AS & A Level Chemistry - 9701 Paper 3 2015 Winter Zone 1

In this experiment you will determine the ionic equation for the reaction of acidified potassium manganate(VII) with potassium iodide. Excess potassium iodide is used and the reaction produces iodine. The amount of iodine produced is measured by titration with sodium thiosulfate. FA 1 is 0.0180 mol dm^-3 potassium manganate(VII), KMnO_4.
FA 2 is 1.00 mol dm^-3 sulfuric acid, H_2SO_4.
FA 3 is 0.500 mol dm^-3 potassium iodide, KI.
FA 4 is 0.100 mol dm^-3 sodium thiosulfate, Na_2S_2O_3.
Starch indicator (a) Method - Pipette 25.0 cm^3 of FA 1 into a conical flask. - Use the measuring cylinder to add 25 cm^3 of FA 2 to the conical flask. - Use the measuring cylinder to add 20 cm^3 of FA 3 to the conical flask. - Fill the burette with FA 4. - Carry out a rough titration. When the colour of the mixture becomes yellow/orange, add a few drops of starch indicator. Then titrate until the mixture goes colourless. - Record all your burette readings in the space below. The rough titre is ......................... cm^3. - Carry out as many accurate titrations as you think necessary to obtain consistent results. - Make sure 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. Keep FA 1 and FA 2 for use in Question 3 and FA 4 for use in Question 2.

(b) From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations. Show clearly how you have obtained this value. Volume of FA 4 required is ...................... cm^3.

(c) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the number of moles of sodium thiosulfate in the volume of FA 4 calculated in (b). moles of Na_2S_2O_3 = .............................. mol (ii) Use the equation below to calculate the number of moles of iodine that reacted with the sodium thiosulfate in the titration. $$I_2 + 2Na_2S_2O_3 \rightarrow Na_2S_4O_6 + 2NaI$$ moles of I_2 = ............................... mol (iii) Use information on page 2 to calculate the number of moles of potassium manganate(VII) in FA 1 used in the titration. moles of KMnO_4 = ............................... mol (iv) From your answers to (ii) and (iii), calculate the number of moles of iodine produced by the reaction of 2.00 moles of potassium manganate(VII) with excess potassium iodide. moles I_2 = ............................... mol (v) Using your answer to (iv), put a tick next to the ionic equation that represents the reaction between FA 1 and FA 3. - 2MnO_4^- + 2I^- + 16H^+ \rightarrow I_2 + 2Mn^6+ + 8H_2O ............ - 2MnO_4^- + 4I^- + 16H^+ \rightarrow 2I_2 + 2Mn^5+ + 8H_2O ............ - 2MnO_4^- + 6I^- + 16H^+ \rightarrow 3I_2 + 2Mn^4+ + 8H_2O ............ - 2MnO_4^- + 8I^- + 16H^+ \rightarrow 4I_2 + 2Mn^3+ + 8H_2O ............ - 2MnO_4^- + 10I^- + 16H^+ \rightarrow 5I_2 + 2Mn^2+ + 8H_2O ............ - 2MnO_4^- + 12I^- + 16H^+ \rightarrow 6I_2 + 2Mn^+ + 8H_2O ............ (vi) Prove that the iodide ion has been oxidised in the equation that you selected in (v).

(d) (i) The error in calibration of the pipette you used is ±0.06 cm^3. Calculate the percentage error when measuring FA 1, using the pipette. percentage error = ...................... % (ii) A student suggested that the experiment would be more accurate if a pipette was used to measure solution FA 3. State and explain whether you agree with the student.

