AS & A Level Chemistry - 9701 Paper 3 2010 Summer Zone 4

Read through question 1 before starting any practical work.

You are provided with the following reagents.
• weighing bottles/tubes labelled FB 1, FB 2 and FB 3; each containing a different mass of sodium hydrogencarbonate, NaHCO₃
• additional solid sodium hydrogencarbonate (approximately 10 g)
• FB 4, 3.0 mol dm⁻³ hydrochloric acid, HCl

The reaction of sodium hydrogencarbonate with hydrochloric acid is endothermic.

By measuring the maximum temperature decrease when the different masses of sodium hydrogencarbonate react with hydrochloric acid you are to determine the enthalpy change of neutralisation for 1 mol of NaHCO₃ with HCl.

(a) Method

• Weigh the bottle/tube containing the sodium hydrogencarbonate labelled FB 1.
• Support the plastic cup in the 250 cm³ beaker.
• Use the measuring cylinder to transfer 30 cm³ of FB 4 into the plastic cup.
• Place the thermometer in the acid in the plastic cup and record the steady temperature of the acid.
• Add the contents of the weighed tube, FB 1, to the acid in the plastic cup, a little at a time with constant stirring.
• You should add the solid as quickly as possible – taking care to minimise any acid spray from the plastic cup.
Avoid breathing any fumes from the experiment.
• Record the minimum temperature obtained in the reaction.
• Reweigh the emptied tube, FB 1, containing any remaining solid that was not tipped from the tube.
• Empty and rinse the plastic cup. Rinse the thermometer. Shake dry the plastic cup.
• Repeat the experiment using tubes labelled FB 2 and FB 3. In each experiment use 30 cm³ of FB 4.

Carry out two further experiments.
Using the empty weighing bottles/tubes, labelled FB 5 and FB 6, weigh two further masses of sodium hydrogencarbonate. Choose masses to enable you to plot an appropriate graph of temperature change against mass of sodium hydrogencarbonate.

Results
Record your results in an appropriate form showing, for each experiment, the measurements of mass and temperature and the calculated temperature fall. [9]

(b) Use the grid below to plot a graph of decrease in temperature (y-axis) against the mass of sodium hydrogencarbonate added (x-axis).
Draw a line of best fit through the plotted points.
You should consider if the best-fit line passes through the origin (0,0) of the graph.



(c) Explain why the mass of NaHCO₃ is plotted on the x-axis rather than on the y-axis.
.................................................................................................................. [1]

(d) Construct the balanced equation for the reaction of NaHCO₃ with hydrochloric acid.
.................................................................................................................. [1]

(e) Calculate the gradient of your graph.
Show all of your working clearly, both construction lines on the graph and working in the calculation.

[3]

(f) Although there is insufficient acid in 30 cm³ of FB 4 to neutralise 1 mol of NaHCO₃ it is possible to calculate the theoretical fall in temperature for this reaction.
Use your answer from (e) to calculate this value.
[A_r: C, 12.0; H, 1.0; Na, 23.0; O, 16.0]

The theoretical fall in temperature for 1 mol of NaHCO₃ = ....................... °C [1]

(g) Calculate the theoretical enthalpy change for the neutralisation of 1 mol of NaHCO₃ by hydrochloric acid. Give your answer in kJ mol^-1 and include the correct sign for the reaction.
[4.3 J are absorbed or released when the temperature of 1 cm³ of solution changes by 1 °C.]

ΔH = ........................... kJ mol^-1 [2]

(h) Suggest two ways in which your apparatus could be modified to reduce transfer of heat from the surroundings to the solution in the apparatus.

modification 1 .................................................................................................. ................................................................................................................

modification 2 .................................................................................................. ................................................................................................................ [1]

(i) State and explain why the experiment would be more accurate if the volumes of FB 4 had been measured using a burette instead of a measuring cylinder.
.................................................................................................................. .................................................................................................................. [1]

(j) The mass of NaHCO₃ used in a further experiment and its associated temperature change are shown in the tables below.
• The mass was obtained on a balance reading to 1 decimal place.
• The thermometer used was graduated at 1 °C.

Complete the table to show the errors in these results.

[Table_1]
mass of NaHCO₃ | 5.6 g
maximum error in a single balance reading | ± .......... g
% error in measured mass | .......... %

temperature change | −12.0 °C
maximum error in a single thermometer reading | ± .......... °C
% error in temperature change | .......... %

[2]

(k) Two students add 6.0 g of sodium carbonate to 50.00 cm³ of 2.0 mol dm⁻³ hydrochloric acid. Each student repeats the experiment a number of times.
The thermometer readings and temperature changes obtained consistently by each student are shown below.

