Chemistry 9701/51 — October/November 2019

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Planning · Analysis, Conclusions and Evaluation

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

Yttrium barium copper oxide, $\text{YBa}_2\text{Cu}_3\text{O}_7$ , is a crystalline compound.

You are to design an experiment in which $\text{YBa}_2\text{Cu}_3\text{O}_7$ is first synthesised and then analysed by titration.

(a)

$\text{YBa}_2\text{Cu}_3\text{O}_7$ can be synthesised by reacting $\text{Y}_2\text{O}_3$ , $\text{BaCO}_3$ and $\text{CuO}$ using the following method.

Place solid $\text{Y}_2\text{O}_3$ , $\text{BaCO}_3$ and $\text{CuO}$ together in a mortar and grind the mixture well with a pestle.
Transfer the mixture to a porcelain crucible and place this in an oven set at $920\text{ }^\circ\text{C}$ .
Heat the mixture for 12 hours, then allow the crucible and its contents to cool slowly in the oven to below $100\text{ }^\circ\text{C}$ before removing it.

The equation for the reaction is given.

\text{Y}_2\text{O}_3 + 4\text{BaCO}_3 + 6\text{CuO} + \frac{1}{2}\text{O}_2 \rightarrow 2\text{YBa}_2\text{Cu}_3\text{O}_7 + 4\text{CO}_2

(i)

$\text{YBa}_2\text{Cu}_3\text{O}_7$ contains Y, Ba and Cu in the molar ratio of $1:2:3$ .

Calculate the minimum masses of $\text{BaCO}_3$ and $\text{CuO}$ that are needed to react with $0.750\text{ g}$ of $\text{Y}_2\text{O}_3$ , to give a $\text{Y}:\text{Ba}:\text{Cu}$ ratio of $1:2:3$ .

$[A_r: \text{Y}, 88.9; \text{Ba}, 137.3; \text{Cu}, 63.5; \text{O}, 16.0; \text{C}, 12.0]$

(ii)

State what should be done once the solid product has cooled to ensure that the highest possible yield of $\text{YBa}_2\text{Cu}_3\text{O}_7$ has been produced.

(b)

$\text{YBa}_2\text{Cu}_3\text{O}_7$ contains some copper ions in the unusual +3 oxidation state.

The proportion of $\text{Cu}^{3+}$ in $\text{YBa}_2\text{Cu}_3\text{O}_7$ can be determined by titration.

Step 1
A sample of $\text{YBa}_2\text{Cu}_3\text{O}_7$ is reacted with an excess of concentrated aqueous $\text{HBr}$ . $\text{Cu}^{3+}$ ions are reduced to $\text{Cu}^{2+}$ ions and $\text{Br}_3^-$ ions are formed. $2\text{Cu}^{3+}(\text{s}) + 3\text{Br}^-(\text{aq}) \rightarrow 2\text{Cu}^{2+}(\text{aq}) + \text{Br}_3^-(\text{aq})$
Step 2
A solution of $1.0\text{ mol dm}^{-3}$ sodium citrate is added to the mixture from Step 1. The resulting mixture is then neutralised with a minimum volume of concentrated $\text{NH}_3(\text{aq})$ .
Step 3
Excess $\text{I}^-$ is added which reacts with $\text{Br}_3^-$ to form $\text{I}_2$ . $\text{Br}_3^-(\text{aq}) + 2\text{I}^-(\text{aq}) \rightarrow 3\text{Br}^-(\text{aq}) + \text{I}_2(\text{aq})$
Step 4
The $\text{I}_2$ is titrated with a standard solution of $\text{S}_2\text{O}_3^{2-}$ and starch solution as an indicator. $2\text{S}_2\text{O}_3^{2-}(\text{aq}) + \text{I}_2(\text{aq}) \rightarrow \text{S}_4\text{O}_6^{2-}(\text{aq}) + 2\text{I}^-(\text{aq})$

The concentration of $\text{I}_2(\text{aq})$ can therefore be determined and hence the concentration of $\text{Br}_3^-(\text{aq})$ . From this the amount of $\text{Cu}^{3+}(\text{s})$ can be determined.

The table gives some electrochemical data.

reduction process	$E^\ominus / \text{V}$
$\text{I}_2 + 2\text{e}^- \rightleftharpoons 2\text{I}^-$	+0.54
$\text{Cu}^{2+} + \text{I}^- + \text{e}^- \rightleftharpoons \text{CuI}$	+0.86
$\text{O}_2 + 4\text{H}^+ + 4\text{e}^- \rightleftharpoons 2\text{H}_2\text{O}$	+1.23

Use these data and the information given above to answer the following questions.

(i)

The citrate anion forms an insoluble complex with $\text{Cu}^{2+}$ and so removes $\text{Cu}^{2+}$ from solution.

Explain why this is necessary.

(ii)

Explain why it is necessary to neutralise the mixture in Step 2.

(iii)

When starch indicator is added in Step 4, the mixture turns blue-black due to the presence of $\text{I}_2(\text{aq})$ . The end-point of the titration with $\text{S}_2\text{O}_3^{2-}(\text{aq})$ is a colourless solution.

The number of moles of $\text{S}_2\text{O}_3^{2-}(\text{aq})$ needed for complete reaction with $\text{I}_2(\text{aq})$ can be calculated from the mean titre value. Hence the moles of $\text{I}_2(\text{aq})$ can be determined.

State the expression for the moles of $\text{Cu}^{3+}$ in the sample of $\text{YBa}_2\text{Cu}_3\text{O}_7$ . Use $A$ to represent the number of moles of $\text{I}_2(\text{aq})$ in Step 4.

(c)

(i)

Calculate the mass of hydrated sodium citrate, $\text{Na}_3\text{C}_6\text{H}_5\text{O}_7\cdot2\text{H}_2\text{O}$ , that would be required for the preparation of $250.0\text{ cm}^3$ of a solution of $1.0\text{ mol dm}^{-3}$ citrate ions, $\text{C}_6\text{H}_5\text{O}_7^{3-}$ .

$[M_r: \text{Na}_3\text{C}_6\text{H}_5\text{O}_7\cdot2\text{H}_2\text{O}, 294.0]$

(ii)

A student places the mass of $\text{Na}_3\text{C}_6\text{H}_5\text{O}_7\cdot2\text{H}_2\text{O}$ calculated in (c)(i) into a beaker.

Describe how the student can prepare exactly $250.0\text{ cm}^3$ of a solution of $1.0\text{ mol dm}^{-3}$ citrate ions from the sample in the beaker.

Give the name and capacity, in $\text{cm}^3$ , of any apparatus used.

(d)

A different student records the following titration data in Step 4.

experiment	rough	1	2
final reading / $\text{cm}^3$	21.20	24.60	47.75
initial reading / $\text{cm}^3$	0.00	3.10	25.30
titre / $\text{cm}^3$	21.20	21.50	22.45

Identify the problem with the student's titration method and suggest how it could be improved.

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