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 pr