2022 MI H2 Chem Prelim Paper 4 - Mark Scheme

2022 PU3 H2 Chemistry EOY Paper 4 Mark Scheme Question Answer Marks 1 (a) All masses consistently recorded to 2 (or 3) decimal places. and correct mass headers Mass of bottle + FA 1 Mass of bottle + residual of FA 1 Mass of FA 1 Mass of empty bottle Mass of bottle + FA 1 Mass of FA 1 *Inform students to be always reminded to weigh residual mass M1 Mass of FA 1 used correctly calculated Mass of FA 1 used 5.00 ±0.05 M2 (b)(i) A table including the appropriate header and units for: • Initial burette readings • Final burette readings • volume added M3 All accurate burette readings are recorded to the nearest 0.05 cm3. Do not award this mark if: 50.00cm3 is used as an initial burette reading M4 Accuracy Titre/mass ratio: 4.05 ± 0.50 M5 (b)(ii) Correct calculation of the average of two consistent readings within 0.10 cm3 difference M6 (c)(i) amount of I2 = 0.5 (ans from b(ii)/1000) x 0.10 = A mol M7 (ii) amount of Cu2+ in 25.0 cm3 = A × 2 = B mol M8 (iii) amount of CuSO4.nH2O in 250 cm3 = 10 × B = C mol M9 (iv) Mr = mass of FA 1 / C = D Must be 1dp, will affect M27 M10 n = (D -159.6) / 18.0 n must be an integer M11 (d) Due to the slow release of I3- from the starch-I3- complex, the end point would have exceeded the equivalence point, resulting in a larger titre value. This leads to larger amount of CuSO4.nH2O hence the Mr would have been smaller than the actual value. OR The I2 is trapped by the starch complex and hence resulting in less I3- a smaller titre value This leads to a smaller amount of CuSO4 and hence Mr will be larger M12 M13

2 Question Answer Marks 2(a) Table with correct headers and units • volumes of FA 5, FA 6 and deionised water • time • 1/t • lg (1/t) • lg (VFA 5) M14 Record all data with correct precision ● volumes to 0.5 cm3 ● t to nearest second ● calculated values to 3 sf M15 Complete set of volume of FA 5 and time readings for 5 experiments and all values of t increase as volume of FA 5 decreases. Data for experiments 1 and 2 must be included M16 Choose 3 other well-spaced values for the volume of FA 5. All volumes must differ by at least 5 cm3 M17 Correctly calculate 1/t, lg (1/t) and lg (VFA 5) values for all stated experiments. M18 (b)(i) Axes correct way round, with correct labels and appropriate scale. Scale must be chosen so that plotted points occupy at least half the graph grid in both x and y directions Both accepted, must be in logical numerical sense M19 All plotted points correct to within ± ½ small square M20 A best fit line is drawn with anomalous points excluded M21 (b)(ii) Correct calculation of gradient of the graph with working shown. The m value is equal to the gradient of the line Must show accurate coordinates falling on the line or triangle or best both M22 (c) ● Correct calculation of lg(1/15) = –1.18 ● Correct reading of lg( VFA 5) value from graph to within ±½ small square. ● Correct calculation of volume of FA 5 using value for lg(VFA 5) [2] for all 3 points correct [1] for 2 points correct M23 M24 (d) Selects Experiment 1 and explains that time, t, is smallest value and so has greatest % error Says that reaction time is fastest and hence it is the most significant OR M25

