9701 Chemistry November 2025 Question Paper 33

1 The thiosulfate ion, S2O3 2–, decomposes when an acid is added. S2O3 2–(aq) + 2H+(aq) S(s) + SO2(aq) + H2O(l)

The rate of this reaction can be investigated by measuring how long it takes for the solid sulfur forming to obscure the print on the insert.

You will investigate how the concentration of the thiosulfate ion affects the rate of the reaction.

Note: A small amount of sulfur dioxide gas may be formed in the experiment. It is very important that you avoid inhaling any fumes. As soon as each experiment is complete, add the reaction mixture to the quenching bath and rinse the beaker thoroughly.

FA 1 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3.

FA 2 is 2.00 mol dm–3 hydrochloric acid, HCl.

(a) Method

Experiment 1 • Fill a burette with FA 1. • Run 40.00 cm3 of FA 1 into the 100 cm3 beaker. • Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 2. • Add the FA 2 to the FA 1 in the beaker and start timing immediately. • Stir the mixture once and place the beaker on the printed insert. • View the printing on the insert from above through the solution. • Stop timing when the print on the insert becomes obscured. • Record this reaction time to the nearest second in the space for results. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker so it is ready to use in Experiment 2.

Experiment 2 • Refill the burette with FA 1. • Fill the second burette with distilled water. • Run 20.00 cm3 of FA 1 into the 100 cm3 beaker. • Run 20.00 cm3 of distilled water into the same beaker. • Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 2. • Add the FA 2 to the FA 1 in the beaker and start timing immediately. • Stir the mixture once and place the beaker on the printed insert. • View the printing on the insert from above through the solution. • Stop timing when the print on the insert becomes obscured. • Record this reaction time to the nearest second in the space for results. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker so it is ready to use in the next experiment.

Experiments 3–5 • Carry out three further experiments to investigate how using different volumes of FA 1 affects the reaction time.

Note: the combined volumes of FA 1 and distilled water must always be 40.00 cm3.

Do not use a volume of FA 1 that is less than 15.00 cm3.

Record all your results in a table. You should include the volume of FA 1, the volume of distilled water, the reaction time and the reaction rate for each of your five experiments.

The rate of reaction can be calculated using the following formula. rate = 1000 reaction time

Results

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(b) (i) On the grid in Fig. 1.1, plot the rate (y-axis) against the volume of FA 1 (x-axis).

Start each axis at the origin (0, 0).

Ring any anomalous points. Draw a line of best fit. I II III IV V VI VII VIII , , Fig. 1.1 [3] I II III , ,

(ii) Use your graph in Fig. 1.1 to determine the time it would take for the print to be obscured if 7.50 cm3 of FA 1, 32.50 cm3 of water and 10.0 cm3 of FA 2 had been used.

Show clearly on the graph how you worked out your answer.

time for printing to be obscured = s [2]

(c) A student carries out the same method as given in (a) but using a different concentration of acid. The student’s calculated values for the rate are plotted on the grid in Fig. 1.2. 0.00 0 10 20 30 40 50 60 5.00 10.00 15.00 20.00 25.00 volume of FA 1 rate vs volume of FA 1 30.00 35.00 40.00 45.00 rate Fig. 1.2

State whether the student’s results show that the rate is directly proportional to the volume of FA 1 used.

Explain your answer [2]

[Total: 15] , ,

2 When hydrated sodium thiosulfate is dissolved in water the temperature of the liquid changes. You will carry out an experiment to determine the enthalpy change, ΔH, when one mole of hydrated sodium thiosulfate dissolves in water.

FA 3 is hydrated sodium thiosulfate, Na2S2O3•5H2O.

(a) Method • Support the cup in the 250 cm3 beaker. • Use the 50 cm3 measuring cylinder to transfer 30.0 cm3 of distilled water into the cup. • Measure the temperature of the water in the cup. Record this in the space for results. • Weigh the container with FA 3. Record the mass. • Tip all the FA 3 into the cup. • Stir the mixture until the minimum temperature is obtained. Record this temperature. • Weigh the container with any residual FA 3. Record the mass. • Calculate and record the mass of FA 3 added. • Calculate and record the temperature change. Results

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(b) Calculations

(i) Calculate the amount, in mol, of hydrated sodium thiosulfate added.

amount of Na2S2O3•5H2O = mol [1]

(ii) Calculate the energy change, in J, in your experiment.

energy change = J [1] I II III IV , ,

(iii) Calculate the enthalpy change, ΔH, in kJ mol−1, when 1.00 mol of hydrated sodium thiosulfate dissolves in water. Show your working.

ΔH = kJ mol−1

sign value [1]

(c) The value calculated in (b)(iii) can be used to determine the enthalpy change, ΔHr , for the following reaction. Na2S2O3(s) + 5H2O(l) Na2S2O3•5H2O(s)

Outline the method of one further experiment you would need to carry out to obtain the data necessary to calculate the value of ΔHr .

Show how you would use your results from this experiment and (b)(iii) to calculate ΔHr .

