Redox and electrode potentials

• Reduction


  Gain of electrons OR decrease in oxidation number
• Oxidation


  Loss of electrons OR increase in oxidation number
• Important oxidation numbers


  • Uncombined element: 0
  • Combined hydrogen: +1
  • Combined oxygen: -2
  • Ion of element: Ionic charge
• Balancing redox reactions


  1. Step 1: Balance the reduction and oxidation half-reactions

  2. Step 2: Add the half-reactions together

  3. Step 3: Finalise the balanced equation
• Oxidising agent


  Takes electrons from the species being oxidised, contains the species that is reduced
• Reducing agent


  Adds electrons to the species being reduced, contains the species that is oxidised
• Redox reaction between Cu and Ag ions

  • 2Ag+(aq) + Cu(s) → Cu2+(aq) + 2Ag(s)
• Ag+ is the oxidising agent - Ag has oxidised Cu
• Cu is the reducing agent - Cu has reduced Ag+
• Construction of redox equations using half-equations


  1. Balance the electrons

  2. Add and cancel electrons

  3. Cancel any species that are on both sides of the equation
• Redox equation


  Equation for a redox reaction
• Construction of redox equations using oxidation numbers

  1. Assign oxidation numbers to all atoms in the equation

  2. Balance the increase in oxidation number during oxidation with the decrease in oxidation number during reduction

  3. Balance any remaining atoms
• In a redox reaction, the overall increase in oxidation number during oxidation must equal the overall decrease in oxidation number during reduction
• In aqueous redox reactions, H2O is often formed. Other likely products are H+ and OH- ions, depending on the conditions used
• Manganate (VII) redox titrations


  Titrations involving the reduction of MnO4- (aq) ions
• Procedure for manganate (VII) redox titrations

  1. Add a standard solution of KMnO4 (aq) to the burette

  2. Add a measured volume of the solution being analysed to the conical flask, along with an excess of dilute sulfuric acid

  3. During the titration, the manganate(VI) solution reacts and is decolourised as it is being added

  4. The end point is judged by the first permanent pink colour, indicating an excess of MnO4- ions

  5. Repeat the titration until concordant titres are obtained
• Reading the meniscus in manganate (VII) titrations


  Burette readings are read from the top, rather than the bottom, of the meniscus
• In a manganate(VI) titration, a student obtained a titre of 22.30 cm3. Each burette reading taken had an uncertainty of ±0.05 cm3