Edexcel IAL Chemistry 16.15 structured and unstructured redox titrations

Track each mole ratio through the titration chain, then scale from titre to flask or sample before calculating concentration, percentage mass or oxidation state.

Exam points

Calculate moles from titre and use redox ratios such as MnO4- to Fe2+ or thiosulfate to iodine.
Scale from aliquot to volumetric flask before finding concentration, mass or percentage purity.
Deduce an oxidation state from titration mole ratios and the manganate(VII) half-equation.

[Maximum number: 5]

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.

(a)

The procedure in Stage 3 is shown:
- the iodine produced in each flask from Stage 2 is titrated with $0.095 \mathrm{~mol} \mathrm{dm}^{-3}$ 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.

[ 5 ]

(i)

The redox equation for the reaction between copper(II) ions and iodide ions is shown.

2 \mathrm{Cu}^{2+}(\mathrm{aq})+4 \mathrm{I}^{-}(\mathrm{aq}) \rightarrow 2 \mathrm{CuI}(\mathrm{~s})+\mathrm{I}_{2}(\mathrm{aq})

The reaction of sodium thiosulfate solution with iodine is shown.

2 \mathrm{~S}_{2} \mathrm{O}_{3}^{2-}(\mathrm{aq})+\mathrm{I}_{2}(\mathrm{aq}) \rightarrow \mathrm{S}_{4} \mathrm{O}_{6}^{2-}(\mathrm{aq})+2 \mathrm{I}^{-}(\mathrm{aq})

Calculate the percentage by mass of copper in the 2.53 g of brass.

\left[A_{r} \mathrm{Cu}=63.5\right]

[ 3 ]

calculation of moles thiosulfate in titre

- calculation of mass of copper in $25 \mathrm{~cm}^{3}$ as stoichiometry of Cu: thiosulfate is 1: 1

- calculation of \% of copper by mass in sample

$\frac{26.55}{1000} \times 0.095=2.52225 \times 10^{-3} / 0.00252225(\mathrm{~mol})$ TE on (c)(iii)

$2.52(225) \times 10^{-3} \times 63.5=0.160163(\mathrm{~g})$ $\frac{0.160163}{2.53} \times 10 \times 100=63.305 / 63.31 / 63.3 / 63 \%$

TE on M2 unless greater than 100 \%

Ignore SF except 1 SF

Correct answer with some working scores

(ii)

Nitric acid reacts with iodide ions to form iodine.

In another experiment the nitric acid was not neutralised before the start of Stage 2.

Explain the effect on the value obtained for the percentage of copper in the sample.

[ 2 ]

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16.15—Carry out both structured and unstructured titration calculations involving redox reactions question 1

Question (a)

Question (i)

Question (ii)