5.2.3 (a-e)

Question 1

Define:

Oxidising Agent

Reducing Agent

Answer 1

Oxidising Agent: Takes electrons from species being oxidised and becomes reduced itself

Reducing Agent: Gives electerons to species being reduced, losing electrons itself

Question 2

How do you construct redox equations using half equations?

Answer 2

See notes and diagram below

1. Write out the half equations
2. Multiply the relevant equations so they have the same number of electrons
3. Combine the 2 half equations and cancel any species that appear on either side

Question 4

How do you construct redox equations using oxidation states?

Answer 4

See notes

Question 5

How do you predict products of redox tranfer?

Answer 5

See notes and diagram

Question 6

What is the practical procedure for a Manganate (VII) Titration?

Answer 6

MnO₄^- are reduced and the other chemical oxidised

1. Add a standard solution** of KMnO₄ to a **burette
   
   • It is self-indicating and requires no indicator
2. Use a volumetric pipette to add a known volume of the solution to be tested** to a **conical flask
   
   • Add excess dilute H₂SO₄ to provide the H⁺ ions to reduce the MnO₄^- ions
3. As the manganate is added it decolourises, the end-point is when the first permanent pink colour is visible
4. Repeat for concordant titres

Question 7

How should burette readings be taken when using KMnO₄^-?

Answer 7

• It is deep purple and difficult to see the bottom of the meniscus
• So, take readings from the top of the meniscus

Question 8

What is the redox equation for manganese (VIII) and iron (II)?

Answer 8

REDUCTION:

MnO₄^-_(aq) + 8H⁺_(aq) + 5e^- → Mn²⁺_(aq) + 4H₂O_(l)

OXIDATION:

5 X {Fe²⁺ + e^- → Fe³⁺} to balance the electrons

OVERALL REDOX

MnO₄^-_(aq) + 8H⁺_(aq) + 5Fe²⁺_(aq) → Mn²⁺_(aq) + 4H₂O_(l) + 5Fe³⁺_(aq)

Question 9

What can an I₂/S₂O₃^2- titration determine?

(Iodine/thiosulfate)

Answer 9

• ClO^- content in bleach
• Cu²⁺ content in copper compounds
• Cu content in copper alloys

Question 10

What is the practical procedure for an iodine/thiosulphate titration?

Answer 10

1. Add a standard solution of sodium thiosulfate (Na₂S₂O₃) to a burette
2. Prepare a solution of the oxidising agent to be analysed
   
   • Use a pippette and add to a conical flask and add excess KI
   • Oxidising agent reacts with iodide ions making iodide turning the solution yellow-brown
3. Titrate this solution with the sodium thiosulphate, the iodine will reduce back to I^- making the yellow brown colour fade gradually

USE A STARCH INDICATOR as the reaction cannot self indicate

Question 11

How does a starch indicator work?

Answer 11

• The end-point is near when iodine has faded enough to be a pale straw colour
• Add starch indicator (a deep blue-black colour forms)
  
  • As more sodium thiosulfate is added, the blue black fades
• At the end point, all the iodine reacts & the blue black disappears as all iodine is reduced to I^- ions