Edexcel A Level Chemistry:复习笔记8.2.1 Redox Titration

Core Practical 11: Redox Titration

 

Redox Titrations

  • In a titration, the concentration of a solution is determined by titrating with a solution of known concentration.
  • In redox titrations, an oxidizing agent is titrated against a reducing agent
  • Electrons are transferred from one species to the other
  • Indicators are sometimes used to show the endpoint of the titration
  • However, most transition metal ions naturally change colour when changing oxidation state
  • There are two common redox titrations you should know about manganate(VII) titrations and iodine-thiosulfate titrations
  • Iron has variable oxidation states and this characteristic can be exploited in redox titrations of Fe2+ with MnO4- or Cr2O72-
  • Both the manganate(VII) ion and dichromate(VI) ions can be used as strong oxidising agents to find the amount of iron(II) ions in solution
  • You need to be able to perform these separate redox titrations with Fe2+ and understand the colour changes in each example

Potassium manganate(VII) titrations

  • In these redox titrations the manganate(VII) is the oxidising agent and is reduced to Mn2+(aq)
  • The iron is the reducing agent and is oxidised to Fe2+(aq) and the reaction mixture must be acidified, to excess acid is added to the iron(II) ions before the reaction begins
  • The choice of acid is important, as it must not react with the manganate(VII) ions, so the acid normally used is dilute sulfuric acid
    • As it does not oxidise under these conditions and does not react with the manganate(VII) ions

     

  • You could be asked why other acids are not suitable for this redox titration in the exam so make sure you understand the suitability of dilute sulfuric acid

Table Explaining Why Other Acids are not Suitable for the Redox Titration

8.2.4-Table-for-unsuitable-acids

 

Indicator and end point

  • Potassium permanganate acts as its own indicator, as the purple potassium permanganate solution is added to the titration flask from the burette and reacts rapidly with the Fe2+(aq)
    • The burette used in this practical should be one with white numbering not black, as you would struggle to read the values for your titres against the purple colour of the potassium permanganate if black numbering was used

     

  • The manganese(II) ions, Mn2+(aq), have a very pale pink colour but they are present in such a low concentration that the solution looks colourless
  • As soon as all of the iron(II), Fe2+(aq), ions have reacted with the added manganate(VII) ions, Mn7+(aq), a pale pink tinge appears in the flask due to an excess of manganate(VII) ions, Mn7+(aq)

8.2.4-Redox-titration

Redox titration colour change for potassium permanganate and iron(II) ions

 

Worked Example

Equations

Find the stoichiometry for the reaction and complete the two half equations:

MnO4- (aq) + 5e- + 8H+ (aq) → Mn2+ (aq) + 4H2O (l)

Fe2+ (aq) → Fe3+ (aq) + e-

Answers:

Balance the electrons:

MnO4- (aq) + 5e- + 8H+ (aq) → Mn2+ (aq) + 4H2O (l)

5Fe2+ (aq) → 5Fe3+ (aq) + 5e-

Add the two half equations:

MnO4- (aq) + 8H+ (aq) 5Fe2+ (aq) → Mn2+ (aq) + 4H2O (l) + 5Fe3+ (aq)

Exam Tip

General sequence for redox titration calculations

  1. Write down the half equations for the oxidant and reductant
  2. Deduce the overall equation
  3. Calculate the number of moles of manganate(VII) or dichromate(VI) used
  4. Calculate the ratio of moles of oxidant to moles of reductant from the overall redox equation
  5. Calculate the number of moles in the sample solution of the reductant
  6. Calculate the number of moles in the original solution of reductant
  7. Determine either the concentration of the original solution or the percentage of reductant in a known quantity of sample

Iodine-Thiosulfate Titrations

  • A redox reaction occurs between iodine and thiosulfate ions:

2S2O32– (aq) + I2 (aq) → 2I–(aq) + S4O62– (aq)

  • The light brown/yellow colour of the iodine turns paler as it is converted to colourless iodide ions
  • When the solution is a straw colour, starch is added to clarify the end point
  • The solution turns blue/black until all the iodine reacts, at which point the colour disappears.
  • This titration can be used to determine the concentration of an oxidizing agent, which oxidizes iodide ions to iodine molecules
  • The amount of iodine is determined from titration against a known quantity of sodium thiosulfate solution

Redox titration calculations

  • Common redox titration calculations can include:
    • Manganate(VII) titrations can be used to determine:
      • The percentage purity of iron supplements

Percentage purity =

      • The formula of a sample of hydrated ethanedioic acid
    • Iodine / thiosulfate reactions can be used to determine:
      • The amount of chlorate(I), ClO-, in bleach
      • The amount of copper(II) ions, Cu2+, in copper(II) compounds
      • The copper content of alloys

Worked Example

Analysis of iron tablets

An iron tablet, weighing 0.960 g was dissolved in dilute sulfuric acid. An average titre of 28.50 cm3 of 0.0180 mol dm-3 potassium manganate(VII) solution was needed to reach the endpoint.

What is the percentage by mass of iron in the tablet?

Answer:

    • MnO4- (aq) + 8H+ (aq) + 5Fe2+ → Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l)
    • 1 : 5 ratio of MnO4- : Fe2+
    • Number of moles of MnO4- (aq) 5.13 x 10-4 moles
    • Moles of iron(II) = 5 x 5.13 x 10-4 = 2.565 x 10-3 moles
    • Mass of iron(II) = 56.0 x 2.565 x 10-3 = 0.14364 g
    • Percentage by mass = 15.0%

Worked Example

Analysis of household bleach

Chlorate(I) ions, ClO-, are the active ingredient in many household bleaches.

10.0 cm3 of bleach was made up to 250.0 cm3. 25.0 cm3 of this solution had 10.0 cm3 of 1.0 mol dm-3 potassium iodide and then acidified with 1.0 mol dm-3 hydrochloric acid.

ClO- (aq) + 2I- (aq) + 2H+ (aq) → Cl- (aq) + I2 (aq) + H2O (l)

This was titrated with 0.05 mol dm-3 sodium thiosulfate solution giving an average titre of 25.20 cm3.

2S2O32- (aq) + I2 (aq) → 2I- (aq) + S4O62- (aq)

What is the concentration of chlorate(I) ions in the bleach?

Answer:

    • One mole of ClO- (aq) produces one mole of I2 (aq) which reacts with two moles of 2S2O32- (aq)
      • Therefore, 1 : 2 ratio of ClO- (aq) : S2O32- (aq)
    • Number of moles of S2O32- (aq)1.26 x 10-3 moles
    • Number of moles of I2 (aq) and ClO- (aq) in 25.0 cm3 6.30 x 10-4 moles
    • Number of moles of ClO- (aq) in 250.0 cm3 = 6.30 x 10-4 x 10 = 6.30 x 10-3 moles
    • The 250.0 cm3 was prepared from 10.0 cm3 bleach
      • 10 cm3 bleach = 6.30 x 10-3 moles of ClO- ions
      • 1.0 dm3 bleach = 0.630 moles of ClO- ions
      • Therefore, the concentration of ClO- ions in the bleach is 0.630 mol dm-3

Exam Tip

  • You are expected to be able to perform similar calculations on non-familiar redox titrations
  • These could include:
    • Using other oxidising agents such as acidified potassium dichromate(VI) instead of potassium manganate(VII)
    • Completing titrations with potassium manganate(VII) where it is reduced

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