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You are provided with the following reagents.
FA 1, 2.0 mol \, dm^{-3} sulfuric acid, H_2SO_4
FA 2, aqueous sodium hydroxide, 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, NaOH, in FA 2
• the enthalpy change when 1 mol of H_2SO_4 is neutralised by NaOH
(a) Method
• Fill the burette with FA 1.
• Support the plastic cup in the 250 cm^3 beaker.
• Run 10.00 cm^3 of FA 1 from the burette into the plastic cup.
• Measure 10 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 cm^3 of FA 1.
Use 20 cm^3, 30 cm^3, 40 cm^3 and 50 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 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 cm^3 of FA 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 .............. 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, H_2SO_4, are contained in 10.00 cm^3 of FA 1.
10.00 \, cm^3 \, \text{of \ FA \ 1 \ contain \ } ............... \text{mol \ of \ } H_2SO_4.
(ii) Calculate how many moles of NaOH are required to neutralise the amount of H_2SO_4 calculated in (i) above.
The sulfuric acid in 10.00 \, cm^3 \, \text{of \ FA \ 1 \ is \ neutralised \ by \ } ............... \text{mol \ of \ NaOH}.
(g) Use the equation below to calculate the concentration of NaOH in FA 2.
\text{concentration \ of \ NaOH \ (mol \ dm$^{-3}$) = answer \ to \ (f)(ii) } \times \frac{1000}{\text{volume \ of \ FA \ 2 \ (cm}^3\text{) \ from \ (c)}}
The concentration of 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 mol H_2SO_4 is neutralised by NaOH. 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 \text{cm}^3 of solution changes by 1 °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) Complete the sections below.
(i) The maximum error in taking a temperature reading on a thermometer with graduations at 1 °C is .................. °C.
(ii) The temperature rise when 30 cm^3 of FA 2 is added to 10.00 cm^3 of FA 1 is .................. °C.
(iii) Calculate the maximum percentage error due to the thermometer when measuring the temperature rise in (ii) above.
The maximum percentage error = ............... \%.
530
599
528
Solutions FA 3, FA 4 and FA 5 each contain a Group 2 halide. Solution FA 6 contains a potassium salt.
You will carry out tests to deduce the following.
- the anion present in FA 6
- the solution containing the chloride ions
- the solution containing barium ions
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, identify the halide ion present.
The reagents are .......................................... followed by ................................................. [1]
(b) Use your chosen reagents to carry out tests on FA 3, FA 4 and FA 5. Record your results in an appropriate form in the space below. [2]
(c) From the results of the tests in (b) state which solution contains the chloride ion, $Cl^-$.
Solution ............... contains the chloride ion. Explain the evidence that supports your conclusion.
..............................................................................................................................................
....................................................................................................................................... [1]
(d) Carry out the following tests on each of the solutions FA 3, FA 4 and FA 5. Record your observations below.
test | observations
--- | ---
FA 3 | FA 4 | FA 5
To 1 cm depth of solution in a test-tube, add 2 cm depth of aqueous sodium hydroxide.
To 1 cm depth of solution in a test-tube, add 2 cm depth of aqueous ammonia.
To 1 cm depth of solution in a test-tube, add 1 cm depth of FA 6. [3]
(e) To 1 cm depth of FA 6 in a test-tube add 1 cm depth of dilute sulfuric acid. observation
..............................................................................................................................................
..................................................................................................................................... [1]
(f) From your observations in (d) and (e) you should be able to identify the anion in FA 6 and which of the solutions FA 3, FA 4 or FA 5 contains barium cations.
The anion present in FA 6 is ....................................... . $Ba^{2+}$ ions are contained in solution ............................ .
Explain how your observations support your conclusions for
(i) the anion present in FA 6, ............................................................................
............................................................................................................................................
(ii) the solution containing $Ba^{2+}$ ions. ..............................................................
............................................................................................................................................ [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) FA 7, FA 8, FA 9 and FA 10 are organic compounds. Each contains one of the following different functional groups.
- primary alcohol
- tertiary alcohol
- aldehyde
- ketone
You are to react some 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 ½ 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 FA 7, FA 8, FA 9 and FA 10 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
--- | ---
FA 7 | FA 8 | FA 9 | FA 10
acidified potassium dichromate(VI)
2,4-DNPH reagent
Tollens’ reagent [3]
(h) State which of the solutions contains a tertiary alcohol. Explain the observations leading to your conclusion.
FA ............... contains the tertiary alcohol.
explanation ..............................................................................................................................
................................................................................................................................................
State which of the solutions contains the aldehyde. Explain the observations leading to your conclusion.
FA ............... contains the aldehyde.
explanation ..............................................................................................................................
................................................................................................................................................ [2]
523
593
588