Do not carry out your experiment. method calculation [4]

[Total: 11] , , Qualitative analysis For each test you should record all your observations in the spaces provided. Examples of observations include: • colour changes seen • the formation of any precipitate and its solubility (where appropriate) in an excess of the reagent added • the formation of any gas and its identification (where appropriate) by a suitable test. You should record clearly at what stage in a test an observation is made. Where no change is observed, you should write ‘no change’. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. If any solution is warmed, a boiling tube must be used. If a solid is heated, a hard-glass test-tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted. , ,

3 Half-fill the 250 cm3 beaker with water and place it on a tripod and gauze. Heat the water until boiling then switch off your Bunsen burner. This will be your hot water bath for use in (b)(i). Start (a)(i) while the water is heating.

(a) FA 4 is an aqueous solution containing three ions. One ion is not listed in the Qualitative analysis notes. FA 5 is an aqueous solution containing two ions. The cation in FA 5 is listed in the Qualitative analysis notes. The anions in both FA 4 and FA 5 contain sulfur.

(i) Carry out the following tests using a 1 cm depth of either FA 4 or FA 5 in a test-tube.

Record your observations in Table 3.1. Table 3.1 test observations FA 4 FA 5 Test 1 Add aqueous sodium hydroxide. Test 2 Add aqueous ammonia. Test 3 Add aqueous sodium carbonate. Test 4 Add aqueous barium chloride or aqueous barium nitrate. Test 5 Add acidified aqueous potassium manganate(VII).

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(ii) From your observations in (a)(i), deduce the identities of the ions present in FA 4 and FA 5. Give the formula of each ion in Table 3.2. If you are unable to identify an ion write ‘unknown’. Table 3.2 ions present FA 4 FA 5 cations anions

[3] , ,

(b) (i) Both FA 6 and FA 7 are one of propan-1-ol, propan-2-ol, methanoic acid or ethanoic acid. You will carry out tests to investigate the identities of FA 6 and FA 7. For each test use a 1 cm depth of FA 6 or FA 7 in a test-tube. Record your observations in Table 3.3. Table 3.3 test observations FA 6 FA 7 Test 1 Add a few drops of acidified aqueous potassium manganate(VII), then place the test-tube in the hot water bath. Test 2 Add a 2 cm depth of aqueous iodine followed by drops of aqueous sodium hydroxide until the colour just disappears. If no reaction is visible, place the test-tube in the hot water bath. Test 3 Add aqueous sodium carbonate.

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(ii) From your observations in (b)(i) deduce the identities of FA 6 and FA 7. Give reasons for your answers.

FA 6 is reasons FA 7 is reasons [2] , ,

(c) A student carrying out a similar set of tests identifies two unknown compounds to be ethanol and propanoic acid.

Write an equation for the reaction between these two compounds [1]

[Total: 14] , , Qualitative analysis notes 1 Reactions of cations cation reaction with NaOH(aq) NH3(aq) aluminium, Al 3+(aq) white ppt. soluble in excess white ppt. insoluble in excess ammonium, NH4 +(aq) no ppt. ammonia produced on warming – barium, Ba2+(aq) faint white ppt. is observed unless [Ba2+(aq)] is very low no ppt. calcium, Ca2+(aq) white ppt. unless [Ca2+(aq)] is very low no ppt. chromium(III), Cr3+(aq) grey-green ppt. soluble in excess giving dark green solution grey-green ppt. insoluble in excess copper(II), Cu2+(aq) pale blue ppt. insoluble in excess pale blue ppt. soluble in excess giving dark blue solution iron(II), Fe2+(aq) green ppt. turning brown on contact with air insoluble in excess green ppt. turning brown on contact with air insoluble in excess iron(III), Fe3+(aq) red-brown ppt. insoluble in excess red-brown ppt. insoluble in excess magnesium, Mg2+(aq) white ppt. insoluble in excess white ppt. insoluble in excess manganese(II), Mn2+(aq) off-white ppt. rapidly turning brown on contact with air insoluble in excess off-white ppt. rapidly turning brown on contact with air insoluble in excess zinc, Zn2+(aq) white ppt. soluble in excess white ppt. soluble in excess 2 Reactions of anions anion reaction carbonate, CO3 2– CO2 liberated by dilute acids chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)) bromide, Br –(aq) gives cream / off-white ppt. with Ag+(aq) (partially soluble in NH3(aq)) iodide, I–(aq) gives pale yellow ppt. with Ag+(aq) (insoluble in NH3(aq)) nitrate, NO3 –(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO2 –(aq) NH3 liberated on heating with OH–(aq) and Al foil; decolourises acidified aqueous KMnO4 sulfate, SO4 2–(aq) gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids); gives white ppt. with high [Ca2+(aq)] sulfite, SO3 2–(aq) gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids); decolourises acidified aqueous KMnO4 thiosulfate, S2O3 2–(aq) gives off-white/pale yellow ppt. slowly with H+ , , 3 Tests for gases gas test and test result ammonia, NH3 turns damp red litmus paper blue carbon dioxide, CO2 gives a white ppt. with limewater hydrogen, H2 ‘pops’ with a lighted splint oxygen, O2 relights a glowing splint , ,