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
• What is the formula for the hydrated form of oxalic acid?


  (COOH)₂·2H₂O
• How is the molar mass of (COOH)₂ calculated?

  By summing atomic masses: 90.0 g/mol
• What is the mass of water in (COOH)₂·2H₂O?

  36.0 g
• What does the value of x represent in (COOH)₂·xH₂O?

  The number of water molecules in the hydrate
• What is the purpose of redox titrations?

  To analyze different substances through oxidation-reduction
• What reducing agent is analyzed using manganate(VII) titrations?

  MnO₄⁻ is reduced to Mn²⁺
• What can KMnO₄ be replaced with in redox titrations?

  Acidified dichromate(VI), H⁺/Cr₂O₇²⁻
• What are the steps in iodine/thiosulfate titrations?

  1. Iodine is reduced by thiosulfate ions.
  2. Iodine reacts with potassium iodide to form iodine.
  3. Titrate with sodium thiosulfate until pale straw color.
  4. Add starch indicator to identify end-point.
• What is the oxidation reaction in iodine/thiosulfate titrations?

  2S₂O₃²⁻ → S₄O₆²⁻ + 2e⁻
• What is the reduction reaction in iodine/thiosulfate titrations?

  I₂ + 2e⁻ → 2I⁻
• What is the overall reaction in iodine/thiosulfate titrations?

  2S₂O₃²⁻ + I₂ → 2I⁻ + S₄O₆²⁻
• Why is starch indicator added towards the end of the titration?

  To enhance visibility of the end-point
• What are the applications of iodine-thiosulfate titrations?

  • Determine O₂ content in bleach
  • Analyze Cu²⁺ in copper compounds
  • Measure Cu content in copper alloys
• What is the active ingredient in household bleach?

  Chlorate(I) ions, ClO⁻
• What happens when ClO⁻ ions react with I⁻ and H⁺ ions?

  They form Cl⁻, I₂, and H₂O
• How many moles of S₂O₃²⁻ are equivalent to 1 mole of ClO⁻?

  2 moles of S₂O₃²⁻
• What is the procedure for analyzing bleach using titration?

  Add bleach, KI, and titrate with Na₂S₂O₃
• What is the mean titre of Na₂S₂O₃ in the example?

  24.20 cm³
• What is the first step in analyzing the concentration of ClO⁻ in bleach?

  Calculate the amount of Na₂S₂O₃ that reacted
• How is the concentration of ClO⁻ ions calculated from the titration results?

  Using the volume and molarity of Na₂S₂O₃
• What is the reaction for copper(II) ions with iodide ions?

  2Cu²⁺ + 4I⁻ → 2CuI + I₂
• What is the relationship between Cu²⁺ and S₂O₃²⁻ in titrations?

  1 mol Cu²⁺ is equivalent to 1 mol S₂O₃²⁻