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 reacted with CaCO3 = 0.0100 – 0.00400 = 0.00600 mol mole ratio of H2SO4 : CaCO3 1 : 1 no. of moles of CaCO3 = 0.00600 mol mass of CaCO3 = 0.00600 × (40 + 12 + 48) = 0.6 g % purity of CaCO3 = 0.6 0.835 ×100% = 71.9 % (3sf) 1 1 1 1 1m for excess H2SO4 that reacted with KOH 1m for amount of H2SO4 that reacted with CaCO3 1m for mass of CaCO3 1m for percentage purity of CaCO3 (b) (i) Point Q was the equivalence point where nitric acid was completely neutralised by sodium hydroxide. At R, sodium hydroxide is in excess. The presence of free moving Na+ and OH– ions from excess sodium hydroxide act as mobile charge carriers to conduct electricity. Thus, electrical conductivity of solution increases. 1 To state the presence of ions from EXCESS sodium hydroxide

(ii) Similar to curve for experiment, except for an initial pH value of 4. Same volume of NaOH used and same height at end of reaction. 1 (iii) As ethanoic acid is a weak acid, it ionises partially in water to produce hydrogen ions. Nitric acid, on the other hand, is a strong acid which ionises completely to produce hydrogen ions. Since ethanoic acid has a lower concentration of hydrogen ions, it has a higher pH 1 1 1m for difference in strength of acids 1m pH value to concentration of hydrogen ions Question Solutions Marks Remarks 5 (a) Iron(II) ion is a catalyst as it is chemically unchanged in the overall reaction / is regenerated at the end of step 2. 1 (b) (i) Aqueous sodium hydroxide could be added to the mixture of i ron(II) ion s and aqueous iodine, to form insoluble i ron(II) hydroxide. Aqueous sodium hydroxide is added in excess until no further change is seen. Filtration can be carried out to remove the solid from the mixture. 1 1 Accept use of aqueous ammonia 1m not awarded if reagent is not added in excess (ii) A black solid is formed and the solution turns into a lighter shade of brown. Iodine is more reactive than astatine, and will displace astatine from sodium astatide solution. 1 1 No mark awarded if explanation is incorrect.

Question Solutions Marks Remarks 6 (a) (i) Z, W, X, Y 1 (ii) 3X (s) + 2W(NO3)3 (aq) → 3X(NO3)2 (aq) + 2W (s) 1 (b) Zinc 1 (c) (i) No gas is produced for metal Z. Metal Z, which is less reactive than copper, must an unreactive metal and does not react with steam. X is more reactive than W and will produce more gas than W during the fixed time interval. 1 1 (ii) Fused/anhydrous calcium chloride 1 (d) Metal V is below Y in the Periodic Table. Since metal V is more reactive than Y, the electrostatic forces of attraction between the nucleus and valence electrons for V must be weaker. This implies that the valence electron is further from the positively charged nucleus, and V has more electron shells than Y. 1 1 1m on linking reactivity to forces of attraction 1m on concluding the no. of electron shells

Question Solutions Marks Remarks 7 (a) (i) Carbon monoxide and oxides of nitrogen react with each other to form nitrogen gas and carbon dioxide through redox reactions. Unburnt hydrocarbons combust / burn / react with oxygen in air to form carbon dioxide and water vapour 1 1 type of reaction and products must be stated clearly for mark to be awarded (ii) When the temperature of the car engine is higher, nitrogen gas will react with the oxygen present in the air to form more oxides of nitrogen. 1 (b) Ammonia is oxidised as the oxidation state of nitrogen increases from –3 in NH3 to 0 in N2 Nitrogen dioxide is reduced as the oxidation state of nitrogen decreases from +4 in NO2 to 0 in N2. Since oxidation and reduction occurs simultaneously, this is a redox reaction. 1 1 Deduct 1m if last sentence on both oxidation and reduction occurring is not stated

Question Solutions Marks Remarks 8 (a) C2H2 1 (b) (i) 2C4H6 + 11O2 → 8CO2 + 6H2O accept C 4H6 + 11 2 O2 → 4CO2 + 3H 2O but must be in mixed fractions 1 (ii) Add a few drops of aqueous bromine to separate test-tubes of butyne and butane. Butyne: Aqueous bromine decolourises Butane: Aqueous bromine remains brown. 1 Test and results for both butyne and butane must be included for 1m (iii) Addition reaction OR 1 1 or any structure that makes sense (such as double addition where only single bonds and 2 hydroxyl groups and present) (c) 1 (d) No, the bromine atoms are not on adjacent carbon atoms and cannot be broken to from an alkyne. 1

(e) 1 1

Question Solutions Marks Remarks 9 (a) Experiment 1: anode: 4OH– (aq) → 2H2O (l) + O2 (g) + 4e– cathode: Ag+ (aq) + e– → Ag (s) Observations & Explanations: Effervescence observed at the platinum electrod