Chemistry 9701/52 February/March 2019: question paper and mark scheme

Question

The reaction between hydrogen peroxide, H2O2(aq), and iodide ions, I–(aq), takes place in acidic conditions. H2O2(aq) + 2I–(aq) + 2H+(aq) I2(aq) + 2H2O(l) reaction 1 The rate of this reaction can be found by measuring the time taken for a given amount of iodine, I2(aq), to form. This is done by adding a known amount of thiosulfate ions, S2O32–(aq), and allowing the I2(aq) formed in reaction 1 to react with the S2O32–(aq). I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O62–(aq) reaction 2 After the S2O32–(aq) ions have all reacted in reaction 2, any further I2(aq) formed in reaction 1 can be detected using an indicator. A student carried out a series of experiments to determine the order of reaction with respect to the concentration of I–(aq) ions in reaction 1. The student prepared the following solutions. solution A 0.100 mol dm–3 KI(aq) solution B 0.0500 mol dm–3 Na2S2O3(aq) The student also had access to the following chemicals. solution C 0.100 mol dm–3 H2O2(aq) 0.2 mol dm H2SO4(aq) –3 distilled water a suitable indicator (a) The student prepared solution A in a 250 cm3 volumetric flask. (i) The student used a balance accurate to two decimal places and a weighing boat. A weighing boat is a small container used to hold solid samples when they are weighed. Determine the mass, in g, of KI needed to prepare 250.0 cm3 of solution A. [Ar: K, 39.1; I, 126.9] mass = …… g [2] (ii) The student weighed the empty weighing boat. The student then added solid KI to the weighing boat until the mass of KI calculated in (i) was reached. The student transferred all of the KI from the weighing boat into a 100 cm3 beaker. Describe how the student could check that the mass of KI transferred into the 100 cm3 beaker was exactly the same as the mass calculated in (i). . …… . …… [1] (iii) The student dissolved the KI in the 100 cm3 beaker in distilled water and transferred the solution formed into a 250 cm3 volumetric flask. Distilled water was added to the volumetric flask until the volume of the solution was exactly 250 cm3. Care was taken to avoid parallax errors. Describe: ●● how the student should transfer all the KI solution from the beaker into the 250 cm3 volumetric flask ●● how the student should fill the volumetric flask exactly up to the 250 cm3 mark. . …… . …… . …… . …… [2] (b) The student rinsed a burette with solution A before filling it with solution A. Explain why this improves the accuracy of the results. . …… . …… [1] (c) The student was given a solution of 0.400 mol dm–3 Na2S2O3(aq). Determine the volume, in cm3, of this solution that should be added to a 100 cm3 volumetric flask to prepare 100.0 cm3 of solution B. Give your answer to two decimal places. volume = …… cm3 [1] (d) Experiment 1 was carried out using a series of steps. step 1 The student used a measuring cylinder to measure 25 cm3 of 0.2 mol dm–3 H2SO4(aq). This was transferred to a conical flask. step 2 The student added 20.00 cm3 of distilled water from a burette to the conical flask. step 3 The student added 5.00 cm3 of solution A from a burette to the conical flask. step 4 The student added 5.00 cm3 of solution B from a burette to the conical flask. step 5 The student added 1.0 cm3 of indicator from a teat pipette to the conical flask. step 6 The student used a burette to add 10.00 cm3 of solution C to a small beaker. The contents of the beaker were added to the conical flask and a stopclock was started immediately. The stopclock was stopped when the I2 formed caused the indicator to change colour. In Experiments 2–6 the student repeated steps 1–6 but using the volumes of distilled water and solution A given in the table. The student carried out two trials of each experiment. volume of time for the indicator volume of volume of volume of volume of to change colour, t distilled experiment H2SO4(aq) solution A, v solution B indicator /s water / cm3 / cm3 / cm3 / cm3 / cm3 trial 1 trial 2 1 25.0 20.00 5.00 5.00 1.0 218 220 2 25.0 15.00 10.00 5.00 1.0 112 113 3 25.0 12.50 12.50 5.00 1.0 100 4 25.0 10.00 15.00 5.00 1.0 77 76 5 25.0 5.00 20.00 5.00 1.0 59 59 6 25.0 0.00 25.00 5.00 1.0 47 49 (i) In Experiment 3, trial 2, the indicator changed colour as soon as the student added solution C to the conical flask. No results were recorded for Experiment 3, trial 2. Suggest which step the student did not carry out in Experiment 3, trial 2. . …… [1] (ii) Suggest why the results shown in the table could be considered reliable. . …… . …… [1] (iii) What was the percentage error in the burette reading for measuring the volume of solution A in Experiment 5? percentage error = …… % [1] (iv) Suggest why a measuring cylinder was used to measure the volume of H2SO4(aq) rather than a more accurate piece of apparatus, such as a burette. . …… [1] (v) For Experiments 1–6, state: ● the independent variable …… ● the dependent variable. …… [2] (e) The rate equation can be written as rate = k[I–]n where [I–] is proportional to the volume of solution A and n is the order of reaction with respect to I–. (i) Use only the results of Experiments 1–6 given in (d) to complete the table where: ● v is the volume of solution A used in cm3 ● tav is the average time taken in trial 1 and trial 2 in s. Give all values to three significant figures. experiment v / cm3 log v tav / s (1/tav) / s–1 log(1/tav) 1 5.00 2 10.00 3 12.50 100 0.0100 4 15.00 5 20.00 6 25.00 [2] (ii) Rate can be expressed as (1/tav). The rate equation can be expressed as shown. log(1/tav) = nlog v + c where: ● c is a constant ● v is proportional to [I–]. On the grid: ● Plot a graph of log(1/tav) against log v. Use a cross (×) to plot each data point. ● Draw a line of best fit. [2] (iii) Use your graph to determine the gradient of the line of best fit. State the coordinates of both points you used in your calculation. Give the gradient to three significant figures. Determine the order of reaction with respect to I–(aq). co-ordinates 1 …… co-ordinates 2 …… gradient = …… order of reaction with respect to I–(aq) = …… [3] –1.60 –1.70 –1.80 –1.90 –2.00 log (1/tav) –2.10 –2.20 –2.30 –2.40 –2.50 0.6 0.7 0.8 0.9 1.00 1.10 1.20 1.30 1.40 log v [Total: 20]