In this experiment you will investigate how the rate of reaction between sodium thiosulfate and hydrochloric acid is affected by the concentration of the acid.
When aqueous thiosulfate ions react with hydrogen ions, H⁺, in any acid, a pale yellow precipitate of sulfur is formed. The ionic equation for this reaction is given below.
$\text{S}_2\text{O}_3^{2-}(\text{aq}) + 2\text{H}^+(\text{aq}) \rightarrow \text{S}(\text{s}) + \text{SO}_2(\text{aq}) + \text{H}_2\text{O}(\text{l})$
The rate of the reaction can be determined by measuring the time taken to produce a fixed quantity of sulfur.
FA 4 is 0.10 mol dm^-3 sodium thiosulfate, Na₂S₂O₃.
FA 5 is 0.20 mol dm^-3 hydrochloric acid, HCl.
(a) Method
Record all your measurements, in an appropriate form, in the space below.
Experiment 1
• Use the larger measuring cylinder to transfer 40 cm³ of FA 4 into the 100 cm³ beaker.
• Rinse the larger measuring cylinder thoroughly with water, then add 30 cm³ of FA 5 to the beaker and start timing immediately.
• Stir the mixture once and place the beaker on top of the printed insert page provided.
• Look down through the solution in the beaker at the print on the insert.
• Stop timing as soon as the precipitate of sulfur makes the print on the insert invisible.
• Record the reaction time to the nearest second.
• Empty and rinse the 100 cm³ beaker.
• Dry the outside of the beaker ready for Experiment 2.
Experiment 2
• Rinse the larger measuring cylinder, then use it to transfer 40 cm³ of FA 4 into the 100 cm³ beaker.
• Use the smaller measuring cylinder to add 10 cm³ of distilled water to the beaker.
• Use the same measuring cylinder to add 20 cm³ of FA 5 to the mixture in the beaker and start timing immediately.
• Stir the mixture once and place the beaker on top of the printed insert page provided.
• Stop timing as soon as the print on the insert becomes invisible.
• Record the reaction time to the nearest second.
• Empty and rinse the 100 cm³ beaker.
• Dry the outside of the beaker ready for Experiment 3.
Experiment 3
• Carry out the reaction using a mixture of 40 cm³ of FA 4, 20 cm³ of distilled water and 10 cm³ of FA 5.
• Measure and record the reaction time to the nearest second.
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(b) (i) The 'rate of reaction' can be represented by the formula below.
$\text{'rate of reaction'} = \frac{1000}{\text{reaction time}}$
Use this formula to calculate the 'rate of reaction' for Experiments 1 and 3. Give the unit.
"rate of reaction" for Experiment 1 ................................ unit ..............
"rate of reaction" for Experiment 3 ................................ unit ..............
(ii) Calculate the initial concentrations of hydrochloric acid in the reaction mixtures in Experiments 1 and 3.
initial concentration of HCl in Experiment 1 = .......................... mol dm^-3
initial concentration of HCl in Experiment 3 = .......................... mol dm^-3
(iii) How is the 'rate of reaction' affected by the concentration of hydrochloric acid in the mixture?
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(iv) Predict how the reaction time measured in Experiment 1 would have been affected if the experiment had been carried out using 0.20 mol dm^-3 sulfuric acid instead of 0.20 mol dm^-3 hydrochloric acid. Explain your answer.
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(v) Predict how the reaction time measured in Experiment 3 would have been affected if the experiment had been carried out in a 250 cm³ beaker instead of a 100 cm³ beaker. Explain your answer.
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(a) FA 6 is a sodium compound containing one anion listed on page 11.
Dissolve the FA 6 provided in about 15 cm³ of distilled water in a boiling tube. Carry out the following tests and record your observations in the table below.

[Table_1]

(i) To a 1 cm depth of the solution of FA 6 in a test-tube, add a few drops of aqueous barium chloride or aqueous barium nitrate, then add dilute hydrochloric acid.

(ii) To a 1 cm depth of the solution of FA 6 in a test-tube, add an equal volume of aqueous hydrogen peroxide, then add a few drops of aqueous barium chloride or aqueous barium nitrate, then add dilute hydrochloric acid.

(iii) To a 1 cm depth of the solution of FA 6 in a boiling tube, add an equal volume of FA 2, sulfuric acid, then heat the mixture gently and cautiously.

(iv) To a 1 cm depth of the solution of FA 6 in a test-tube, add an equal volume of aqueous sodium hydroxide, then add a few drops of FA 1, aqueous potassium manganate(VII), then add FA 2, sulfuric acid.

(v) Identify the anion in FA 6, and state one piece of evidence for your identification.
anion ...............................................
evidence ............................................................
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(vi) Give the chemical equation for the reaction between FA 6 and hydrogen peroxide, H₂O₂, in test (ii). State symbols are not required.
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(b) FA 7, FA 8, FA 9 and FA 10 each contain one cation from the list on page 10. You will attempt to identify the cations by testing with aqueous sodium hydroxide and aqueous ammonia. In each case, use a 1 cm depth of the solution in a test-tube.

(i) Complete the table below.

[Table_2]

(ii) Use your observations to identify, as far as possible, the cation present in each solution. If alternative identities are possible, state this clearly.
FA 7 cation ..............................................
FA 8 cation ..............................................
FA 9 cation ..............................................
FA 10 cation ............................................

(iii) Give the ionic equation for the reaction of one of your cations with a few drops of sodium hydroxide. State symbols are not required.
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(iv) The precipitates obtained when alkalis are added to solutions of certain cations are sometimes difficult to see. Suggest how, using no additional apparatus, the experiment could be repeated in a way that would make these precipitates more visible.
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