Chemistry 9701/31 — May/June 2010

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 EvaluationQualitative AnalysisFree sample

Read through question 1 before starting any practical work.

You are provided with the following reagents.

FA 1, $2.0\text{ mol dm}^{-3}$ sulfuric acid, $\text{H}_2\text{SO}_4$
FA 2, aqueous sodium hydroxide, $\text{NaOH}$

The reaction of sulfuric acid with sodium hydroxide is exothermic.
In separate experiments you will add increasing volumes of FA 2 to a fixed volume of
FA 1. In each experiment you will measure the maximum temperature rise. As the volume of
FA 2 is increased, this maximum temperature rise will increase and then decrease.

By measuring the maximum temperature rise for different mixtures of the two reagents you
are to determine the following.

the concentration of sodium hydroxide, $\text{NaOH}$ , in FA 2
the enthalpy change when $1\text{ mol}$ of $\text{H}_2\text{SO}_4$ is neutralised by $\text{NaOH}$

(a)

Method

Fill the burette with FA 1.
Support the plastic cup in the $250\text{ cm}^3$ beaker.
Run $10.00\text{ cm}^3$ of FA 1 from the burette into the plastic cup.
Measure $10\text{ cm}^3$ of FA 2 in a measuring cylinder.
Place the thermometer in the FA 2 in the measuring cylinder and record the steady temperature of the solution.
Tip the FA 2 in the measuring cylinder into the plastic cup, stir and record the maximum temperature obtained in the reaction.
Empty and rinse the plastic cup. Rinse the thermometer. Shake dry the plastic cup.
Carry out the experiment four more times. Each time use $10.00\text{ cm}^3$ of FA 1.
Use $20\text{ cm}^3$ , $30\text{ cm}^3$ , $40\text{ cm}^3$ and $50\text{ cm}^3$ of FA 2 in these different experiments.

Carry out two further experiments.
Choose volumes of FA 2 which will allow you to investigate more precisely the volume of FA 2 that produces the highest temperature rise when added to $10.00\text{ cm}^3$ of FA 1.

Results
Record your results in an appropriate form showing, for each experiment, the volumes of solution used, temperature measurements and the temperature rise.

(b)

Use the grid below to plot a graph of temperature rise ( $y$ -axis) against the volume of FA 2 added ( $x$ -axis).

Draw a line of best fit through the points where the temperature rise is increasing and another line through the points where the temperature rise is decreasing.

The intersection of these lines represents the temperature rise for the volume of FA 2 that exactly neutralises the sulfuric acid present in $10.00\text{ cm}^3$ of FA 1.

Fig. 1.1

(c)

Read from the graph the volume of FA 2 that gives the maximum temperature rise.

The volume of FA 2 giving the maximum temperature rise is .................... $\text{cm}^3$ .

(d)

Explain why the temperature rise is plotted on the $y$ -axis rather than on the $x$ -axis.

(e)

Construct the balanced equation for the reaction of sulfuric acid with sodium hydroxide.

(f)

(i)

Calculate how many moles of sulfuric acid, $\text{H}_2\text{SO}_4$ , are contained in $10.00\text{ cm}^3$ of FA 1.

$10.00\text{ cm}^3$ of FA 1 contain .................... mol of $\text{H}_2\text{SO}_4$ .

(ii)

Calculate how many moles of $\text{NaOH}$ are required to neutralise the amount of $\text{H}_2\text{SO}_4$ calculated in (i) above.

The sulfuric acid in $10.00\text{ cm}^3$ of FA 1 is neutralised by .................... mol of $\text{NaOH}$ .

(g)

Use the equation below to calculate the concentration of $\text{NaOH}$ in FA 2.

\text{concentration of NaOH (mol dm}^{-3}\text{)} = \text{answer to (f)(ii)} \times \frac{1000}{\text{volume of FA 2 (cm}^3\text{) from (c)}}

The concentration of $\text{NaOH}$ in FA 2 = .................... $\text{mol dm}^{-3}$ .

(h)

Read the maximum temperature rise from the graph and use this to calculate the enthalpy change when $1\text{ mol}$ of $\text{H}_2\text{SO}_4$ is neutralised by $\text{NaOH}$ . Give your answer in $\text{kJ mol}^{-1}$ and include the correct sign for the reaction.

[4.3 J are absorbed or released when the temperature of $1\text{ cm}^3$ of solution changes by $1\text{ }^\circ\text{C}$ . Remember that separate volumes of FA 1 and FA 2 were mixed together.]

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

(i)

A student suggested that the accuracy of the experiment would be improved if the volume of FA 2 had been measured using a burette rather than a measuring cylinder.
Suggest an advantage and a disadvantage of using a burette in the procedure.

advantage:

disadvantage:

(j)

Identify two further significant sources of error, other than the measurement of volume, in the experiments used for measuring temperature rise.

error 1:

error 2:

(k)

(i)

The maximum error in taking a temperature reading on a thermometer with graduations at $1\text{ }^\circ\text{C}$ is .................... $^\circ\text{C}$ .

(ii)

The temperature rise when $30\text{ cm}^3$ of FA 2 is added to $10.00\text{ cm}^3$ of FA 1 is .................... $^\circ\text{C}$ .

(iii)

Calculate the maximum percentage error due to the thermometer when measuring the temperature rise in (ii) above.

The maximum percentage error = .................... %.

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Q2Qualitative Analysis · Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation14M

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