CourseCourseConceptConceptDocumentDocument

Edexcel A-Level Chemistry A2 16.17 Core Practicals 13a and 13b Carry Out Redox Titrations with Both I Iron Ii Ions and Potassium Manganate Vii Ii Sodium Questions

Practise Edexcel IAL Chemistry 16.17 by identifying redox titration endpoint colours and adding starch at the pale-yellow iodine stage.

Syllabus
First assessment 2019
Course
Chemistry YCH11
Level
A2

Exam points

  • State endpoint colours for manganate(VII) and iodine-thiosulfate titrations.
  • Add starch near the pale-yellow iodine endpoint to avoid a difficult endpoint.

Edexcel A-Level Chemistry A2 16.17 Core Practicals 13a and 13b Carry Out Redox Titrations with Both I Iron Ii Ions and Potassium Manganate Vii Ii Sodium Questions question 1

[Maximum number: 2]

Brass is an alloy of copper. The percentage by mass of copper in a sample of brass can be determined by a three-stage process.

Stage 1 Oxidation of the copper to copper(II) ions with excess concentrated nitric acid.
Stage 2 Reduction of the copper(II) ions to copper(I) ions with excess iodide ions.
Stage 3 Titration of the iodine produced in Stage 2 against a standard solution of sodium thiosulfate.

Question (a)

(a)

The procedure in Stage 3 is shown:
- the iodine produced in each flask from Stage 2 is titrated with 0.095 moldm−30.095 \mathrm{~mol} \mathrm{dm}^{-3} sodium thiosulfate solution until the iodine colour is pale yellow
- a few drops of starch indicator are added
- sodium thiosulfate is added drop by drop with swirling until the end-point is reached
- the titration is repeated until two concordant titres are obtained.

[ 2 ]

Question (i)

(i)

Give a possible reason why the starch indicator is added when the iodine colour is pale yellow rather than at the start of the titration.

[ 1 ]

Question (ii)

(ii)

Give the colour change at the end-point of the reaction.

[ 1 ]
All question bank results loaded