RVHS Prelim P4 Soln

River Valley High School Pg 1 of 20 Year 6 H2 Chemistry 9729 Practical Examination 2018 RIVER VALLEY HIGH SCHOOL YEAR 6 PRACTICAL EXAMINATION II H2 CHEMISTRY 9729 23 AUGUST 2018 2 HOURS 30 MINUTES NAME CLASS 6 ( ) INDEX NO. INSTRUCTIONS TO CANDIDATES DO NOT OPEN THIS BOOKLET UNTIL YOU ARE TOLD TO DO SO. Read these notes carefully. Write your name, class and index number in the spaces at the top of this page. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use a 2B pencil for any diagrams or graph. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Shift Laboratory For Examiner’s Use 3 s.f. Units Total 55 _________________________________________________________________________ This Question Paper consists of 14 printed pages and 1 blank page. Answer all the questions in the spaces provided.

River Valley High School Pg 2 of 20 Year 6 H2 Chemistry 9729 Practical Examination 2018 1 To investigate the kinetics of the reaction between iron( III) ions and iodide ions For Examiner’s Use You will investigate rate of oxidation of iodide by iron( III) ions in reaction 2 . The chemical changes in this experiment can be presented by the following equations: Reaction 1: Fe3+(aq) + S2O32–(aq) ⇌ [Fe(S2O3)]+(aq) Reaction 2: 2Fe3+(aq) + 2I–(aq) → 2Fe2+(aq) + I2(aq) Reaction 3: I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) The reaction is started by mixing acidic solution of iron(III) chloride with sodium thiosulfate, potassium iodide, and starch. The iodine, I2, produced in reaction 2 reacts immediately with thiosulfate ions, S2O32– in reaction 3. When all the thiosulfate has been used, the iodine produced will turn starch indicator blue-black. The rate of the reaction can therefore be determined by finding the time for the blue-black colour to appear. FA 1 is 0.030 mol dm –3 iron(III) chloride, FeCl3. FA 2 is 0.060 mol dm–3 potassium iodide, KI. FA 3 is 0.0060 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator You are advised to read the instructions before starting any practical work and draw a table, in an appropriate format, for your results in the space on page 4. You will need to include volume of FA 1, volume of water, reaction time and calculated rate of reaction for each of the five experiments. Record all calculated values to 3 significant figures. (a) Method Experiment 1 1. Use the measuring cylinders to place the following in a 100 cm 3 beaker. • 10 cm 3 of FA 2 • 20 cm 3 of FA 3 • 10 cm 3 of starch indicator 2. Using a measuring cylinder, transfer 20 cm 3 of FA 1 rapidly into the same 100 cm 3 beaker. Start the stopwatch during this addition. 3. Stir the mixture and place the beaker on a white tile. 4. The mixture turns brown and then yellow before turning a blue-black colour. Stop timing when this blue-black colour appears. Record the time to the nearest 0.1 second. 5. Wash the beaker thoroughly with water and carefully dry the beaker.

River Valley High School Pg 3 of 20 Year 6 H2 Chemistry 9729 Practical Examination 2018 Experiment 2 6. Repeat step 1 in Experiment 1. 7. Using a measuring cylinder, add 12 cm 3 of FA 1, and make up the volume to 20 cm 3 using deionised water. Transfer the solution rapidly into the same 100 cm 3 beaker. Start the stopwatch during this addition. 8. Stir the mixture and place the beaker on the white tile. 9. Stop timing when a blue-black colour appears. 10. Wash the beaker thoroughly with water and carefully dry the beaker. Experiments 3−5 Carry out three further experiments to investigate the effect of changing the concentration of Fe 3+(aq) by altering the volume of aqueous FeC l3, FA 1, used. You should use a volume of FA 1 that is at least 12 cm 3 and the total volume of the reaction mixture must always be 60 cm3. (b) Show, by means of calculation, that the change in the concentration of Fe3+, ∆[Fe3+], which occurred when the blue-black colour appeared was 2.00 × 10−3 mol dm−3. For Examiner’s Use Amount of SଶOଷ ଶି = ଶ଴ ଵ଴଴଴ ×0 .006 =1 .20 × 10ିସ mol ∆[Fe3+] = ଵ.ଶ଴×ଵ଴షర లబ భబబబ =2 .00 × 10ିଷ mol dm−3 1