[Table_2]
initial temperature / °C | final temperature / °C | temperature rise / °C
student 1 | 20.0 | 28.0 | 8.0
student 2 | 19.0 | 27.0 | 8.0

Suggest the type of error shown by these results.
.................................................................................................................. [1]

FB 7 and FB 8 are aqueous solutions of salts. One of these contains two cations and one anion. The other contains one cation and one anion. Both FB 7 and FB 8 have a common cation.

You will carry out tests to deduce the following.

• the cations present in each solution
• whether a sulfate ion is present in either solution

At each stage of any test you are to record details of the following.

• colour changes seen
• the formation of any precipitate and the colour of the precipitate

Where gases are released they should be identified by a test, described in the appropriate place in your observations.
You should indicate clearly at what stage in a test a change occurs. Marks are not given for chemical equations.
No additional tests for ions present should be attempted.
If any solution is warmed directly with a Bunsen burner a boiling-tube MUST be used.
Rinse and reuse test-tubes where possible.

(a) Use information from the Qualitative Analysis Notes on page 11 to select a pair of reagents that, used together, determine whether a sulfate ion is present in either solution.

The reagents are $\text{.....................}$ followed by $\text{.....................}$. [1]

(b) Use your chosen reagents to carry out tests on FB 7 and FB 8. Record your results in an appropriate form in the space below. [2]

(c) From your observations in (b) show with a tick which of the following statements is true.

FB 7 contains the sulfate ion
FB 8 contains the sulfate ion
neither solution contains the sulfate ion

Explain the evidence that supports your conclusion.
............................................................................................................................................................................ [1]

(d) Carry out the following tests on the solutions FB 7 and FB 8. Record your observations below.

test	observations
	FB 7	FB 8
To 1 cm depth of solution in a boiling-tube, add 2 cm depth of aqueous sodium hydroxide; then
.................................................................................................................................
warm the solution gently. Care is needed when heating aqueous sodium hydroxide.
To 1 cm depth of solution in a test-tube, add 2 cm depth of aqueous ammonia.

[3]

(e) To 1 cm depth of FB 7 in a test-tube add 1 cm depth of sodium hydroxide. Leave to stand for a few minutes.
observation
............................................................................................................................................................................ [1]

(f) From your observations in (d) and (e) you should be able to identify the common cation in the solutions and the second cation in one of the solutions.
The common cation present in both solutions is $\text{.....................}$.
The second cation contained in one of the solutions is $\text{.....................}$.
Explain how your observations support your conclusions for
(i) the common cation,
..........................................................................................................
.......................................................................................................... [1]
(ii) the second cation.
..........................................................................................................
.......................................................................................................... [1]

Read through the remainder of question 2 before starting further practical work.
Heat a half-full 250 cm$^3$ beaker of water for use as a hot water-bath.

(g) FB 9, FB 10, FB 11 and FB 12 are organic compounds. Each contains one of the following different functional groups.

• primary alcohol
• secondary alcohol
• aldehyde
• ketone

You are to react each of these compounds with some of the following reagents.

• acidified aqueous potassium dichromate(VI)
• 2,4-dinitrophenylhydrazine (2,4-DNPH) reagent
• ammoniacal silver nitrate (Tollens’ reagent)

You are provided with the first two reagents. You must prepare the last of these reagents, Tollens’ reagent, immediately before use. Follow the instructions in the box below.

To 2 cm depth of aqueous silver nitrate in a boiling-tube add \dfrac{1}{2} cm depth of aqueous sodium hydroxide. This will produce a brown precipitate of silver(I) oxide. Add aqueous ammonia a little at a time, with continuous shaking, until the brown precipitate just dissolves. Do not add an excess of aqueous ammonia.

In each of the following tests add a few drops of the reagent to 1 cm depth of FB 9, FB 10, FB 11 and FB 12 in separate test-tubes. In the tests using acidified potassium dichromate(VI) and Tollens’ reagent, if no initial reaction is seen, warm that tube and its contents in your hot water-bath. There is no need to heat any tube to which you have added 2,4-DNPH reagent.
Do not heat any tube with a naked flame.
Record your results in the table below.
Do not carry out tests for the shaded boxes.

reagent	observations
	FB 9	FB 10	FB 11	FB 12
acidified potassium dichromate(VI)
2,4-DNPH reagent
Tollens’ reagent

[3]
(h) State which of the solutions contain alcohols. Explain the observations leading to your conclusion.
FB $\text{..............}$ and FB $\text{..............}$ contain alcohols.
explanation
.................................................................................................. ......................................
.................................................................................................. ......................................
State which solution contains the ketone. Explain the observations leading to your conclusion.
FB $\text{..............}$ contains the ketone.
explanation
.................................................................................................. ......................................
.................................................................................................. ...................................... [2]