Chemistry 9701/32 — 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 · Presentation of Data and Observations · Analysis, Conclusions and Evaluation · 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 sodium carbonate, $\text{Na}_2\text{CO}_3$ , with water and carbon dioxide to form sodium hydrogencarbonate, $\text{NaHCO}_3$ . In step 1 you will react a known mass of sodium hydrogencarbonate with a known volume of dilute hydrochloric acid and find the temperature change. In step 2 you will find the temperature change on adding a known mass of sodium carbonate to a known volume of acid. You will then use your results to calculate the enthalpy change for the reaction.

\text{Na}_2\text{CO}_3(\text{s}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g}) \rightarrow 2\text{NaHCO}_3(\text{s})

FB 1 is sodium hydrogencarbonate, $\text{NaHCO}_3$ .
FB 2 is sodium carbonate, $\text{Na}_2\text{CO}_3$ .
FB 3 is $2.0\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .

(a)

Method

Step 1

\text{NaHCO}_3(\text{s}) + \text{HCl}(\text{aq}) \rightarrow \text{NaCl}(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})

Place the plastic cup in the $250\text{ cm}^3$ beaker.
Use the measuring cylinder to transfer $25\text{ cm}^3$ of the acid, FB 3, into the plastic cup. The acid is in excess.
Weigh the container with FB 1 and record the balance reading in a suitable form below.
Place the thermometer in the acid 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 acid at 1 minute and at 2 minutes.
At $2\frac{1}{2}$ minutes carefully tip all the FB 1, in small portions to avoid spray, into the acid and stir to dissolve it.
Record the temperature of the solution at 3, 4, 5, 6, 7 and 8 minutes.
Reweigh the container with any residual FB 1 and record the balance reading and the mass of FB 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. The scale for temperature should extend at least $1\text{ }^{\circ}\text{C}$ below your lowest recorded temperature.

grid_1

Draw two straight lines of best fit on your graph, one for the temperature of the acid before adding FB 1 and the other for the warming 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{Na}_2\text{CO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow 2\text{NaCl}(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})

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 the acid, FB 3, into the plastic cup. The acid is in excess.
Weigh the container with FB 2 and record the balance reading.
Place the thermometer in the acid and record the initial temperature. Tilt the cup if necessary so that the bulb of the thermometer is fully covered.
Carefully tip all the FB 2, in small portions to avoid spray, into the acid and stir to dissolve it.
Record the highest temperature.
Reweigh the container with any residual FB 2 and record the balance reading and the mass of FB 2 used.

Results

(d)

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 absorbed when FB 1 was added to the acid 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 absorbed = ................. J

(ii)

Calculate the enthalpy change, in $\text{kJ mol}^{-1}$ , when 1 mole of FB 1, $\text{NaHCO}_3$ , reacts with acid. ( $A_r$ : $\text{H}, 1.0$ ; $\text{C}, 12.0$ ; $\text{O}, 16.0$ ; $\text{Na}, 23.0$ )

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

(iii)

Using your results from (c), calculate the heat energy produced when FB 2 was added to the acid 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 produced = ................. J

(iv)

Calculate the enthalpy change, in $\text{kJ mol}^{-1}$ , when 1 mole of FB 2, $\text{Na}_2\text{CO}_3$ , reacts with acid. ( $A_r$ : $\text{C}, 12.0$ ; $\text{O}, 16.0$ ; $\text{Na}, 23.0$ )

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

(v)

Use your answers to parts (ii) and (iv) and the equations for steps 1 and 2 to determine the enthalpy change for the reaction below.

\text{Na}_2\text{CO}_3(\text{s}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g}) \rightarrow 2\text{NaHCO}_3(\text{s})

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

(e)

(i)

Complete the following table.


The balance used in the experiment reads to	................ decimal place(s).
The maximum error in a single balance reading is	................. g.
The maximum error in measuring the recorded mass of FB 2 in (c) would be	............ g.

(ii)

Calculate the maximum percentage error in the mass of FB 2 in step 2.

maximum percentage error in the mass of FB 2 in step 2 = ............... %

(f)

(i)

A student stated that the experiment could be made more accurate by using twice the volume of acid in step 1 and step 2. Suggest whether the student is correct or incorrect and justify your answer.

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

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

(ii)

Another student carried out step 2 twice using the carbonate of a different metal and obtained the following results.

First result: mass used = $2.96\text{ g}$ ; increase in temperature = $4.0\text{ }^{\circ}\text{C}$
Second result: mass used = $3.65\text{ g}$ ; increase in temperature = $5.0\text{ }^{\circ}\text{C}$

The student then used the mean mass and the mean temperature increase 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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