DHS H2 CHEM P2 ANSWERS Prelim

© DHS 2011 9647/02/Answers 1 Dunman High School 2011 Year 6 H2 Chemistry Preliminary Exams – Paper 2 Answers 1 Planning When aqueous sodium chloride, NaC l, is added to aqueous lead( II) nitrate, Pb(NO 3)2, a white precipitate of lead chloride, PbCl2, is produced. A suggested stoichiometric equation is Pb(NO3)2(aq) + 2NaCl(aq)  PbCl2(s) + 2NaNO3(aq) In separate experiments, different volumes of 0.20 mol dm –3 aqueous sodium chloride are added to a fixed volume of 0.10 mol dm –3 aqueous lead( II) nitrate. In each case, the precipitate, PbCl2 will be formed. This precipitate is then filtered, washed with distilled water and thoroughly dried. The mass of the precipitate is recorded. You are to plan an experiment to investigate this reaction in order to confirm or reject the stoichiometry of the equation. (a) (i) By considering the suggested stoichiometric equation, predict how the number of moles of the precipitate, PbC l2, will change as the number of moles of NaC l added increases. Prediction: No. of moles of PbCl2, increases as no. of moles of NaCl added Increases. (ii) Sketch the graph which would result if NaC l is the limiting reagent . Start your graph with no NaCl added. (b) You are provided with 250 cm 3 of 0.20 mol dm –3 aqueous sodium chloride and some solid lead(II) nitrate. (i) Outline how you would prepare 250 cm 3 of 0.10 mol dm –3 aqueous lead( II) nitrate. [Ar: N, 14.0; O, 16.0; Pb, 207.0] • Weigh 8.275 g of solid Pb(NO3)2 into an empty weighing bottle / beaker. • Transfer and dissolve it in some distilled/deionised water in a beaker. • Rinse weighing bottle with distilled water and transfer the washings to the beaker. • Ensure all solid has dissolved. PbCl2 /mol NaCl /mol 0

© DHS 2011 9647/02/Answers 2 • Transfer to a 250 cm 3 standard/volumetric/graduated flask using a funnel and glass rod. • Rinse beaker with distilled water and transfer the washings to the flask • Add distilled water to the standard/volumetric/graduated flask to the 250 cm3 mark • Shake thoroughly to ensure a homogenous mixing. (ii) You will be carrying out five sets of experiments to obtain different masses of precipitate. Give a step by step description of how you would carry out one such experiment. You need to state only the following:  the volumes of each solution to be used,  how the volumes will be measured, and  how you would dry the precipitate. • Measure 40 cm 3 of Pb(NO 3)2 into an beaker using a measuring cylinder/burette/pipette • Measure 5 cm3 of NaCl using a measuring cylinder/burette/pipette and • Dry the precipitate by: o allowing water to evaporate o pressing between filter papers o warming in oven o adding propanone (c) In the table below  enter appropriate headings to show recorded and calculated data. The headings should include appropriate units,  enter the volumes from your plan in (b),  enter suitable volumes for four further experiments. experiment volume of Pb(NO3)2 / cm3 volume of NaCl / cm3 mass of PbCl2 / g no. of moles of PbCl2 no. of moles of NaCl 1 40 5 0.139 0.001 0.0005 2 40 10 3 40 20 4 40 30 5 40 35 1.112 0.008 0.004 (d) How would you ensure that at the end of each experiment the precipitate is thoroughly dried? Repeat drying/heating process until a constant mass is obtained.

