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The Finkelstein reaction is a nucleophilic substitution reaction in which a halogen atom in a halogenoalkane is replaced by another halogen atom. The reaction is carried out using dry propanone as a solvent.
One example of the Finkelstein reaction is given.
$\text{CH}_3\text{CH}_2\text{CH}_2\text{Br} + \text{NaI} \rightleftharpoons \text{CH}_3\text{CH}_2\text{CH}_2\text{I} + \text{NaBr}$
(a) (i) Explain why it is important for dry propanone to be used as a solvent for this reaction.
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(ii) The solubilities of NaBr and NaI in propanone are shown.
[Table_1]
Use this information to explain why, although the reaction between $\text{CH}_3\text{CH}_2\text{CH}_2\text{Br}$ and NaI is reversible, the reaction produces a very high yield.
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(b) Some safety information for the organic compounds used in this reaction is shown.
[Table_2]
Identify two different precautions, other than using protective equipment such as gloves, a lab coat or eye protection, that should be taken when carrying out this experiment. Explain each answer.
1 precaution ..................................................................................................................................................
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explanation ..................................................................................................................................................
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2 precaution ..................................................................................................................................................
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explanation ..................................................................................................................................................
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(c) A student plans an experiment to show that the rate of the reaction is proportional to the concentration of NaI.
Propanone is used as the solvent in this reaction.
$\text{CH}_3\text{CH}_2\text{CH}_2\text{Br(pr)} + \text{NaI(pr)} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{I(pr)} + \text{NaBr(s)}$
(pr) = substance is dissolved in propanone
The student plans to record the time it takes for the solid formed to obscure a cross on a piece of paper below the conical flask, as shown.
To carry out this experiment, the following materials are available.
• $\text{CH}_3\text{CH}_2\text{CH}_2\text{Br(l)}$
• $\text{NaI(s)}$
• dry propanone, $\text{CH}_3\text{COCH}_3\text{(l)}$
• usual laboratory apparatus
(i) Calculate the masses of $\text{NaI(s)}$ and $\text{CH}_3\text{COCH}_3\text{(l)}$ that would be needed to make $150\text{cm}^3$ of $0.50\text{mol}\text{dm}^{-3}$ NaI(pr). Assume $150\text{cm}^3$ of propanone are required. Give your answers to one decimal place.
The density of $\text{CH}_3\text{COCH}_3\text{(l)}$ is $0.79\text{g}\text{cm}^{-3}$.
$[A_r: \text{Na}, 23.0; \text{I}, 126.9]$
mass of $\text{NaI(s)} =$ ................................. g
mass of $\text{CH}_3\text{COCH}_3\text{(l)} =$ ................................. g [2]
(ii) Part of the table the student used to record data is given.
Complete the table with appropriate volumes that the student could have used in four further experiments.
[Table_3] [2]
(iii) The student uses the same experimental set-up each time.
In this experiment, identify the dependent variable.
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(iv) Write an expression to show how the student could calculate the rate of the reaction.
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(v) Identify the major source of inaccuracy of measurement in this reaction.
Suggest an improvement to the experiment to make it more accurate.
inaccuracy .................................................................................................................................................
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improvement .............................................................................................................................................
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(d) The reaction between $\text{CH}_3\text{CH}_2\text{CH}_2\text{Br}$ and NaI proceeds via an $S_N2$ mechanism.
The student repeated the experiment in (c) using an isomer of $\text{CH}_3\text{CH}_2\text{CH}_2\text{Br}$ that reacts via both $S_N1$ and $S_N2$ mechanisms.
State and explain whether the student will be able to show that the rate of this reaction is proportional to the concentration of NaI.
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511
557
571
(a) (i) The solutions in the half-cells need to be kept at 40°C.
Explain how you would do this.
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(a) (ii) If the temperature was maintained at 40°C, over time the reading on the voltmeter would change.
Suggest one reason why.
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(b) Complete the fourth and fifth columns of the table.
Give each answer to two decimal places.
| experiment | [Ag^+(aq)] / mol dm^-3 | E_{cell}/V | -log [Ag^+(aq)] | 16.1E_{cell}/V |
| 1 | 0.00100 | -0.097 | | |
| 2 | 0.00500 | -0.140 | | |
| 3 | 0.0100 | -0.159 | | |
| 4 | 0.0250 | -0.171 | | |
| 5 | 0.0500 | -0.202 | | |
| 6 | 0.100 | -0.221 | | |
| 7 | 0.200 | -0.239 | | |
| 8 | 0.500 | -0.264 | | |
| 9 | 1.50 | -0.294 | | |
(c) (i) On the grid on page 9, plot a graph of 16.1E_{cell} against -log [Ag^+(aq)].
Draw a line of best fit.
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(ii) Circle the single most anomalous point on the graph.
The temperature of the half-cells was maintained at 40°C. Suggest what error in the experimental set-up may have caused the anomaly.
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(iii) Use your graph and the equation to determine a value for log C_{sat}.
16.1E_{cell} = log C_{sat} - log [Ag^+(aq)]
log C_{sat} = ........................................................ [1]
(d) (i) For experiment 6, calculate the volume of 2.0 mol dm^-3 Ag^+(aq) needed to prepare exactly 250.0 cm^3 of Ag^+(aq) in a 250.0 cm^3 volumetric flask.
volume = .................................. cm^3 [1]
(ii) Name a suitable piece of apparatus which could be used to measure the volume calculated in (i).
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(e) (i) The relationship between the solubility product, K_{sp}, and the concentration of the saturated solution of silver chloride, C_{sat}, is shown.
$$ K_{sp} = C_{sat}^2 $$
Use this equation and your answer to (c)(iii) to calculate a value for K_{sp}.
Give your answer to three significant figures.
(If you have no answer for (c)(iii), use a value of -4.20 but this is not the true value.)
K_{sp} = ........................................ [2]
(e) (ii) The solubility of AgCl increases with temperature.
Using the axes below, sketch a graph to show how the K_{sp} of AgCl varies with temperature.
Label both axes.
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(f) The salt bridge in a concentration cell commonly contains a solution of one of the following compounds.
potassium chloride potassium nitrate sodium chloride
Identify which, if any, of these compounds would not be suitable for use in the salt bridge in this experiment.
Explain your answer.
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553
579
516
