PRSS Sc(Chem) Paper 1/3 AK

PASIR RIS SECONDARY SCHOOL 2025 Preliminary Examination SECONDARY 4 EXPRESS / 5 NORMAL ACADEMIC SCIENCE (CHEMISTRY) 5086 / 5088 MARKING SCHEME MCQ Question 21 22 23 24 25 26 27 28 29 30 Answer C D D D C B C A A B Question 31 32 33 34 35 36 37 38 39 40 Answer C C C D D B B A A B SECTION A 1 ai C 1m aii B 1m aiii E 1m aiv A 1m b number of electrons number of neutrons number of protons 1634𝑆𝑆 16 18 16 818𝑂𝑂 2– 10 10 8 Many students did not manage to get the correct number of electrons for the oxide ion. These students often assume that number of electrons is the same as the number of protons (8) or did a subtraction to end up with 6 instead. 4m 2 a fractional distillation Some students gave incomplete answers citing ‘distillation’ without the word ‘fractional’ or identified incorrectly the method as ‘simple distillation’ instead. 1m b A: to measure the temperature of the vapour Many students wrote “measure the temperature” without specifying what temperature are they measuring. This answer is ambiguous as there are students who gave incorrect ideas such as “measure temperature of solution”. B: to (cool and) condense the vapour into the distillate 1m 1m

This part of the question was attempted. c the alcohols are flammable / may catch fire easily This question was poorly attempted. Many students gave incorrect ideas such as electric heater not being able to reach the boiling points of the alcohols, or that heating with the Bunsen burner may break the glass of the round-bottomed flask. 1m d the boiling point of water is lower than that of butanol OR the boiling point of butanol is higher than that of water OR the water would have boiled off Many students gave vague / incomplete answers e.g. indicating that the boiling points of butanol and pentanol is high but not making reference to the water. The key idea here is that water (in the water bath) would have boiled but the boiling points of butanol and pentanol is still not reached yet, and thus cannot be separated. 1m 3 ai Neutralisation Well done 1m aii H+(aq) + OH–(aq) → H2O(l) correct chemical formula correct state symbols Poorly attempted 1m 1m bi exothermic 30 cm3 Generally well done 1m 1m bii Reaction is already completed / All hydrochloric acid has been reacted when 30 cm3 of aqueous NaOH is added. Not enough HCl to react with NaOH suggesting HCl is the LR [1] Limiting reagent is used up - not specific [0.5] Rej: there is excess NaOH unreacted causing the temp to decrease 1m ci no. of moles of hydrochloric acid = 1.5 × ( 25 1000) = 0.0375 mol Poorly attempted 1m cii no. of moles sodium hydroxide no. of moles of hydrochloric acid = 0.0375 mol Poorly attempted Allow ecf 1m ciii concentration of sodium hydroxide = 0.0375 / ( 30 1000) = 1.25 mol/dm3 Allow ecf 1m

Some students did not use the volume of NaOH from the graph, used 25 cm3 as the volume 4 a i. copper / Cu ii. copper(II) oxide / CuO iii. hydrogen gas / H2 iv. copper(II) nitrate / Cu(NO3)2 Poorly attempted Rej: Copper(II) for i, No roman numeral for compound 1m 1m 1m 1m b CuCO3(s) → CuO(s) + CO2(g) OR CuO(s) + H2(g) → Cu(s) + H2O(l) OR CuO(s) + 2HNO3(aq) → Cu(NO3)2(aq) + H2O(l) OR Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s) correct chemical formula correct coefficients correct state symbols Poorly attempted Allow ecf 1m 1m 1m c dark blue / deep blue Only emphasise on colour 1m 5 ai All the sulfuric acid has been used up / reacted away, hence no more new gas produced. Rej: Some students did not specify the limiting reactant as sulfuric acid 1m aii 3 minutes Well done 1m

aiii steeper gradient than original graph plateau earlier at same volume (38 cm3) Some students missed this qn but generally well done 1m 1m b Zinc carbonate powder is smaller in size than the small pieces of zinc carbonate. There is larger (total) surface area (to volume ratio) of contact between reactant particles / more surface area exposed for reactant particles to collide in zinc carbonate powder. The particles in zinc carbonate powder will collide more frequently and increases the frequency of effective collisions. Hence, the rate of reaction increases. Phrasing of answers missing key terms e.g. frequency of effective collisions and the use of comparative terms e.g. more/larger SA Missing rate of reaction increases -0.5 1m 1m ci H+ Not very well done. Students end up giving hydroxide instead of hydrogen ion 1m cii colourless to red Poorly attempted. Students did not realise that the colour change should be in the solution not the Universal indicator. 1m 6 ai ZnO is reduced as the oxidation state of zinc decreased from +2 in ZnO to 0 in Zn. CO is oxidised as the oxidation state of carbon increased +2 in CO to +4 in CO2. Since oxidation and reduction occurred at the same time, the reaction is redox. Some students have difficulty explaining in terms of oxidation states 1m 1m aii Zinc oxide has a giant ionic structure^ while carbon monoxide has a simple covalent structure^. A large amount of energy# is needed to overcome the strong electrostatic forces of attraction between oppositely charged ions* , while only a small amount of energy# is needed to overcome the weak intermolecular forces of attraction between carbon monoxide molecules*.

Hence, the melting point of zinc oxide is very high and it exists as a solid while the boiling point of carbon monoxide is very low and it exists as a gas at room temperature and pressure. Compare type of structures^ Compare amount of energy# Compare bonds/forces* 0.5m per point Rej: ionic bonding vs covalent bonding only. Some students have misconceptions eg. ionic bonding as weak intermolecular forces / weak covalent bonds / electromagnetic forces / strong electrostatic forces of attraction between molecules etc 1m 1m 1m bi cobalt, zinc, magnesium, barium Moderately well done Some students gave the opposite sequence 1m bii Zinc has a higher melting point / is denser / harder compared to lithium. Some students gave chemical property instead. Can cut easily does not answer to the qn of physical property Not very well done 1m 7 a correct number of bonded electrons correct number of all other valence electrons Some students are able to do this while others attempted with the dot and cross diagram of water instead. 1m 1m b Particles in a liquid are close together in a disorderly manner while particles in a gas are spread far apart randomly. Particles in a liquid slide over each other throughout the liquid volume while particles in a gas move at high speeds, occupying every available space. OR Particles in a gas are moving faster than particles in a liquid. Particles in a gas are further apart than those in a liquid. 0.5m per point 1m 1m

Some students missed either description of arrangement or motion/ used foreign terms / wrong terms. c (Colour change of KMnO4 shows) it is a reducing agent as it can be oxidised / KMnO4 is reduced. (Colour change of KI shows) it is an oxidising agent as it can be reduced / KI is oxidised. Some students mistaken RA as OA vice versa. Need to show observations. 1m 1m di Diagram with suitable apparatus Labelled apparatus and chemicals Many students either gave incomplete diagrams or did not label A: gas collection using inverted measuring cylinder R: gas syringe / gas jar with no interval markings No penalty for not including oxygen gas labelling in the diagram 1m 1m dii no. of moles of oxygen gas produced = 48 ÷ 24000 = 0.00200 mol Many students forgot to co