AS & A Level Chemistry - 9701 Paper 3 2013 Summer Zone 3

You are to determine the enthalpy change of the reaction between hydrochloric acid and sodium hydroxide by adding various volumes of acid and alkali and measuring the change in temperature.

FA 1 is 0.950 mol dm$^{-3}$ hydrochloric acid, HCl.
FA 2 is aqueous sodium hydroxide, NaOH.

(a) Method

• Support the plastic cup in a 250 cm$^{3}$ beaker.
• Using a measuring cylinder, transfer 25 cm$^{3}$ of FA 1 into the cup and measure the temperature of the acid. Tilt the cup if necessary to cover the bulb of the thermometer.
• Record this initial temperature.

• Use a second measuring cylinder to transfer 10 cm$^{3}$ of FA 2 and 25 cm$^{3}$ of water into a 100 cm$^{3}$ beaker.
• Add this mixture to the plastic cup and stir.
• Measure the maximum temperature reached and record this maximum temperature in the table below.
• Rinse out the plastic cup and shake it to remove excess water.
• Repeat the experiment, using the volumes of FA 1, FA 2 and water shown in the table. Record the maximum temperature for each experiment.

The identity of a metal, M, can be found by titrating a solution of its carbonate with hydrochloric acid of known concentration.
FA 3 is a solution of the metal carbonate, $M_2CO_3$, of concentration $6.90\text{ g dm}^{-3}$.
You are to dilute the hydrochloric acid that you used in Question 1 and then titrate the carbonate solution with this acid.
(a) Method
**Dilution of the acid**
● Pipette $25.0\text{ cm}^3$ of $FA\;1$ into the $250\text{ cm}^3$ volumetric (graduated) flask labelled $FA\;4$.
● Add distilled water to make the total volume $250\text{ cm}^3$.
● Stopper the flask and mix the contents thoroughly.
**Titration**
● Fill the burette with diluted hydrochloric acid, $FA\;4$.
● Use a clean pipette to transfer $25.0\text{ cm}^3$ of $FA\;3$ into a conical flask.
● Titrate $FA\;3$ with $FA\;4$ using the indicator provided.
● Perform a rough titration and record your burette readings in the space below.
The rough titre is .................... $\text{cm}^3$.
● Carry out as many accurate titrations as you think necessary to obtain consistent results.
● Make certain any recorded results show the precision of your practical work.
● Record, in a suitable form below, all of your burette readings and the volume of $FA\;4$ added in each accurate titration.

(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value.
25.0 cm³ of FA 3 required .................... cm³ of FA 4 [1]
(c) Calculation
The equation for the reaction between hydrochloric acid and the metal carbonate is given below.
$$M_2CO_3 + 2HCl \rightarrow 2MCl + CO_2 + H_2O$$
(i) Calculate the number of moles of hydrochloric acid present in the volume in (b).
moles of HCl = .................... mol
(ii) Hence, calculate the number of moles of $M_2CO_3$ present in 25.0 cm³ of FA 3.
moles of $M_2CO_3$ = .................... mol
(iii) Calculate the concentration of $M_2CO_3$ in FA 3 in mol dm$^{-3}$.
concentration of $M_2CO_3$ = .................... mol dm$^{-3}$
(iv) Use your answer to (iii), and the fact that FA 3 contains 6.90 g dm$^{-3}$, to determine the relative atomic mass, $A_r$, of M.
$A_r$ of M = ..............
(v) Use your answer to (iv) and the Periodic Table on page 16 to suggest the identity of M.
M is .................... [5]

(d) The concentration of a carbonate solution could be found using either the method in Question 1 or that in Question 2.
(i) Suggest, and explain, which of the methods is more accurate.
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(ii) For the method that you think is less accurate, suggest an improvement to the practical procedure that could be made to improve the accuracy.
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(a) You are provided with a solid, FA 5. FA 5 is a mixture that contains two anions and two cations.

To all your sample of FA 5 in a boiling tube add 3 cm depth of distilled water. Shake the tube and filter the contents. Keep the solid residue for tests in (b) and the filtered solution for tests in (c).

(b) (i) Open up the filter paper and scrape the residue into a boiling tube. Add dilute nitric acid, $\text{HNO}_3$, using a dropping pipette until the solid just disappears. Record your observations and keep the solution for tests in (ii).

observations ............................................................................................................
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(ii) Divide the solution from test (i) equally into three test-tubes.

To the first test-tube add aqueous sodium hydroxide, NaOH, until in excess. Record your observations.

observations ............................................................................................................
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Which cations, from those listed in the Qualitative Analysis Notes on page 12, would give these observations?
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(c) To 1 cm depth of filtered solution from (a) in a test-tube add 1 cm depth of dilute nitric acid followed by a few drops of aqueous silver nitrate. Record your observation.

observation .......................................................................................................
Which further reagent could be added to this test-tube to help you to confirm the nature of the anion present?
reagent ..................................................................................................................
Carry out a test using this additional reagent. Record your observation and conclusion about the anion present.
observation ............................................................................................................
The anion present is ................................................................... .

(d) Using your observation in (b)(i) state which other anion is present in FA 5.

The anion present is ................................................................................................... .

(e) Solutions FA 6 and FA 7 each contain one of the ions sulfite, $\text{SO}_3^{2-}$, sulfate, $\text{SO}_4^{2-}$, nitrite, $\text{NO}_2^{-}$, or nitrate, $\text{NO}_3^{-}$.

(i) Carry out the tests in the table below to identify which ion is present in each solution.

[Table]
To 1 cm depth of solution in a boiling tube, add a small piece of aluminium foil and 1 cm depth of aqueous sodium hydroxide. Warm the mixture with care.

To 1 cm depth of solution in a test-tube, add a few drops of aqueous barium chloride or barium nitrate, then
.........................................................................................................................................
add dilute hydrochloric acid.

To 1 cm depth of solution in a test-tube, add 1 cm depth of dilute hydrochloric acid.

(ii) From your observations, identify the anion present in each solution.

FA 6 contains ................................................

FA 7 contains ................................................

(iii) What type of reaction takes place when a positive observation is seen with aluminium foil and aqueous sodium hydroxide in (i)?
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