oxidation and redox

• oxidation is the process of electron loss

  it involves an increase in oxidation number
• reduction is the process of electron gain

  it involves a decrease in oxidation number
• all uncombined elements have an oxidation number of zero
• the oxidation numbers of the elements in a compound add up to zero
• oxidation number of a monoatomic ion is equal to the ionic charge

  e.g Zn2+ = +2 oxidation state
• in a polyatomic ion the sum of the individual oxidation numbers of the elements adds up to the charges on the ion
• group 1 metals have +1 oxidation number
• group 2 metals have +2 oxidation number
• Al has +3 oxidation number
• H has +1 oxidation number except in metal hydrides where it is -1
• fluorine has a -1 oxidation number
• Cl, Br and I have a -1 oxidation number except in compounds with oxygen and fluorine
• O has -2 oxidation number except in peroxides where it is -1 and in compounds with fluorine
• a reduction half equation only shows the parts of a chemical equation involved in reduction

  the electrons are on the left
• an oxidation half equation only shows parts of a chemical equation involved in oxidation

  the electrons are on the right
• reducing agents are electron donors that cause another element to reduced

  they are oxidised themselves in the reaction
• balancing redox equations

  1. work out oxidation numbers for element being oxidised/reduced
  2. add electrons equal to the change in oxidation number (for reduction add to reactants, for oxidation add to products)
  3. check that the sum of charges on both sides are equal
• if the substance being oxidised or reduced contains a varying amount of O, half equations are balanced by adding H+ and H2O
• to make a full redox equation combine a reduction half equation with an oxidation half equation
• in full redox equations the two combined equations must have the same number of electrons to cancel out