Redox

• Reducing agents


  Electron donors
• Oxidising agents


  Electron acceptors
• Oxidation


  1. Process of electron loss

  2. Involves an increase in oxidation number
• Reduction


  1. Process of electron gain

  2. Involves a decrease in oxidation number
• Rules for assigning oxidation numbers

  • All uncombined elements have an oxidation number of zero
  • The oxidation numbers of the elements in a compound add up to zero
  • The oxidation number of a monoatomic ion is equal to the ionic charge
  • In a polyatomic ion, the sum of the individual oxidation numbers of the elements adds up to the charge on the ion
  • Several elements have invariable oxidation numbers in their common compounds
• Note the oxidation number of Cl in CaCl2 = -1 and not -2 because there are two Cl's
• Always work out the oxidation per one atom of the element
• Oxidation number of Fe in FeCl3

  +3
• Redox equations and half equations

  1. Br2 (aq) + 2I- (aq) → I2 (aq) + 2 Br- (aq)

  2. Br2 (aq) + 2e- → 2 Br- (aq)

  3. 2I- (aq) → I2 (aq) + 2 e-
• Oxidising agent


  Bromine water. It is an electron acceptor
• Reducing agent


  Iodide ion. It is an electron donor
• Oxidising agent (or oxidant)


  The species that causes another element to oxidise. It is itself reduced in the reaction.
• Reducing agent (or reductant)


  The species that causes another element to reduce. It is itself oxidised in the reaction.
• Reduction half equation


  Only shows the parts of a chemical equation involved in reduction. The electrons are on the left.
• Oxidation half equation


  Only shows the parts of a chemical equation involved in oxidation. The electrons are on the right.
• Balancing redox equations


  1. Work out oxidation numbers for element being oxidised/ reduced

  2. Add electrons equal to the change in oxidation number

  3. Check to see that the sum of the charges on the reactant side equals the sum of the charges on the product side
• Writing half equations (more complex)

  1. Balance the change in O.N. with electrons

  2. Add H2O in products to balance O's

  3. Add H+ in reactants to balance H's in H2O

  4. Check to see that the sum of the charges on the reactant side equals the sum of the charges on the product side
• Combining half-equations


  1. To make a full redox equation combine a reduction half equation with an oxidation half equation

  2. To combine two half equations there must be equal numbers of electrons in the two half equations so that the electrons cancel out