Chemistry 9701/51 — May/June 2020

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

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Topics Analysis, Conclusions and Evaluation · Planning

Q1Analysis, Conclusions and EvaluationPlanningFree sample

Trichloromethane and propanone are both organic liquids. The molecules within each liquid are attracted to each other by relatively weak permanent dipole-dipole interactions.

When trichloromethane is mixed with propanone a strong electrostatic attraction forms between the two different molecules.

Fig. 1.1

A student plans to perform an experiment to investigate the strength of this electrostatic attraction by finding the temperature change when equal volumes of trichloromethane and propanone are mixed together.

(a)

(i)

State and explain your prediction for the temperature change for this experiment.

(ii)

The student is given only the following equipment and chemicals for the experiment.

$1 \times 25\text{ cm}^3$ beaker
$2 \times$ thermometers
$2 \times 25\text{ cm}^3$ measuring cylinders
$50\text{ cm}^3$ trichloromethane
$50\text{ cm}^3$ propanone

Outline the method the student should use in this one experiment to find the temperature change when trichloromethane is mixed with propanone. Give details of the volumes of liquids used and any readings taken.

volumes used

readings taken

method used

(b)

The apparatus used leads to significant heat loss.

State one improvement the student could make to the apparatus to reduce heat loss.

(c)

Trichloromethane and propanone are both volatile and flammable.

State one relevant precaution that should be taken when carrying out this experiment.

(d)

State one change, apart from reducing heat loss, that could be made to improve the accuracy of this experiment.

(e)

In another experiment, a student uses $37.50\text{ g}$ of trichloromethane and $19.75\text{ g}$ of propanone and determines that the energy released is $1.67\text{ kJ}$ .

(i)

Calculate the number of moles of each compound in this mixture.

$M_r$ trichloromethane = 119.5
$M_r$ propanone = 58.0

moles of trichloromethane = .............................. mol

moles of propanone = .............................. mol

(ii)

Calculate the enthalpy change, $\Delta H$ , of the electrostatic attraction formed between trichloromethane and propanone. You must include a sign in your answer.

\Delta H = \text{.............................. kJ mol}^{-1}

(f)

Suggest an experiment the student could carry out to test whether the number of moles of trichloromethane affects the temperature change.

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