3 [Turn over Selects Experiment 2 and explains that volume of FA 5 used (20 cm3) is smallest and so has greatest % error (e) amount of H+ = 0.05 × 2 × 40 × 10–3 = 0.00400 mol [H+] = 0.00400 / (65/1000) = 0.0615 mol dm–3 M26 Show correct units and appropriate significant figures in all final answers in 1(c), 2(e) Penalise only if answer is correct M27 Show all relevant workings in 1(c), 2(e) M28 deducted for any blank questions M28 (f)(i) The value of t will be lower. This is because the [H +] is higher than expected, so rate of reaction is faster. M29 (ii) Measuring volumes of FA 4, FA 6 and deionized water separately and mixing to make solution 1 M30 Question Answer Marks 3(a) • presence of partially filled d-orbitals • splitting of d orbitals into 2 groups with a small energy gap • absorbs light energy for electrons from lower energy d orbital to be promoted to higher energy d orbital • light not absorbed is seen as the colour of the complex 2m - all 3 points 1m – any correct 2 points M31 M32 (b) violet M33 (c) • Calculation of vol of stock solution needed: Vol of stock solution needed = 100×2 5 = 40 cm3 • Procedure for preparation of standard solution of 2.00 mol dm- 3 1. Fill the burette with the stock solution of 5.00 mol dm -3 [Ni(en)3]2+ 2. Run 40.00 cm3 of the solution into a 100 cm3 volumetric flask 3. Make up to the mark with deionised water 4. Stopper and shake the volumetric flask to obtain a homogeneous solution. Label this solution P. • Preparation of a suitable range of diluted solutions 1. Fill the burette with solution P 2. Run 15.00 cm3 of P into a 100 cm3 beaker. 3. Using another burette, transfer 5.00 cm 3 of deionised water into the same beaker. Stir the solution with a glass rod to obtain a homogeneous solution. 4. Repeat steps 2 and 3 with different volumes of solution P and deionised water as shown in the table below (Expt 3 to 5). Expt Volume of Volume of Conc of [Ni(en)3]2+ Absorbance / A

4 solution P/ cm3 deionised water /cm3 prepared / mol dm-3 1 20.00 0.00 2.00 2 15.00 5.00 1.50 3 10.00 10.00 1.00 4 5.00 15.00 0.50 5 0.00 20.00 0.00 • Outline of how the results would be obtained 1. Use the spectrometer to measure the absorbance of each of the solutions of different concentrations prepared and record the absorbance value. 2. Plot a graph of the absorbance against concentration and draw a best-fit line. This is the calibration line. • Determining concentration of solution X 1. Use the spectrometer to measure and rec ord the absorbance of [Ni(en)3]2+ in solution X 2. Using the calibration line drawn, read off the concentration of [Ni( en)3]2+ corresponding to the absorbance. 1. correct volume of stock solution used to prepare the standard solution. M34 2. correct procedure for preparing the standard solution, including top up to the mark, stopper and shake M35 3. use of suitable apparatus with stated capacity for the preparation of standard solution o burette for measuring out 40.00 cm3 stock solution o 100 cm3 volumetric flask M36 4. correct dilution concept for preparation of range of diluted solutions using 2.00 mol dm-3 solution M37 5. select a suitable range of concentrations for the diluted solutions (at least 3 more solutions with concentrations not less than 0.3 mol dm-3 apart) M38 6. measurement of absorbance value using the diluted solutions M39 7. plotting of graph (mention axis) M40 8. sketch of the calibration line graph (straight line graph passing through the origin) (if sketch is correct and with appropriate axes, M40+M41) M41 9. description of use of calibration line to determine concentration of [Ni(en)3]2+ in solution X M42

5 [Turn over 4 observations with FA 8 observations with FA 9 (a)(i) Add 1 cm 3 of dilute hydrochloric acid to ½ spatula of the solid sample in a test tube. Blue / Blue -green / green solution is observed. Cream / Off-white ppt is observed (optional) Yellow ppt / solid is formed on warming. (No need to test for SO2) FYI: (ii) Add 2 cm3 of silver nitrate solution to ½ spatula of the solid sample in a test tube. If need be, filter the resultant mixtures. White ppt in blue solutio n formed. A white / yellow ppt and eventually forms black / brown ppt. Do not accept red-brown ppt Do not accept brown solution Can accept eventually brown ppt (iii) Add 1 cm 3 of potassium iodide solution to ½ spatula of the solid sample in a t