Oxidation and reduction

• Oxidation


  Loss of electrons
• Reduction


  Gain of electrons
• Oxidation and reduction always occur together
• Oxidation-reduction reactions are also called redox reactions
• Definitions of oxidation and reduction in terms of electron transfer are broader than previous definitions in terms of oxygen and hydrogen
• Oxidising agent


  Accepts electrons, gets reduced
• Reducing agent


  Donates electrons, gets oxidised
• Oxidising agents


  • Hydrogen peroxide
  • Sodium hypochlorite
• Reducing agents

  • Carbon monoxide
  • Sulfur dioxide
• Reducing action of sulfur dioxide is slowly reversed by oxygen in the air
• Reducing agent


  A substance that removes oxygen from iron ore to convert it to iron metal
• Reducing agents


  • Carbon monoxide
  • Sulfur dioxide
• Reducing action of sulfur dioxide

  1. Converts the yellow colour of the dye in straw to a colourless form

  2. Slowly reversed by the oxygen in the air, causing paper and straw to turn from white to yellow with age
• Hydrogen peroxide is a very useful bleaching agent, it removes the natural pigments in hair to a blonde colour
• Oxidation


  The loss of electrons
• Reduction


  The gain of electrons
• Oxidation-reduction reactions do not necessarily involve oxygen, and not all involve electron transfer to form ionic compounds
• Oxidation number


  A number assigned to atoms to help track the movement of electrons during chemical reactions, especially in covalent compounds
• Rules for assigning oxidation numbers

  • The oxidation number of any uncombined element is zero
  • The oxidation number of an ion is the same as its charge
  • The sum of oxidation numbers in a compound must be zero
  • Oxygen always has an oxidation number of -2 except in peroxides (-1) and OF2 (+2)
  • Hydrogen is +1 in compounds except metal hydrides (-1)
  • Halogens are -1 in compounds unless bonded to more electronegative atoms