Copy of SCSS Prelim 2024 Chem P3 MS

1 SWISS COTTAGE SECONDARY SCHOOL SECONDARY 4 CHEMISTRY 6092 PRELIMINARY EXAMINATION 2024 PAPER 3 MARK SCHEME Question Skill MS Mark Total 1(a) PDO MMO all temperature readings recorded to 0.5 C temperature changes correctly calculated (to 1 d.p.) and ‘+’ sign is included full set of data + correct trend (rise in temperature decreases with deceasing volume of P used) temperature rise for first reading is greater than 10 C 1 1 1 1 [4] 1(b) PDO correct axis labels and units appropriate scale + size of plotted area covers at least 50% of the graph paper all points correctly plotted correct line of best fit, allow ecf from wrongly plotted points accept both straight line and curved line as long as the line is the best fit anomalous points must be circled 1 1 1 1 [4] 1(c) ACE rise in temperature decreases as the volume of P decreases / rise in temperature increases as the volume of P increases greater volume of P used means greater number of moles of aci d to react with the magnesium, leading to more energy given off 1 1 [2] 1(d) ACE volume of P = 0.0065 1.000 = 6.5 cm3 correct calculation and unit temperature change correctly read with unit and shown clearly on the graph 1 1 [2] 1(e) ACE Answer 1 His prediction was correct. As the amount of magnesium ribbon that reacted with the acid doubled, the amount of energy produced would also be doubled. Thus, the rise in temperature also doubled. The prediction is only correct if it is assumed that the acid is not used up in the reaction. / there is no heat loss to the surroundings. / the pieces of magnesium ribbon have exactly the same length. 1 1 [2]

2 Answer 2 His prediction was not correct. Although the amount of magnesium ribbon added to the acid doubled, there may not be sufficient acid to react completely with the magnesium. / the acid could be the limiting reagent. / the magnesium ribbon s could be added in excess. Thus, amount of energy produced may not be doubled. Thus, the rise in temperature may not be doubled. He assumed that there is excess acid present to react completely with the magnesium ribbons. / the acid is not used up in the reaction. / magnesium ribbons reacted completely with the acid. R: the pieces of magnesium ribbon have exactly the same length. 1 1 1(f) MMO ACE any two observations with relevant explanation observation: Effervescence is seen. / Bubbles are seen. explanation: The reaction produces hydrogen gas. observation: Magnesium dissolves to form a colourless solution. explanation: Soluble magnesium chloride is formed. 1 1 1 1 max [4] 1(g) ACE error: A measuring cylinder was used to measure the volumes of P and water. This was not accurate / precise as it can only measure up to 0.1 cm3. improvement: A burette should be used instead to measure the volumes. A burette can measure up to 0.05 cm3. error: The temperature of the solution was measured using a laboratory thermometer which was not accurate / precise. It can only measure up to 0.5 C. improvement: A temperature sensor and data logger should be used instead. It can give temperature readings up to 0.01 C. error: Each experiment (Experiments 1 to 5) was done only once. Hence, the temperature readings taken were not reliable. improvement: Each experiment (Experiments 1 to 5) should be repeated twice. The average temperature change can then be calculated for greater reliability. error: The measuring cylinder w as washed after each use. As it still contained a very small amount of water, the actual volume of the acid used would be lower than it should be. improvement: Use a dry measuring cylinder for each experiment. 1 1 1 1 1 1 1 1 max [2] Total: 20

3 2(a)(i) PDO MMO Results table: • Correct headings (initial burette reading, final burette reading and volume of R added) and units • Initial reading is plausible e.g. not 50.00 cm3 • Correct calculations to get volume R • All burette readings recorded to nearest 0.05 cm3 Titration Results: Supervisor’s reading = 21.00 cm3 • Accuracy: If average titre is within 0.20 cm3 of Supervisor’s average value scores 2 marks If Average titre is within 0.30 cm3 of Supervisor’s average value scores 1 mark • Concordance: At least two titre values are within 0.20 cm3 Allow ecf from calculation error 1 1 2 1 [5] 2(a)(ii) MMO Average volume of R in 2 d.p. (titre s used should be identified by a tick in the table or in a calculation) Unit must be correct. 1 [1] 2(b)(i) ACE no. of mol of aqueous sodium thiosulfate = 0.0500 x (average volume of R/1000) 1 [1] 2(b)(ii) ACE no. of moles iodine = 1 2 x no. of moles of aqueous sodium thiosulfate 1 [1] 2(b)(iii) ACE no. of mol of chlorate(I) ions in 25.0 cm3 solution Q = no. of moles of iodine 1 [1] 2(b)(iv) ACE PDO no. of mol of chlorate(I) ions in 1000 cm3 of solution Q = no. of moles of chlorate(I) ions in 25.0 cm3 of solution Q x 1000 25 no. of mol of chlorate(I) ions in 100 cm3 commercial bleach = no. of mol of chlorate(I) ions in 1000 cm3 of solution Q concentration of chlorate(I) ions in commercial bleach = no. of mol of chlorate(I) ions in 100 cm3 commercial bleach x 1000 100 Statement to describe calculation is insisted, because calculation involves 2 steps. Students must describe clearly which concentration they are calculating. appropriate significant figures (3 s.f.) in final answers in (b)(i) – (iv) appropriate units in final answer in (b)(iv) mol/dm3 ‘mol’ not insisted for (b)(i) – (iii) 1 1 1 1 [4]

4 2(c)(i) MMO Colourless solution turned yellowish-brown. / brown. / yellow. / dark brown. Effervescence was seen. The gas relighted a glowing splint. The gas is oxygen. 1 1 [2] 2(c)(ii) ACE Solution T contains an oxidising agent. Awarded only if correct observation ( yellowish-brown / brown solution) is recorded in 2(c)(i). 1 [1] 3 P Answer 1 [2]: (P) Procedure - include apparatus and measurements taken • measurement of mass using electronic balance before and after heating • heat, cool and measure mass of residue until constant mass is achieved • Measure the mass of each metal carbonate using an electronic balance. • Heat each carbonate strongly. Cool the residue and measure the mass using an electronic balance. • Continue heating, cooling and measuring the mass of the residue until constant mass is achieved. [2]: (T) Treatment of results of this experiment • calculation of the no. of moles of metal carbonate and metal oxide correctly described • ratio of no. of moles of metal carbonate to no. of moles of metal oxide correctly described • show clearly how the relative atomic mass es of X and Y can be compared Let, mass of XCO3 = mx mass of YCO3 = my After heating, the residue left behind is metal oxide. Let, mass of residue XO = mx’ mass of residue YO = my’ 1 mole of metal carbonate produces 1 mole of metal oxide. Let, Ar of X = x Ar of Y = y mx = mx’ x + 12 + 3(16) x + 16 Thus, the value of x is determined. my = my’ y + 12 + 3(16) y +16 1 1 1 1 [4]

5 Thus, the value of y is determined. Comparison of the calculated values of x and y will tell us which metal has a higher relative atomic mass. Alternative calculation Let, mass of XCO3 = mx mass of YCO3 = my After heating, the residue left behind is metal oxide. Let, mass of residue XO = mx’ mass of residue YO = my’