MI 2020 Prelim Paper 4 mark scheme

2020 PU3 H2 MI Prelim P4 Mark Scheme 1(a) M1 Construct a table for 5 results and Correct headings and units for volumes of FA1, distilled water and time. M2 All times recorded to the nearest second and Volumes of FA1 and distilled water to nearest 0.05 cm3 M3 Three further experiments chosen with intervals not less than 1 cm3 and no volume less than 6 cm3. M4 Water added to make total volume of FA 1 and water constant in each experiment and no other changes in volume. M5 Time increases with decrease in volume FA 1. 1(b)(i) M6 Amount of S2O32– = 20 1000 x 0.010 = 0.000200 mol 1(b)(ii) M7 Amount of H2O2 = 0.000100 mol 1(b)(iii) M8 Change in concentration of H2O2 = 0.000100 65 1000 = 0.00154 mol dm-3 1(b)(iv) M9 Rates correctly calculated using 0.00154 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 × 105 Values in 3 sig fig Units for rate in mol dm−3 s−1 1(c) M10 M11 M12 Axes labelled – rate/ mol dm-3 s-1 on y-axis and volume of FA 1 / cm3 on x-axis. Scales to use at least half of each axis Correct plotting – all points recorded plotted and within half a small square and within correct small square. Draws a line of best fit (can be straight line or curve). Straight lines must be straight (single line with no kinks, drawn using a ruler) or a smooth curve (gradual change in gradient). 1(d) M13 Analyse the order based on the shape of graph. Straight line: 1st order 1(e) M14 To ensure an equal amount of iodine (or H2O2) has reacted for each experiment before the solution turns dark blue in each experiment. 1(f) M15 States that excess I2 left in the flask will react with the S2O32- in the new experiment. As there is lesser S2O32- for the new experiment, less I2 is reacted, the time taken to produce the I2 will be shorter in the second experiment and hence observe the blue colour faster.

1(g)(i) M16 M17 M18 Preparation of the 100 cm3 of 1.00 mol dm-3 aqueous iodine standard solution Amount of I2 needed = (100/1000) x 1 = 0.100 Mass of I2 needed = 0.100 x (126.9 x 2) = 25.38 g 1. Weigh accurately about 25.38 g of I2 using a clean, dry weighing bottle using a mass balance by recording the mass of the I2 and weighing bottle as shown in the table below. Mass reading table Mass of weighing bottle with I2 / g x Mass of weighing bottle with residual mass / g y Mass of I2 added/ g x - y 2. Transfer the weighed solid into a 100 cm3 beaker and dissolve it using distilled water and stir. 3. Transfer the solution and washings into a 100 cm3 volumetric flask. 4. Reweigh the empty weighing bottle and record its mass. 5. Top up to the mark with distilled water. Stopper, invert and shake well to obtain a homogeneous solution. Determination of the correct mass of I2 Correct procedure for weighing Correct procedure of preparation of standard solution 1(g)(ii) 5 different solutions with different concentrations of I2 (aq) were prepared, as shown in the following table: Solution Volume of 1.0 moldm-3 of I2 (aq) / cm3 Volume of deionized water / cm3 Total volume / cm3 [I2] / mol dm-3 Absorbance value 1 20.00 0.00 20.00 1.000 2 10.00 10.00 20.00 0.500 3 5.00 15.00 20.00 0.250 4 2.50 17.50 20.00 0.125 5 1.00 19.00 20.00 0.050 1. Fill the 50cm3 burette with 1.00 mol dm-3 I2 standard solution. 2. To prepare 0.500 mol dm-3 of I2 (aq), add 10.00 cm3 of 1.00 mol dm-3 I2 standard solution into a 50 cm3 beaker. 3. Using a separate burette, add 10.00 cm 3 of deionised water into the beaker and stir using a glass rod to obtain a homogenous solution.

