Redox, Rusting and Iron

• Redox 


  A redox reaction is one in which oxidation and reduction occur at the same time
• Redox reactions

  • oxidation
  • reduction
• Oxidation can be defined as

  • gain of oxygen
  • loss of hydrogen
  • loss of electrons
• Reduction can be defined as

  • loss of oxygen
  • gain of hydrogen
  • gain of electrons
• Reactions

  • can be described as oxidation or reduction in terms of the change in oxygen or hydrogen content
  • others can only be described in terms of electrons lost or gained
  • reduction is the reverse of oxidation
• Redox and displacement reactions 


  a displacement reaction is when a more reactive metal replaces a less reactive metal in a compound. It involves the transfer of electrons.Two types:

  1. solid metal reacts with a solution containing metal ions
  2. solid metal reacts with a solid metal oxide

  One will lose electrons and one will gain eletrons

  When both occur, it is a redox reaction
• Displacement reactions involving solutions 


  Mg metal reacts when placed in a solution of copper(11) sulfate. Explain in terms of electrons, why this reaction is redox

  • Mg + CuSO4 -> MgSO4 + Cu - balanced equation
  • Mg + Cu2+ + Cu - ionic equation
  • SO4- - spectator ion
  • Mg -> Mg2+ + 2e- = oxidation as it loses electrons
  • Cu2+ + 2e- -> Cu = reduction as it gains electrons

  The reaction is redox because both oxidation and reduction occurs
• Displacement reactions between solids 


  When a solid metal compound is heated with a solid metal, displacement occurs- it is a redox reaction
• Example of displacement reaction between solids 


  Zinc powder is mixed with black copper (11) oxide powder and heated in a crucible. The reaction produces a blue-green glow and a yellow solid, which changes to white on cooling. Explain in terms of oxygen content, why its redox

  • Zn+CuO->ZnO+Cu
  • Zinc gains oxygen which is oxidation
  • Copper (11) oxide loses oxygen- reduction
  • Redox is oxidation and reduction occurring at the the same time
• The cause of rusting

  When iron is exposed to air and moisture, it rusts
• Rust 


  hydrated iron (111) oxide, sometimes written as Fe2O3.xH2O
• Steel 


  an alloy of iron containing between 0.2% and 2% carbon. Steel is stronger than iron. The iron is steel nuts
• Investigation to determine factors that cause rusting

  • 3 test tubes, each containing iron nail
  • Test tube 1 has air and water present
  • Test tube 2 has air removed from water via boiling so only water is present. Layer of olive oil prevents gases from air dissolving in water
  • Test tube 3 contains anhydrous calcium chloride which removes water vapour from air so only air is present. The nail is suspended to prevent contact between it and the calcium chloride
  • Test tubes are left for several days and the iron nail rusts only in Test tube 1. Indicates air and water are required for rusting
• Prevention of rusting 


  Rust is unsightly and dangerous because it weakens the metal. Iron and steel are used extensively in construction, so rusting is a major problem due to the cost of replacing the structure.

  Rusting can be prevented in a variety of ways. Methods fall into 3 groups
• Preventing surface of iron coming into contact with water and air


  • Using barrier or protective layer
• Protective layers


  • Paint used to protect cars, bridges and railings
  • Oil or grease used to protect tools and machinery
  • Plastic coatings used to cover bike handlebars, garden chairs and dish racks
• Car manufacturers are using plastic in cars
• Plating


  Iron is plated with another metal
• Plated materials


  • Cans for food made from steel and coated on both sides with thin layer of tin
  • Chromium used to coat steel, giving it a shiny, attractive appearance
• Tin coating


  Unreactive and non-toxic, deposited on steel by electrolysis
• Chromium coating


  Used for some vehicle bumpers and bike handlebars, can be applied by electrolysis
• 2. putting a more reactive metal in contact with iron or steel

  • The more reactive metal reacts first, leaving iron intact
  • Bars of Mg are attached to side of ships, oil rig and underwater pipes. Mg corrodes and must be replaced with fresh Mg. Called sacrificial protection
  • iron can be coated in zinc. Called galvanising. Zinc is more reactive and oxidise readily, forming a layer of zinc oxide
  • galvanising protects by sacrificial protection if the surface is scratched and zinc oxide provides barrier towards air and water
• 3. Alloying 


  an alloy is a mixture of two or more elements at least one is metal, resulting in a mixture with metallic properties

  • often stronger and more resistant to corrosion than the pure metals they are made from
  • Stainless steel is resistant to corrosion
• Extraction of iron in a blast furnace

  • The solid material put into the blast furnace is called the charge
  • it is made up of iron ore (haematite), limestone (calcium carbonate) and coke (carbon)
  • Hot air is blasted in through pipes near the bottom of the blast furnace
  • Reduction of iron ore happens because the iron (111) oxide loses oxygen or Fe3+ ions gain electrons to form Fe atoms
• 5 reactions involved in extraction of iron

  • Production of reducing agent carbon monoxide occurs in 2 of them
  • Reduction of iron ore occurs in 1 of them
  • Removal of acidic impurities occurs in 2 of them
• Production of reducing agent

  1. Coke burns in oxygen from hot air to produce carbon dioxide

  • C+O2->CO2

  2. Carbon dioxide reacts with more coke to produce the reducing agent carbon monoxide

  • CO2+C->2CO
• Reduction of iron ore to iron 


  3. Iron(111) oxide reacts with carbon monoxide to produce molten iron and carbon dioxide

  • Fe2O3+3CO->2Fe+3CO2

  This is the oxidation and reduction stage of the extraction of iron. Carbon and carbon monoxide reduces iron(111) oxide to iron. The carbon monoxide is oxidised as it gains oxygen. Iron(111) ions gain electrons which is reduction
• Removal of acidic impurities 


  4. Calcium carbonate thermally decomposes to form calcium oxide and carbon dioxide

  • CaCO3 -> CaO + CO2

  5. Acidic silicon dioxide impurities react with calcium oxide to form molten slag (calcium silicate)

  • SiO2 + CaO -> CaSiO3

  The molten slag and molten iron fall to the bottom of the furnace. Iron is denser than slag so it sinks below the slag. They are taped off separately at the base of blast furnace
• Why is iron used in construction of buildings and bridges?

  Iron is a strong metal