ZHSS 2026 Chem Prelim P2 MS

2026 ZHSS 4E Pure Chem (6092) Preliminary Examination Section A (70 m) 1 (a) H 1 (b) C 1 (c) F 1 (d) A 1 (e) D 1 [Total:5] 2 (a) Carbon allotrope A (diamond): It has a giant molecular / covalent structure in which each carbon atom is covalently bonded to four other carbon atoms by strong covalent bonds. [1] A large amount of energy is needed to break these bonds, making diamond very hard and suitable for use as a drill bit.[1] Carbon allotrope B (graphite):It has a giant molecular / covalent structure made up of layers of carbon atoms. [1] The weak forces of attraction between the layers allow them to slide over one another easily lubricant [1], making graphite slippery and suitable for use as a lubricant. 4 (b) The property is electrical conductivity. [1] The diagram only shows the arrangement of atoms and covalent bonds. It does not show the mobile electrons (or charge carriers) [1] for electrical conductivity. 2 (c)(i) [1] – number of shared electrons [1] – number of valence electrons on C & H 2 (ii) In methane, each carbon atom shares one electron with each hydrogen atom to form four single covalent bond.[1] Both carbon and hydrogen atom achieve a stable (noble gas) electronic configuration. 1 [Total:9]

3 (a) [1]: volume of gas must be lower (no values needed) [1]: initial gradient must be steeper 2 (b) (i) 3CaCO3 +2H3PO4 → Ca3(PO4)2 + 3H2O + 3CO2 1 (ii) No. of moles of CaCO3 = 6/100 = 0.06 mol No. of moles of H3PO4 = (40/1000) × 2 = 0.08 mol [1] Mole ratio of CaCO3:H3PO4 = 3:2 0.08 mol of H3PO4 requires 0.12 mol of CaCO3, but only 0.06 mol of CaCO3 is available. CaCO3 is limiting reagent. [1] * No e.c.f. 2 (c) There is a lower concentration of H+ ions in weak acid as it partially dissociates. [1] The decrease in concentration of H + ions would decrease the number of H + ions per unit volume [1], leading to decrease frequency of collision between leading to an decrease in the frequency of effective collision . This lower/decrease the rate of reaction. both [1] 3 [Total:8] 4 (a) Cobalt oxide (CoO) and Bismuth oxide (Bi₂O₃) [1] Both metal oxides form grey solids after heating, which are the metals formed. [1] And the liquid was also observed on the side of the glass tube, showing that water was produced. The metal oxides have been reduced by hydrogen to form metals. 2 time/s volume of gas/ dm3 1.20 experiment 1 experiment 2 0.48

(b) Gallium, Cobalt, Lead and Bismuth 1 (c) Co(s) + Pb²⁺(aq) → Co²⁺(aq) + Pb(s) [1]: Correct ionic equation [1]: Correct state symbols 2 (d) (i) Cl2: 0; Cl⁻: -1, ClO-: +1; ClO₃⁻: +5 2 correct – [1] 2 (d) (ii) In the chlorate(I) ion, chlorine in ClO- has an oxidation number of +1. Some ClO- ions are reduced, where the oxidation number of chlorine decreases from +1 in ClO- to –1 in to Cl⁻. Therefore, ClO- acts as an oxidising agent. [1] Other ClO- ions are oxidised, where the oxidation number of chlorine increases from +1 in ClO- to +5 in ClO₃⁻. Therefore, ClO- acts as a reducing agent. [1] 2 [Total:9] 5 (a) N2 + 3H2 ⇋ 2NH3 [1] Since 1 mole of gas occupies the same volume of gas at room temperature. From the equation: 1 mole of nitrogen reacts with 3 moles of hydrogen, hence a ratio of 1: 3 is most efficient. [1] 2 (b) At a lower pressure, the particles are further apart. There are less particles per unit volume. [1] Therefore, there is a lower frequency of collisions, leading to a lower frequency of effective collisions between reacting particles, [1] resulting in a lower rate of reaction. 2 (c) Lower pressure would reduce the percentage yield of ammonia. [1] Hence, less ammonia leaves [1] the reactor and more nitrogen and hydrogen would be left unreacted and leave the reactor. [1] 3 (d) - To allow the unreacted hydrogen and nitrogen to react again to produce more ammonia. - To reduce the wastage of nitrogen & hydrogen / reduce the cost of production / improve efficiency of the process. 2 [Total:9]

