Oxidation, Reduction and Redox

• Reduction and oxidation involve the transfer of electrons
• Oxidation


  Loss of electrons
• Reduction


  Gain of electrons
• The reaction where calcium is completely burnt in oxygen involves the reduction and oxidation of the elements involved
• This reaction is called a redox reaction
• Calcium is being oxidized


  Calcium is losing electrons and forming Ca2+ ions
• Oxygen is being reduced


  Oxygen is gaining electrons and forming O2- ions
• Reducing agent


  A chemical that loses electrons and oxidizes itself
• Oxidizing agent


  A chemical that gains electrons and is itself reduced
• Calcium is the reducing agent in this reaction
• Oxygen is the oxidizing agent in this reaction
• Oxidation number


  A number assigned to an element that represents its degree of oxidation
• Rules for assigning oxidation numbers

  • Uncombined elements have an oxidation number of 0
  • The oxidation number of an ion is equal to its charge
  • Group 1 elements have an oxidation number of +1
  • Group 2 elements have an oxidation number of +2
  • Aluminium has an oxidation number of +3
  • Hydrogen has an oxidation number of +1 except in hydrides where it is -1
  • Chlorine has an oxidation number of -1 except when bonded to oxygen or fluorine
  • Fluorine always has an oxidation number of -1
  • Oxygen generally has an oxidation number of -2 except in peroxides where it is -1
• Calculating oxidation numbers


  • N in NH3: N is -3 since H is +1 and there are 3 of them
  • S in H2S: S is -2 since H is +1 and there are 2 of them
  • O in O2: O is 0 since it is an uncombined element
  • O in H2O2: O is -1 since this is a peroxide
  • S in SO4(2-): S is +6 since O is -2 and there are 4 of them, making the total charge -2
• Transition metals have variable oxidation states
• Oxidation states of iron

  • Fe2O3: Fe is +3
  • FeO: Fe is +2
• The oxidation states of the elements must add up to the overall charge of the compound
• Iron has different oxidation states, very typical of transition metals
• Vanadium oxide, vanadium must be +4 to balance the charge
• Another vanadium compound, vanadium must be +5 to balance the charge
• Vanadium is a transition metal, it has different oxidation states
• Reduction


  Decrease in oxidation number
• Oxidation


  Increase in oxidation number
• 2 Na + Cl- will form 2 NaCl, sodium is oxidised from 0 to +1, chlorine is reduced from 0 to -1
• Sodium is the reducing agent, chlorine is the oxidising agent
• Balancing half equations


  1. Write species before and after

  2. Balance atoms apart from O and H

  3. Balance O with H2O

  4. Balance H with H+

  5. Balance charges with e-
• Half equations must have electrons
• Combining two half equations to form a full ionic equation, ensure electrons balance
• Final ionic equation should have no electrons, it shows both reduction and oxidation (redox)