GYSS 2024 4E Chemistry Prelims Paper 1&2_Marking Scheme

Guangyang Secondary School Chemistry 6092 Secondary Four Express Preliminary Examination MARK SCHEME Paper 1 Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 D D B C B D A B C C Q11 Q12 Q13 Q14 Q15 Q16 Q17 Q18 Q19 Q20 D B A C A A C D B D Q21 Q22 Q23 Q24 Q25 Q26 Q27 Q28 Q29 Q30 C A C D D D A C A B Q31 Q32 Q33 Q34 Q35 Q36 Q37 Q38 Q39 Q40 D C B C D A B D D C

Paper 2 SECTION A Question Solutions Marks Remarks 1 (a) barium chloride, zinc sulfate 1 both answer for 1m (b) sulfur dioxide 1 (c) silver chloride, zinc carbonate 1 both answer for 1m (d) ammonium carbonate 1 Question Solutions Marks Remarks 2 (a) (i) Labelled arrow of electron highly deflected towards positive plate 1 (ii) Electron is negatively charged with a very small / negligible mass 1 both points 1m (b) (i) Unlike n eutrons, protons are positively- charged and will be repelled by the positively-charged nucleus. 1 both points 1m (ii) No change in chemical properties as the new isotope / atom formed has the same electronic configuration (or same number of valence electrons) as the original element. Reject no change in chemical properties as they are isotopes. 1 award marks even if the term isotope is not used

Question Solutions Marks Remarks 3 (a) Typical: Bonding in the compound is covalent and involves the sharing of electrons which is expected as boron and fluorine are both non-metals. Not typical: The valence electron shell of boron only has 6 valence electrons , instead of a complete octet electronic configuration. 1 1 (b) 1m for correct electronic structure 1m for square brackets and charge number Accept if donation of electron pair includes electron from caesium atom. 2 deduct 1m if key not included (c) Boron trifluoride has a simple molecular structure while caesium tetrafluoroborate has a giant ionic crystal lattice structure. There are weak intermolecular forces of attraction between boron trifluoride molecules, which only require a small amount of energy to overcome. There are strong electrostatic forces of attraction between caesium cations and tetrafluoroborate anions. Thus, a larger amount of energy is required to overcome these forces of attraction within the ionic compound. Hence, boron trifluoride exists as a gas and caesium tetrafluoroborate exists as a solid at rtp. 3 1m: both structures 1m: bonding present in both substances 1m: comparison of energy required

Question Solutions Marks Remarks 4 (a) CaCO3 + H2SO4 → CaSO4 + H2O 2KOH + H2SO4 → K2SO4 + 2H2O no. of moles of KOH = 25.00 1000 × 0.320 = 0.00800 mol mole ratio of KOH : H2SO4 2 : 1 no. of moles of excess H2SO4 = 0.008 ÷ 2 = 0.00400 mol total no. of moles of H2SO4 = 50.00 1000 × 0.200 = 0.0100 mol no. of moles of H2SO4