Question

A student was given a sample of an unknown Group 2 chloride. The student dissolved 3.172 g of the unknown Group 2 chloride in distilled water in a beaker and added an excess of aqueous silver nitrate, AgNO3(aq), to the beaker. A white precipitate of silver chloride formed. (a) Write the ionic equation, including state symbols, for the reaction occurring. . …… [1] (b) To separate the filtrate from the residue, filtration can be carried out using gravity or by using reduced pressure. The student decided to filter the mixture under reduced pressure. (i) Complete the labelled diagram to suggest how the student could filter the mixture under reduced pressure. to suction pump (to reduce the pressure in the conical flask) filtrate [2] (ii) Suggest one major advantage of filtering the mixture under reduced pressure compared with filtering using gravity. . …… [1] (c) The student rinsed the residue, transferred it to a crucible and placed it in a warm oven to dry it. (i) What should the student do to ensure that the drying process is complete? . …… [1] (ii) The student recorded the masses shown in the table. Complete the table to calculate the mass of dry silver chloride formed. Use this value to determine the number of moles of silver chloride formed. [Ar: Cl, 35.5; Ag, 107.9] mass of crucible + dry silver chloride / g 24.898 mass of empty crucible / g 19.162 mass of dry silver chloride / g moles of silver chloride formed = …… mol [1] (iii) Use your answer to (ii) to calculate the mass of one mole of the Group 2 chloride and hence identify the Group 2 metal present in the chloride. If you were unable to calculate an answer in (ii), assume that 0.0304 mol of silver chloride formed. This is not the correct value. [Ar: Be, 9.0; Mg, 24.3; Ca, 40.1; Sr, 87.6; Ba, 137.3] mass of one mole of the Group 2 chloride = …… g identity of the Group 2 metal = …… [3] (iv) State and explain how the number of moles of silver chloride formed in (ii) would change if the student used tap water instead of distilled water to dissolve the Group 2 chloride. . …… . …… [1] [Total: 10]