AS & A Level Chemistry - 9701 Paper 3 2011 Winter Zone 4

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 mol dm^-3 sodium hydroxide, NaOH.
FB 2 is an aqueous solution of an acid.
(a) Method
• Fill a burette with FB 1. [Care: FA1 is corrosive]
• Support the plastic cup in a 250 cm³ beaker.
• Run 10.0 cm³ 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 cm³ 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 cm³ of FB 1 and 35 cm³ 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
[Table_1]
[8]
(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.

[4]
(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 = ................... °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 = ................... cm³
(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 J are required to raise the temperature of 1 cm³ of any solution by 1°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 kJ mol^-1 and include the relevant sign.
enthalpy change of neutralisation = ........ ........ kJ mol^-1
[7]
(d) Identify a source of error, other than heat loss, in the experimental method. Suggest an improvement which would reduce this source of error.
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[2]
(e) (i) Calculate the concentration of hydrogen ions, H⁺, in mol dm^-3, present in FB 2.
concentration of hydrogen ions in FB 2 = ...................... mol dm^-3
(ii) If the acid present in FB 2 is sulfuric acid, calculate its concentration.
concentration of sulfuric acid = ...................... 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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[4]

(a) (i) FB 3, FB 4 and FB 5 are aqueous solutions of sodium compounds. None of these compounds contains sulfur.

To about 1 cm depth of each of the solutions in separate test-tubes add the same depth of dilute sulfuric acid.

To another 1 cm depth of each of the solutions in separate test-tubes add a few drops of aqueous lead(II) nitrate.

Record your observations for these tests in an appropriate form in the space below. Using the Qualitative Analysis Notes printed on pages 10 and 11 and your observations identify the anions in FB 3 and FB 4.

FB 3 contains ......................................................
FB 4 contains ...................................................... [6]

(ii) Select a reagent or pair of reagents that would enable you to determine the identity of the anion in FB 5.

reagent(s) ......................................................................................................................

Carry out a test on FB 5 using the reagent(s) given above. Record your observations below.

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The anion present in FB 5 is ............................................................. [3]

(b) You are provided with solid FB 6. Complete the following table.

\begin{array}{|c|c|} \hline \text{test} & \text{observations} \\ \hline \text{(i)} & \text{To 1 cm depth of aqueous hydrogen peroxide in a test-tube,} \\ & \text{add a very small spatula measure of FB 6.} \\ \hline \text{(ii)} & \text{To 1 cm depth of aqueous iron(II) sulfate in a boiling tube,} \\ & \text{add the same depth of dilute sulfuric acid.} \\ & \text{Add a very small spatula measure of FB 6 to the tube.} \\ & \text{Warm the mixture gently using a Bunsen burner for about 20 seconds,} \\ & \text{then filter the warm mixture and collect the filtrate.} \\ & \text{To the filtrate, add aqueous sodium hydroxide.} \\ \hline \text{(iii)} & \text{To 2 cm depth of aqueous potassium manganate(VII) in a test-tube,} \\ & \text{add the same depth of aqueous sodium hydroxide and then a small spatula measure of FB 6.} \\ & \text{Stir the contents of the test-tube for about 20 seconds. Filter the mixture and collect the filtrate.} \\ & \text{To the filtrate, add dilute sulfuric acid.} \\ \hline \end{array}

Suggest a conclusion that could be made about the chemical behaviour of FB 6 from the observations in (i). Explain the reasons for your answer.

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What conclusion can be made about the chemical behaviour of FB 6 from the observations in (ii)?

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