2024 XMS SEC4 WA1 MS

1 SEC 4E CHEMISTRY 6092 TERM 1 WA1 MARK SCHEME Question 1 2 3 4 5 Answer B C C A B SECTION B [20 marks] 6 (a) titration first second third final reading / cm3 25.80 47.00 32.30 initial reading / cm3 0.00 21.80 6.90 volume of acid used / cm3 25.80 25.20 25.40 best results (✓) ✓ ✓ Deduct (1) for 1 d.p. average volume of acid used = 25.30 cm3 1 1 1 (b) Moles of HNO3 = C × V = 0.095 × 0.0253 = 0.0024 Moles of B(OH)2 = 0.0024 / 2 = 0.0012 1 1 (c) 0.0012 × 10 = 0.012 mol 1 (d) Mass of B(OH)2 = 10.94 – 8.89 = 2.05 g Mr of B(OH)2 = 2.05 / 0.012 = 171 1 1 (e) Ar of B = 171 – 17(2) = 137 B is barium 1 1 10 7 (a) (i) Mass difference between readings decreased during reaction shows that the rate of reaction decreased with time ALLOW: a mathematical calculation of rate per minute NOTE: Mass lost increases with time, but the rate of mass lost decreases with time 1 (ii) Total mass lost = Total mass of CO2 produced = 1.10 g No. of moles of CO2 produced = 025.0=44 10.1 mol No. of moles of CaCO3 = 0.025 mol (1:1) Mass of CaCO3 = 0.025  100 = 2.5 g % by mass of calcium carbonate in the marble sample = %8.20=%100×12 5.2 1 1 1 (b) (i) Experiments 4 and 5 show that varying t he volume of the acid does not affect the reaction time. 1 (ii) More reacting particles have the activation energy / Reacting particles move faster in experiment 3 than 2. Particles collide more frequently / More (effective) collisions per unit time HCl is a monobasic acid / produces 1 H+ ion per acid molecule H2SO4 is a dibasic acid / produces 2 H+ ions per acid molecule ALLOW: H2SO4 produces more H+ ions than HCl More H+ ions per unit volume / Higher concentration of H+ ions in experiment 4 (or 5) than experiment 3. Particles collide more frequently / More (effective) collisions per unit time 1 1 1 1 1 10

2 SECTION C [10 marks] 8 (a) Any three: (MUST quote figures from graph) supply rate is a maximum at 3.6 dm3/min for the 1st minute supply rate decreases steadily to 1.2 dm3/min after 3.5 minutes supply rate decreases further to 0.7 dm3/min after 5 minutes supply rate remains constant at 0.7 dm3/min for next 6 minutes 3 (b) (i) average rate of reaction = gradient = (32 – 24) / 5 = 1.6 (ALLOW 1.5 – 1.7) 1 (ii) moles of NaN3 = 130 / 65 = 2 (1) moles of N2 formed in reaction 1 = 3 (1) moles of Na formed in reaction 1 = 2 moles of N2 formed in reaction 2 = 0.2 total moles of N2 = 3 + 0.2 = 3.2 (1) total volume of N2 = 3.2 × 24 = 76.8 dm3 (1) 4 (c) Any two: (ALLOW other reasonable responses) Both involve decomposition (of solid reactants) Both require a high temperature to start the reaction Both involve irreversible reactions Both involve very fast reactions Both produce harmful / dangerous side products 2 10