Chemistry 9701/52 — May/June 2021

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

Q1Analysis, Conclusions and EvaluationPlanningFree sample

A student is asked to find the enthalpy change for the reaction between anhydrous magnesium sulfate and water.

\text{MgSO}_4(\text{s}) + 7\text{H}_2\text{O}(\text{l}) \rightarrow \text{MgSO}_4\cdot7\text{H}_2\text{O}(\text{s})

This enthalpy change cannot be measured directly.

(a)

Predict whether the enthalpy change for this reaction is positive or negative. Explain the reason for your prediction.

(b)

The student decided to do two separate experiments.

Experiment 1
To find the enthalpy change of solution of anhydrous magnesium sulfate, $\text{MgSO}_4(\text{s})$ , $0.0250$ moles of $\text{MgSO}_4(\text{s})$ are dissolved in $50.0\text{ cm}^3$ distilled water.

Experiment 2
To find the enthalpy change of solution of hydrated magnesium sulfate, $\text{MgSO}_4\cdot7\text{H}_2\text{O}(\text{s})$ , $0.0250$ moles of $\text{MgSO}_4\cdot7\text{H}_2\text{O}(\text{s})$ are dissolved in $50.0\text{ cm}^3$ distilled water.

The results for Experiment 1 are shown in the graph of temperature against time on page 3.

(i)

Draw and extrapolate the cooling curve back to 180 seconds. Determine the temperature change during the reaction.

Fig. 1

(ii)

The anhydrous magnesium sulfate was not added when the timing started.

Explain why.

(iii)

$3.01\text{ g}$ ( $0.0250\text{ mol}$ ) of anhydrous magnesium sulfate is weighed.

Outline the next steps that should be taken in order to obtain the results in Experiment 1.

Write your answer using a series of numbered steps.

(c)

(i)

The student realised that when dissolving $0.0250$ moles $\text{MgSO}_4\cdot7\text{H}_2\text{O}(\text{s})$ , the amount of water present in the compound alters the total volume of water used in Experiment 2.

Calculate the volume of distilled water needed to make the total volume of water $50.00\text{ cm}^3$ in Experiment 2.

Give your answer to the nearest $0.05\text{ cm}^3$ .

Assume that $1.00\text{ cm}^3$ of distilled water has a mass of $1.00\text{ g}$ .

[ $A_r$ : O, 16.0; H, 1.0]

(ii)

State which piece of apparatus should be used to measure the volume of distilled water in (c)(i).

Explain your answer.

(d)

The temperature change when $0.0250$ moles of $\text{MgSO}_4\cdot7\text{H}_2\text{O}(\text{s})$ is added to the water is very small.

Suggest how the student should modify the experimental procedure to make the temperature change larger.

(e)

(i)

The energy released by $0.0250$ moles of $\text{MgSO}_4(\text{s})$ , in Experiment 1, is $2125\text{ J}$ . The energy absorbed by $0.0250$ moles of $\text{MgSO}_4\cdot7\text{H}_2\text{O}(\text{s})$ , in Experiment 2, is $477.5\text{ J}$ .

Calculate the enthalpy change, $\Delta H$ , for the reaction.

Include a sign in your answer. Give your answer to one decimal place.

\text{MgSO}_4(\text{s}) + 7\text{H}_2\text{O}(\text{l}) \rightarrow \text{MgSO}_4\cdot7\text{H}_2\text{O}(\text{s})

(ii)

The student noticed that some $\text{MgSO}_4(\text{s})$ in Experiment 1 was left undissolved.

State and explain the effect this would have on the value of the enthalpy change for the reaction in Experiment 1.

The rest of this paper

1 more questions

Q2Planning · Analysis, Conclusions and Evaluation16M

Loading the full paper…