Chemistry 9701/35 — May/June 2013

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Presentation of Data and Observations · Qualitative Analysis

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

You will determine the enthalpy change, $\Delta H$ , for the reaction of anhydrous magnesium sulfate, $\text{MgSO}_4$ , with water to form hydrated magnesium sulfate, $\text{MgSO}_4\cdot 7\text{H}_2\text{O}$ . In step 1 you will dissolve a known mass of anhydrous magnesium sulfate in a known volume of water and find the temperature change. In step 2 you will find the temperature change on adding a known mass of hydrated magnesium sulfate to a known volume of water. You will then use your results to calculate the enthalpy change for the reaction.

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

FA 1 is anhydrous magnesium sulfate, $\text{MgSO}_4$ .
FA 2 is hydrated magnesium sulfate, $\text{MgSO}_4\cdot 7\text{H}_2\text{O}$ .
distilled water

(a)

Method

Step 1

\text{MgSO}_4(\text{s}) + \text{aq} \rightarrow \text{MgSO}_4(\text{aq})

Place the plastic cup in the $250\text{ cm}^3$ beaker.
Use the measuring cylinder to transfer $25\text{ cm}^3$ of distilled water into the plastic cup.
Weigh the container with FA 1 and record the balance reading in a suitable form in the space below.
Place the thermometer in the water and record the initial temperature in the table of results. Tilt the cup if necessary so that the bulb of the thermometer is fully covered. This is the temperature at time zero. Start timing.
Record the temperature of the water at 1 minute and at 2 minutes.
At $2\frac{1}{2}$ minutes tip all the FA 1 into the water and stir to dissolve.
Record the temperature of the solution at 3, 4, 5, 6, 7 and 8 minutes.
Reweigh the container with any residual FA 1 and record the balance reading and the mass of FA 1 used.
Rinse out the plastic cup and shake it to remove excess water.

Results

Mass

Temperature

Time in minutes	0	1	2	3	4	5	6	7	8
Temperature / $^\circ\text{C}$

(b)

Plot temperature on the $y$ -axis against time on the $x$ -axis on the grid below. You will use the graph to determine the theoretical temperature change at $2\frac{1}{2}$ minutes.

Fig. 1

Draw two straight lines of best fit on your graph, one for the temperature of the water before adding FA 1 and the other for the cooling of the solution once the reaction is complete. Extrapolate the two lines to $2\frac{1}{2}$ minutes and determine the change in temperature at this time.

temperature change at $2\frac{1}{2}$ minutes = ................. $^\circ\text{C}$

(c)

Method

Step 2

\text{MgSO}_4\cdot 7\text{H}_2\text{O}(\text{s}) + \text{aq} \rightarrow \text{MgSO}_4(\text{aq})

Read through the method and prepare a suitable table for your results.
Place the plastic cup in the $250\text{ cm}^3$ beaker.
Use the measuring cylinder to transfer $25\text{ cm}^3$ of distilled water into the plastic cup.
Weigh the container with FA 2 and record the balance reading below.
Place the thermometer in the water and record the initial temperature. Tilt the cup if necessary so that the bulb of the thermometer is fully covered.
Tip all the FA 2 into the water and stir to dissolve.
Record the lowest temperature.
Reweigh the container with any residual FA 2 and record the balance reading and the mass of FA 2 used.

Results

(d)

Calculations

Show your working and appropriate significant figures in the final answer to each step of your calculations.

(i)

Using your answer to (b), calculate the heat energy produced when FA 1 was added to water in step 1.
(Assume that $4.3\text{ J}$ of heat energy changes the temperature of $1.0\text{ cm}^3$ of solution by $1.0\text{ }^\circ\text{C}$ .)

heat energy produced = ................. J

(ii)

Calculate the enthalpy change, in $\text{kJ mol}^{-1}$ , when 1 mole of FA 1, $\text{MgSO}_4$ , is dissolved.
( $A_r$ : $\text{O}, 16.0$ ; $\text{Mg}, 24.3$ ; $\text{S}, 32.1$ )

enthalpy change = ................. $\text{kJ mol}^{-1}$
(sign) (value)

(iii)

Using your results from (c), calculate the heat energy absorbed when FA 2 was added to water in step 2.
(Assume that $4.3\text{ J}$ of heat energy changes the temperature of $1.0\text{ cm}^3$ of solution by $1.0\text{ }^\circ\text{C}$ .)

heat energy absorbed = ................. J

(iv)

Calculate the enthalpy change, in $\text{kJ mol}^{-1}$ , when 1 mole of FA 2, $\text{MgSO}_4\cdot 7\text{H}_2\text{O}$ , is dissolved.
( $A_r$ : $\text{H}, 1.0$ ; $\text{O}, 16.0$ ; $\text{Mg}, 24.3$ ; $\text{S}, 32.1$ )

enthalpy change = ................. $\text{kJ mol}^{-1}$
(sign) (value)

(v)

Use your answers to parts (ii) and (iv) and the equations for the reactions shown in steps 1 and 2 to determine the enthalpy change, in $\text{kJ mol}^{-1}$ , for the reaction below.

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

enthalpy change, $\Delta H$ = ................. $\text{kJ mol}^{-1}$
(sign) (value)

(e)

(i)

Complete the following table.


The maximum error in a single thermometer reading is	................. $^\circ\text{C}$ .
The maximum error in measuring the change in temperature in step 2 is	................. $^\circ\text{C}$ .

(ii)

Calculate the maximum percentage error in the temperature change in step 2.

maximum percentage error in the temperature change in step 2 = ................. %

(f)

(i)

A student suggested that the experiment could be made more accurate by using $50\text{ cm}^3$ of water in step 1 and step 2. State whether the student is correct or incorrect and justify your answer.

The student is .................

because .................

(ii)

Another student carried out step 2 twice for a different hydrated salt and obtained the following results.

First result: mass used = $3.34\text{ g}$ ; drop in temperature = $4.0\text{ }^\circ\text{C}$
Second result: mass used = $4.18\text{ g}$ ; drop in temperature = $5.0\text{ }^\circ\text{C}$

The student then used the mean mass and mean temperature drop when calculating the enthalpy change for the reaction. Explain whether or not the student was justified in using the results in this way, by showing appropriate calculations.

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