River Valley High School Pg 4 of 20 Year 6 H2 Chemistry 9729 Practical Examination 2018 (c) Results The rate of the reaction can be calculated as shown. rate = ∆[୊ୣయశ] ୰ୣୟୡ୲୧୭୬ ୲୧୫ୣ ×1 0଺ Calculate the rate of reaction for each experiment and complete your table. Expt VFA1/ cm3 ܸୌమ୓ /cm3 Reaction time, t /s Rate/ mol dm−3 s–1 1 20.00 0.00 12.9 155 2 12.00 8.00 28.8 69.4 3 14.00 6.00 20.7 96.6 4 16.00 4.00 17.3 115 5 18.00 2.00 14.7 136 2 3 4 5 (d) (i) Plot the rate (y-axis) against the volume of FA 1 (x-axis) on the grid. Draw a line of best fit through the points. Your chosen scales should allow you to extrapolate this line to volume of FA 1 = 0 cm3. You should also identify any points you consider anomalous by drawing a circle around it.

River Valley High School Pg 5 of 20 Year 6 H2 Chemistry 9729 Practical Examination 2018 6 7 8 (ii) Explain why the volume of FA 1 in each experiment can be used as the [Fe3+]. Since total volume is kept constant by adding deionised water, volume of FA 1 used is proportional to [Fe3+]. 9 (iii) Deduce the order of reaction with respect to Fe 3+. Use evidence from your graph to support your deduction.  Observe from the graph, any one of the following: • the rate increase by xxx for every additional xxx cm 3 of FA 1 used • when volume of FA 1 double, rate double (or other multiples) • the graph is linear/gradient is a constant, showing that rate ∝ volume/ concentration of FA 1 / state that it is a graph of rate against [Fe3+]  Therefore, the reaction is first order with respect to Fe 3+. 10

River Valley High School Pg 6 of 20 Year 6 H2 Chemistry 9729 Practical Examination 2018 (e) (i) Use your graph to calculate the time that the reaction would have taken if 4.0 cm 3 of FA 1 had been used. Show your working clearly. If 4.0 cm3 of FA 1 is used, rate = xxx mol dm−3 s−1 Time = ∆[Fe3+] × 106/ rate = xxx s (to 3 sf or nearest seconds). time = .............................. 11 (ii) Calculate the initial concentration of iron( III) ions and the initial concentration of iodide ions if 4.0 cm3 of FA 1 had been used. [Fe3+] = ଴.଴ଷ଴×ସ.଴ ଺଴.଴ = 0.00200 mol dm–3 [I−] = ଴.଴଺଴×ଵ଴.଴ ଺଴.଴ = 0.0100 mol dm–3 initial [Fe3+] = .............................. initial [I−] = .............................. 12 (iii) Given that the reaction is second order with respect to iodide, calculate the rate constant. rate = k[Fe3+][I−]2 k = ௥௔௧௘ [ி௘యశ][ூష]మ = xxx mol–2 dm6 s−1 rate constant = .............................. 13 (f) The complex [Fe(S2O3)]+ formed in reaction 1 is purple. Consider the colour change observed when FA 1 was added to the solution prepared in Step 1 before the blue-black colour appeared. Suggest an explanation for the sequence of colour change in terms of the chemistry involved. The brown solution is due to a mixture of purple complex [Fe(S 2O3)]+ and orange-brown Fe3+(aq). Since the I2 formed reacted immediately with thiosulfate, [S 2O32–(aq)] decreased, and the position of equilibrium in (1) shifts left. Thus, concentration of [Fe(S 2O3)]+decreased. Hence, the colour of the solution gradually fades/ until the solution became pale yellow due to Fe3+. 14 15 [Total: 15]

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