Chemistry 9701/52 — February/March 2016

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

questions

marks

minutes

Topics Analysis, Conclusions and Evaluation · Planning

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

Propanone, $\text{CH}_3\text{COCH}_3$ , is an organic liquid which is soluble in water.

Aqueous propanone reacts with aqueous iodine. The reaction is catalysed by $\text{H}^+(\text{aq})$ ions.

\text{CH}_3\text{COCH}_3(\text{aq}) + \text{I}_2(\text{aq}) \rightarrow \text{CH}_3\text{COCH}_2\text{I}(\text{aq}) + \text{HI}(\text{aq})

The order of reaction with respect to iodine can be determined experimentally.

An experiment is carried out using the following solutions.

solution A, $25.0\text{ cm}^3$ of $1.00\text{ mol dm}^{-3}$ $\text{CH}_3\text{COCH}_3(\text{aq})$
solution B, $25.0\text{ cm}^3$ of $1.00\text{ mol dm}^{-3}$ $\text{H}_2\text{SO}_4(\text{aq})$
solution C, $50.0\text{ cm}^3$ of $0.200\text{ mol dm}^{-3}$ $\text{I}_2(\text{aq})$

The solutions are mixed to start the reaction. At certain time intervals, a $10.0\text{ cm}^3$ portion of the mixture is withdrawn and transferred to a conical flask containing excess sodium hydrogencarbonate, $\text{NaHCO}_3(\text{aq})$ . This prevents any further significant reaction taking place by removing the $\text{H}^+(\text{aq})$ ions. The concentration of unreacted $\text{I}_2(\text{aq})$ in each $10.0\text{ cm}^3$ portion of the mixture can then be determined by titration with aqueous thiosulfate ions, $\text{S}_2\text{O}_3^{2-}(\text{aq})$ .

(a)

State the size and type of apparatus needed to prepare a suitable volume of a standard solution of $1.00\text{ mol dm}^{-3}$ $\text{CH}_3\text{COCH}_3(\text{aq})$ from liquid propanone.

Calculate the mass of propanone needed to prepare this standard solution.

$[A_r: \text{C}, 12.0; \text{H}, 1.0; \text{O}, 16.0]$

(b)

Solutions A, B and C need to be added in a specific order and the clock started as the third solution is added.

(i)

Suggest the best order of adding the solutions.

(ii)

Explain your choice.

(c)

Each $10.0\text{ cm}^3$ portion of mixture removed from the main reaction is added to a separate solution of sodium hydrogencarbonate, $\text{NaHCO}_3(\text{aq})$ , in a conical flask to remove $\text{H}^+(\text{aq})$ ions.

(i)

Which piece of apparatus should be used to transfer each $10.0\text{ cm}^3$ portion of mixture to the conical flask?

(ii)

Suggest two reasons why $\text{NaHCO}_3(\text{aq})$ is preferred to $\text{NaOH}(\text{aq})$ as the reagent used to remove $\text{H}^+(\text{aq})$ ions.

(d)

The unreacted iodine in each $10.0\text{ cm}^3$ portion of the mixture is titrated against $0.100\text{ mol dm}^{-3}$ aqueous thiosulfate ions, $\text{S}_2\text{O}_3^{2-}(\text{aq})$ , to determine the concentration of $\text{I}_2(\text{aq})$ in the mixture at the time that the $10.0\text{ cm}^3$ portion was withdrawn.

\text{I}_2(\text{aq}) + 2\text{S}_2\text{O}_3^{2-}(\text{aq}) \rightarrow 2\text{I}^-(\text{aq}) + \text{S}_4\text{O}_6^{2-}(\text{aq})

(i)

A $10.0\text{ cm}^3$ portion of mixture is removed at $\text{time} = 0$ . This is before any of the $0.200\text{ mol dm}^{-3}$ $\text{I}_2(\text{aq})$ had reacted.

Calculate the volume of $0.100\text{ mol dm}^{-3}$ $\text{S}_2\text{O}_3^{2-}(\text{aq})$ needed to react with the iodine present in this $10.0\text{ cm}^3$ portion of mixture.

(ii)

Suggest the name of a suitable indicator to use in the titration and state its colour change.

(e)

State two variables which must be recorded in this experiment.

For each variable, state the units.

(f)

State one other variable which must be controlled in this experiment.

(g)

The order of reaction with respect to iodine is expected to be first order.

(i)

Use the axes below to draw a sketch graph of how the concentration of iodine changes during the experiment. Label both axes.

Fig. 1.1

(ii)

How could the graph be used to prove that the order of reaction with respect to iodine is first order?

(h)

A student suggested that the temperature at which the experiment was carried out would affect the order of reaction with respect to iodine.

State if the student was correct and explain your answer.

The rest of this paper

1 more questions

Q2Analysis, Conclusions and Evaluation10M

Loading the full paper…