6 (a) CnH2n 1 (b) 1 (c) The boiling points increase down the homologous series, because the forces of attraction between the molecules increase as their size increases.[1] More energy is required to overcome the forces of attraction. [1] 2 (d) (i) The energy absorbed to break the bonds in cyclopropane and oxygen is less than the energy released when bonds are formed in carbon dioxide and water.[1] Therefore, the reaction is exothermic [1] and the enthalpy change is negative. 2 (d) (ii) [1]: general curve with reactants and products [1]: Ea and enthalpy 2 2C3H6 + 9O2 6CO2 + 6H2O Ea ∆H = -2091 kJ/mol Energy Progress of reaction

(d) (iii) n(cyclopropane) = 24 0.00124000 = mol C3H6:CO2 1 : 3 n(CO2) = 0.001 x 3 = 0.003 mol [1] v(CO2) = 0.003 x 24 000 = 72 cm3 [1] 2 [Total:10] Data-based question (12m) 7 (a)(i) Similarity: Both HDPE and LDPE contain carbon and hydrogen only./hydrocarbons/ contain carbon-carbon single bonds, carbon - hydrogen single bonds/ contains monomer ethene. [1] Difference: LDPE contains side chains with different number of carbon atoms while HDPE does not contain any side chain/ LDPE is branched but HDPE is a straight chain. [1] Reject: Different number of C atoms between LDPE and HDPE. 2 (ii) HDPE does not have any side chains resulting in a closer arrangement.[1] This leads to higher mass/molecules per unit volume / can pack closely within the same volume.[1] Therefore, HDPE has a higher density than LDPE. 2 (b)(i) 1 (ii) Poly(lactic acid) is a homopolymer. 1 (c)(i) Mr of ONE repeat unit = 100000 / 962 = 103.95 1 (ii) C H percentage / % 92.3 7.7 no. of moles 92.3 7.69112 = 7.7 7.71 = ratio 7.691 17.691= 7.7 1.00117.691= Empirical formula = CH 1 (iii) Mr of CH = 12 + 1 = 13 Let the molecular formula of one repeat unit of poly(styrene) be (CH)n. n = (relative molecular mass)/(Mr of empirical formula) = 103.95/13 = 7.99 = 8 (round off to whole no.) 1 (iv) Molecular formula of one repeat unit = C8H8 From the Table 7.1, 1

Repeat unit of poly(styrene) with the deduced R formula: (d) Based on the information provided, both mechanical recycling and depolymerisation can be used to recycle RIC 1- PET which is a co polymer. Thus, the statement is not true. 1 (e) Any one of the following: - Recycling plastics reduces the amount of built-up waste in landfills. - Recycling plastics conserves natural resources. - Recycling plastics reduces the amount of plastics required to be incinerated, which releases poisonous gases. 1 [Total:12] 8 (a)(i) Carbon monoxide : toxic, combines/binds with haemoglobin in blood, reduces the ability of haemoglobin to /prevent transport oxygen to the rest of the body. [1] Oxides of nitrogen : react with water /rain water to form acid rain which corrodes buildings and harms aquatic life and plants OR cause lung/eye irritations [1] 2 (ii) 2NO + 2CO → 2CO2 + N2 1 (iii) Carbon dioxide [1] is a greenhouse gas and it contributes to global warming/trap heat within the atmosphere/. [1] Reject: Carbon dioxide degrades the ozone layer and leads to global warming. Reject: sulfur dioxide/ oxides of nitrogen 2 (b) The NO molecules reacted/removed in step 1 (after reacting with ozone) are regenerated in step 2 (in reaction with more ozone). Accept: reproduced/ form back Reject if describe the step 1 and 2 as it is listing information without thinking Reject NO is a catalyst 1 (c) The carbon dioxide released during combustion /burning of biofuel is removed from air by photosynthesis carried out by sugarcane plant [1]. Thus, there is no net gain or loss of /offset/balance off carbon dioxide [1