Chemistry 9701/34 — October/November 2011

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

questions

marks

120

minutes

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

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationQualitative AnalysisFree sample

You are to determine the enthalpy change of neutralisation of sodium hydroxide by an acid and also the concentration of hydrogen ions in the acid. These can be found by measuring the temperature change when solutions of the acid and alkali are mixed.

FB 1 is $1.50\text{ mol dm}^{-3}$ sodium hydroxide, $\text{NaOH}$ .
FB 2 is an aqueous solution of an acid.

(a)

Method

Fill a burette with FB 1. [Care: FB 1 is corrosive]
Support the plastic cup in a $250\text{ cm}^3$ beaker.
Run $10.0\text{ cm}^3$ of FB 1 from the burette into the plastic cup.
Measure and record, in the table below, the temperature of the FB 1 in the cup. You may need to tilt the beaker to ensure that the bulb of the thermometer is covered.
Measure $40\text{ cm}^3$ of FB 2 using the measuring cylinder.
Pour this volume of FB 2 into the plastic cup containing FB 1. Stir carefully and measure the highest temperature obtained.
Record this temperature in the table.
Rinse the plastic cup with water.
Repeat the experiment using $15.0\text{ cm}^3$ of FB 1 and $35\text{ cm}^3$ of FB 2 as shown for experiment 2 in the table.
Carry out experiments 3 to 7 in the same way.
Complete the table for each experiment.

Results

experiment number	1	2	3	4	5	6	7
volume of FB 1 / $\text{cm}^3$	10.0	15.0	20.0	25.0	30.0	35.0	40.0	50.0	0.0
volume of FB 2 / $\text{cm}^3$	40	35	30	25	20	15	10	0	50
initial temperature FB 1 / $^\circ\text{C}$
highest temperature / $^\circ\text{C}$
temperature change / $^\circ\text{C}$								0.0	0.0

(b)

On the grid below plot the temperature change ( $y$ -axis) against the volume of FB 1 ( $x$ -axis). Use all the results in the table including those provided in the final two columns.

Using these points, draw two straight lines that intersect.

Fig. 1.1

(c)

(i)

Use your graph and the intersection of the two lines to determine the largest temperature change which could occur in the reaction between FB 1 and FB 2.

largest temperature change = .................................. $^\circ\text{C}$

(ii)

From your graph, read the volume of FB 1 required to produce the temperature change in (i).

volume of FB 1 required is = ................................ $\text{cm}^3$

(iii)

Calculate how many moles of sodium hydroxide are present in the volume of FB 1 recorded in (ii).

moles of sodium hydroxide present = ................................. mol

(iv)

Use the temperature change from (i) to calculate the amount of heat energy produced in the reaction.

[Assume that $4.3\text{ J}$ are required to raise the temperature of $1\text{ cm}^3$ of any solution by $1^\circ\text{C}$ ]

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

(v)

Use your answers from (iii) and (iv) to calculate the enthalpy change of neutralisation of sodium hydroxide by the acid.

Give your answer in $\text{kJ mol}^{-1}$ and include the relevant sign.

enthalpy change of neutralisation = .......... .............. $\text{kJ mol}^{-1}$

\text{sign} \quad \text{value}

(d)

Identify a source of error, other than heat loss, in the experimental method. Suggest an improvement which would reduce this source of error.

(e)

(i)

Calculate the concentration of hydrogen ions, $\text{H}^+$ , in $\text{mol dm}^{-3}$ , present in FB 2.

concentration of hydrogen ions in FB 2 = ........................ $\text{mol dm}^{-3}$

(ii)

If the acid present in FB 2 is sulfuric acid, calculate its concentration.

concentration of sulfuric acid = ........................ $\text{mol dm}^{-3}$

(iii)

Describe a chemical test by which you could prove that the acid in FB 2 is sulfuric acid.

Do not carry out the test.

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Q2Qualitative Analysis · Presentation of Data and Observations · Analysis, Conclusions and Evaluation15M

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