosion
• Low reactivity – more resistant to corrosion
  • BUT Aluminium - the surface oxides quickly forming a protective layer of aluminium oxide, stops further corrosion

• Many metals can be recycled
  • Melted down to make something new
• Advantages
  • Natural reserves of metal ores will last longer
  • For most metals, less energy is required to recycle than to extract from their ores
    • therefore also cheaper
  • Reduces the need to mine ores and damage the landscape/create noise and dust pollution
  • Produces less pollution than fossil fuelled electrolysis and gases from extracting
  • Less waste metals are filling up landfill sites
• Separated by magnetism (steel, iron) or by hand (others)
• Melted into blocks and sold to manufacturers

• % of metal used that is recycled
  • aluminium – 40%
  • copper –     30%
  • lead –         75%
  • iron/steel –  40%

• Many useful properties
  • Shiny when polished
  • Conduct heat and electricity
  • Malleable (hammered into shape)
  • Ductile (stretched into wires)
  • Different metals useful for different things
• Aluminium
  • Low density
  • Does not corrode
  • Used to make aeroplanes and many cars
• Copper
  • Extremely good electrical conductor
  • Low reactivity
  • Does not react with water
  • Used to make electrical cables, water pipes

• Gold
  • Very unreactive
  • Does not corrode – remains shiny
  • Very attractive in appearance
  • Worked easily into shapes
  • Best electrical conductor
  • Used for jewellery, most electronic devices, printed circuit boards, connection strips, contacts in switches
• Iron and steel
  • Used more than any other metal
  • Cheap to extract by heating with carbon
  • Iron is soft, so it’s mixed into steel with carbon
  • Steel is strong and hard
  • Magnetic
  • Rust – reacts with oxygen and water
  • Used for bridges, cars, electrical products, machinery, frames of buildings

Alloy - Metals mixed with other metals to improve properties for specific applications

• In a pure metal, atoms are the same size
  • Layers of atoms can slide over each other
  • This makes them soft
• In alloys, some atoms of other elements are different sizes
  • Layers are prevented from moving past each other so easily
  • This makes them much harder and stronger

<DIAGRAM>

• Many different alloy steels
  • small amounts of other metals mixed with iron
  • stronger than iron (more force to break them)
  • some resist corrosion
    • stainless steel - alloy of iron, chromium and nickel doesn't corrode at al
• Pure gold is too soft to use in jewellery successfully
  • copper or silver are added to make it harder and stronger
  • the purity is measured in carats or as a fineness

Carats      % of gold/fineness

24              100%       1000

22               91.7%       917

18               75%          750

12               50%          500

  9               37.5%       375