© DHS 2011 9647/02/Answers 3 2 Iodine forms an intense blue complex with st arch and many chemical reactions make use of this property. (a) In the iodine clock reaction, the following set of reactions take place and a colour change involving the iodine–starch comple x happens after a fixed time delay, allowing the kinetics of the reaction to be determined. Step I: 2 I–(aq) + S2O8 2–(aq)  I2(aq) + 2SO4 2–(aq) Step II: I2 + 2S2O3 2–  2I – + S4O6 2– During an experiment to determine the rate equati on for these reactions, different concentrations of K I, Na2S2O8 and Na2S2O3 are mixed according to the following table. The rates of colour change in each solution are shown below. Flask [I –]/ mol dm–3 [S2O8 2–] / mol dm–3 [S2O3 2–] / mol dm–3 rate /s–1 1 0.10 0.10 0.06 0.0222 2 0.10 0.20 0.06 0.0434 3 0.10 0.30 0.06 0.0665 4 0.20 0.15 0.05 0.0685 5 0.20 0.15 0.12 0.0675 6 0.20 0.15 0.18 0.0672 (i) Deduce two sets of variables whose values can be plotted to obtain the order of reaction with respect to either S2O8 2– or S2O3 2–. Plot rate (or relative rate) / s–1 against concentration / moldm–3 (ii) On the grids provided, plot a graph using the variables in (i) and determine the order of reaction with respect to S 2O8 2–. Label the axes and the graph. Graph of [S2O3 2–] against rate Concentration /moldm–3 Rate /s–1 Graph of [S2O8 2–] against rate

© DHS 2011 9647/02/Answers 4 Order of reaction with respect to S2O8 2– 1 (iii) Plot another line on the axes above to determine the order of reaction with respect to S2O3 2–. Order of reaction with respect to S2O3 2– 0 (iv) Using a non–graphical method, deduce the order of reaction with respect to I–. or Let order w.r.t. I– be x. Rate = k [S2O8 2–][I–]x Comparing experiments 2 and 4, x x 20)k(0.15)(0. 10)k(0.20)(0. Rate Rate 4 2  x x 0)(0.15)(0.2 0)(0.20)(0.1 0.0685 0.0434  0.47518=0.5x x =1.07 1 Order of reaction with respect to I– = 1 (Reasoning method) Comparing experiments 1 and 4, When [I–] doubles / increases by 2 times and [S2O8 2–] increases by 1.5 times, while the other concentrations remain constant, rate increases by about 0.0222 0.0685 3 times Since order wrt S2O8 2– is one, order of reaction with respect to I– must be 1 to increase rate by 3 times. (v) Hence or otherwise, write the rate equation and decide if step I could be the rate determining step for the iodine clock reaction. Rate = k [S2O8 2–][I–] No, step I is not the rate determining step. (b) In the determination of the mass percentage of ascorbic acid (C6H8O6) present in vitamin C tablets, the following reaction involving iodine takes place. C6H8O6 + I2  C6H6O6 + 2I– + 2H+ Due to the low solubility of iodine in water, it is generated in situ using potassium

© DHS 2011 9647/02/Answers 5 iodate (V), potassium iodide and sulfuric acid. In this reaction, iodine is the only product. (i) Write a balanced equation for the reaction between IO3 – and I– to obtain I2. 5I– + 6H+ + IO3 –  3I2 + 3H2O Or 5KI + 3H2SO4+ KIO3  3I2 + 3H2O + 3K2SO4 A 3.0 g sample of the tablet is crushed and dissolved in 100.0 cm 3 of dilute sulfuric acid. To this soluti on, 1.08 g of potassium iodate( V) and 4.32 g of potassium iodide is added. A 25.0 cm 3 aliquot is withdrawn and 21.60 cm 3 of 0.1 mol dm–3 thiosulfate is required to titrate with the excess iodine, using starch as an indicator. (ii) Calculate the total amount of iodi ne produced from reacting potassium iodate (V) with potassium iodide. No. of mol of KIO3 = 0.005044 mol No. of mol of KI = 0.02600 mol Since 5I–  IO3 –, IO3 – is the limiting reagent Since IO3 –  3I2, Total amount of iodine produced = 0.005044 x 3 =0.015133 mol = 0.0151 mol (iii) Determine the total amount of excess iodine in the 100 cm3 solution. No. of mol of S2O3 2– = )1.0(1000 60.21 =0.00216 mol in 25 cm3 I2  2S2O3 2– No. of mol of excess I2 in 100 cm3 = )4(2 00216.0 = 0.00432 mol (iv) Hence or otherwise, obtain the mass percentage of ascorbic acid in the sample. No. of mol of iodine reacted with ascorbic acid = 0.015133 – 0.00432 = 0.010813 mol Mass of ascorbic acid = 0.010813 x 176.0 = 1.9030 g Mass perc