M19 M20 4. Repeat the above steps to obtain diluted solutions of different concentrations using the volumes in the table. Prepare at least 3 other solutions of different concentrations with correct calculations Correct procedure with appropriate apparatus 1(g)(iii) Table of results Solution [I2] / mol dm-3 Absorbance value 1 1.000 1.96 x 104 2 0.500 9.80 x 103 3 0.250 4.90 x 103 4 0.125 2.45 x 103 5 0.05 9.80 x 102 Using the spectrometer 1. Add solution 1 into the 1 cm wide glass cuvette and run the spectrometer to record the absorbance value. Repeat for solutions 2-5. 2. Plot a graph of absorbance value versus [I2]. 3. Draw the best -fit straight line passing through the origin. This is the calibration line. Analysing solution FA5 1. Add FA 5 into the 1 cm wide glass cuvette and run the spectrometer. Record the absorbance value, Ax. Absorbance [I2] moldm-3 0 0.025 0.5 1.0

M21 M22 M23 M24 2. Using the graph drawn earlier, draw a horizontal line at value Ax to intersect the calibration line. By drawing a vertical line down from the intersection point, [I2] in FA 5 can be determined. Correct calculation of the absorbance value Correct sketch of calibration line Outline of how to obtain the calibration line Appropriate procedure to determine the unknown concentration of the aqueous iodine solution 1(h) M25 3000 = (1.96 X 104)(c)(1) c = 0.153 mol dm-3 1(i) M26 M27 Toxic vapour since iodine sublimes easily Carry out in fumehood or keep away from high temperature 2(a) M28 Tabulates initial and final burette readings and volume of FA 7 used. Table has correct headers and units. All accurate burette readings rounded to the nearest 0.05 cm3. M29 Has at least two uncorrected titres within 0.10 cm3. (i.e. consistent) Titres labelled “rough” may be included. M30 Check all subtractions in (a). Use the titres, corrected where necessary, to select the “best average” titre to be used as an accuracy standard using the following hierarchy. • value of 2 identical titres • average of titres within 0.05 cm3 • average of titres within 0.10 cm3, etc. Do not round calculated averages to nearest 0.05 cm3. Compare the calculated average to the supervisor’s value (22.05 cm 3). If the difference is < 0.40 cm3, award this mark. (b) M31 Select and calculate correct “average” from titre values within 0.10 cm3. (c)(i) M32 Correctly identifying the types of reaction as acid-base neutralization and nucleophilic substitution. (c)(ii) M33 Correctly calculates amount of HCl = vol of FA 7 from (b) × 0.100 1000 And Amount of KOH is the same. (c)(iii) M34 Correctly calculates amount of KOH added to P = 0.40 × 0.250 = 0.10

(c)(iv) M35 Correctly calculates amount of KOH remaining = answer in (c)(ii) × 10 (c)(v) M36 Correctly calculates amount of KOH reacted with P = answer in (c)(iii) – answer in (c)(iv) And Amount of P = answer ÷ 2 (c)(vi) M37 Correctly calculates Mr of P = 4.50 ÷ second answer to (c)(v) (c)(vii) M38 Expression to show 59 + Ar of X = answer in (c)(vi) M39 Identification of X as halogen with nearest Ar calculated. (d) M40 Explain that Y has p-orbital overlap with the π-electron cloud of the benzene ring, causing the C-Y bond to be stronger hence unable to react with hot aqueous KOH. 3(a)(i) M41 +Na2CO3 : effervescence (of a gas that forms white ppt with limewater) M42 +KMnO4 : purple to colourless M43 + AgNO3 : no ppt M44 + Tollens’ : silver mirror / grey ppt (a)(ii) M45 Correctly identify functional groups as carboxylic acid And either one of Alkene / aromatic alkyl side-chain / 1°/2° alcohol / aldehyde (b)(i) M46 Describe the test: add aq NH3 dropwise until in excess M47 Describe the observation: White ppt formed dissolves in excess NH3(aq) M48 Correctly identify the cation in FA 9 as Zn2+ (ECF from M47) (b) (ii) M49 FA 9 : purple solution remains FA 10 : purple solution turns colourless M50 FA 9 : no ppt formed FA 10 : white ppt formed M51 FA 9 : white ppt formed (soluble in NaOH(aq)) FA 10 : no white ppt formed M52 FA 9 : gas evolve