plmgs p3 p4 answer key

PLMGSS Marking Scheme for 4NA Science Chemistry Prelim Exams 2024 Paper 3 (20 MCQs) 1 2 3 4 5 6 7 8 9 10 B D C B B D A A A B 11 12 13 14 15 16 17 18 19 20 B A D B D D C A C D Paper 4 Section A (22 marks) Qn Answers Marks 1(a)(i) Q Marker’s Comments: Most students could answer this. 1 1(a)(iii) Q and T Marker’s Comments: Most students could answer this. 1 1(c)(i) Marker’s Comments: Students need to use different symbols for the electrons of R and S. Students also need to draw all the electrons, and not just the valence electrons. 1m given if (1) one of the ions is correct, (2) both charges are correct but electronic structures are wrong, (3) R gains one electron and S loses one electron, but missing charges, (4) only valence electrons are drawn. 1; cation 1; anion 1(c)(ii) Any 2 points: Ionic compound/ ionic bonding that exists as a giant ionic crystal lattice It has strong electrostatic forces of attraction between the oppositely charged ions. A large amount of energy is needed to overcome the forces of attraction. Marker’s Comments: Key words in bold are needed to get the marks. More energy is not accepted – need to say large amount of energy is needed. 1; 1; 1 2(a) Chlorine is more reactive than bromine and hence chlorine displaces bromine from its aqueous salt solution, sodium bromide, to form a reddish-brown solution. 1

Marker’s Comments: Not well done. Students could not identify that this is a displacement reaction. 2(b) Cl2 + 2NaBr → 2NaCl + Br2 Marker’s Comments: Not well done. Students forgot that chlorine and bromine exist as diatomic molecules, and forgot the equation for displacement reaction. 1 3(a) Ethanol; propanol; butanol Marker’s Comments: Some students gave the sequence from distil last to distil first. 1 3(b) It provides a large surface area for the vapour to condense on before it reaches its boiling point. Marker’s Comments: Students need to state that there is a large surface for condensation, not just to improve condensation. 1 3(c) Direction of water in/out in the condenser is reversed. Marker’s Comments: Accepted – there should be boiling chips. 1 4(a) Molar mass of Na2CO3 = 2(23)+12+3(16) = 106 g/mol Number of moles of Na2CO3 = 265/106 = 2.50 mol 1; 1 4(b) Mass of water = 2.5 x (2+16) = 45 g 1 Marker’s Comments: Most students did not know to/how to calculate the molar mass of Na2CO3 or H2O. Most were unable to correctly manipulate/ use the formula no. of mol = mass/molar mass, or quoted a wrong formula (such as that involving the number of particles/ Avogadro’s constant).

5(a) All points correctly plotted Straight line passing through all the points Marker’s Comments: Students are reminded to check their plotted points. 1; 1 5(b) –32 or –30 °C [read from student’s graph, allow ECF] Marker’s Comments: A h