catholic high school chemistry prelim 2023 answers

1 2023 CHS S4 Prelim Mark Scheme Paper 1 Paper 2 Section A A1 (a) Put a tick (✓) if a redox reaction would occur. (i) a copper strip added to lead(II) nitrate solution (ii) aqueous chlorine added to potassium iodide solution ✓ (iii) hydrogen gas passed over heated copper(II) oxide ✓ (iv) potassium nitrate solution warmed with sodium hydroxide solution 4 correct – 3m , 3 correct – 2m, 2 correct – 1m (b) Difference: Reaction rate is slower when used with aqueous bromine OWTTE; Explain: Chlorine is more reactive than bromine/ gains electrons more readily (and displaces iodine from iodide solution faster); OWTTE A2 (a) As the number of Cl atoms in the compound incre ases, the boiling point increases. A: has more Cl atom substituted/ less H atoms R: increasing Mr/ molecular size (b) (i) C2H6 (ii) C2H4Cl2 (iiii) 5 (c) and 1m for each correct structure 1 2 3 4 5 6 7 8 9 10 B A C A A B D B C C 11 12 13 14 15 16 17 18 19 20 B B A B B A B D D D 21 22 23 24 25 26 27 28 29 30 D A C D B B B D C C 31 32 33 34 35 36 37 38 39 40 A D C D D D C C D C

2 A3 (a) Carbon dioxide = C; phosphorus pentoxide = B; both has simple molecular/ covalent structure; where a small amount of energy is needed to overcome weak intermolecular forces; phosphorus pentoxide is a larger molecule with stronger intermolecular forces (b) (i) 1 similarity (1m) + 1 difference (1m) Similarity: both has giant molecular/ covalent structure; both consists of only the same type of atoms difference: Silicon Graphite Difference 1 Only has strong covalent bonds between atoms Has both strong covalent bonds and weak intermolecular forces Difference 2 Not layered Layered Difference 3 Tetrahedral structure or 1 atom bonded to 4 other atoms Hexagonal structure or 1 atom bonded to 3 other atoms (ii) No. + A semiconductor needs to have presence of mobile charged carriers such as ions or electrons; All 4 valence electrons are used in bonding (and absence of ions/ contains only atoms) A4 (a) (i) removes carbon dioxide from atmosphere + hence regulating amount of carbon dioxide OWTTE (ii) 2NO + 2CO → 2CO2 + N2 ; 2CO + O2 → 2CO2 A: 2C8H18 + 25O2 → 16CO2 + 18H2O A: 2NO2 + 4CO → 4CO2 + N2 ; R: formation of sulfur dioxide (fuel does not contain S) Still releases large amount of carbon dioxide into the atmosphere + explain how carbon dioxide causes environmental problems (see e.g. below) Global warming/ Melting of ice caps/ rise in sea levels/ flooding in low -lying areas etc

3 (b) (i) The set-up using light will produce gas X but the set -up without light will not produce any gas X (or oxygen gas); Or In the absence of light, carbon dioxide is produced while in the presence of light, gas X / oxygen is produced (+ CO2) I: if students discussed observations such as presence of effervescence R: More gas given off for set-up with light than set-up without light (ii) To determine if a reaction is endothermic or exothermic, we have to consider the difference in both energy taken in / absorbed and energy released / given off; OWTTE A: Light energy used for activation energy A: if explain in terms of bond break/ form A5 (a) C3H7OH + 2[O] → C2H5COOH + H2O or C3H8O + 2[O] → C3H6O2 + H2O (b) (i) C H O % 64.6 10.8 24.6 Ar 12 1 16 No. of moles 64.6 12 = 5.383 10.8 1 = 10.8 24.6 16 = 1.537 Mole ratio 5.383 1.537 = 3.5 10.8 1.537 = 7 1.537 1.537 = 1 Simplest ratio 7 14 2 1m for calculating no. of moles Empirical formula of R = C7H14O2 [1] (ii) Butyl propanoate (c) (i) any suitable metal carbonate or ammonium carbonate (ii) corresponding metal/ ammonium propanoate E.g. NaC2H5COO or C2H5COONa or Ca(C2H5COO)2 or (C2H5COO)2Ca A: molecular formula e.g. NaC3H5O2 A6 (a) Comparison of energy + recognise bond forming releases energy and bond breaking absorbs energy ; [1] Identify forming bonds in correct products + breaking bonds in correct reactants; [1]

4 Suggested answer: More energy is given off/ released when forming b onds in HCl / hydrogen chloride and CH3Cl/ chloromethane than taken in/ absorbed when breaking bonds in methane/ CH4 and chlorine/ Cl2. (b) (i) (ii) −102 kJ/ mol; [1] Both require breaking 1 mole of C−H and 1 mole of Cl−Cl bonds in reactants and both form 1 mole of C−Cl and 1 mole of H−Cl bonds [1] A: 1 C-H bond etc instead of saying 1 mole (iii) • the reactants of the reaction • the products of the reaction • the energy profile and the enthalpy change of reaction, ∆H. • activation energy, Ea A7 (a) (i) electrode 1: 4OH− (aq) → 2H2O(l) + O2 (g) + 4e− ; Electrode 2: 2H+ (aq) + 2e− → H2(g) (ii) filter paper around electrode 1 changes from purple to red; [1] Filter paper around electrode 2 changes from purple to blue; [1] Reference to difference in concentration of H + and OH − + determine acidic/ alkaline; [1] Suggested answer: concentration of OH− < concentration of H+, so solution around electrode 1 is acidic and turns litmus solution red 1m 1m

5 + concentration of H+ < concentration of OH−, so solution around electrode 2 is alkaline and turns litmus solution blue A7 (b) (i) barium nitrate is soluble in water (A: aqueous solution) [1] (ii) A white precipitate was observed on the filter paper. [1] A8 (a) (i) (ii) chlorine gas is soluble in water + so the mass loss is less than expected [1] (iii) The rate of reaction is not constant throughout the reaction OWTTE [1] (b) Conclusion 1: increase the total surface area of MnO2 [1] Conclusion 2: MnO2 is a catalyst + no change in the amount/ mole of limiting reactant / no change in yield. [1] Section B B9 (a) Hydrogen ion has 0 electrons, 0 neutrons and 1 proton only [1] (b) (i) H2SO4 → 2H+ + SO42- Conc. of H+ ions = 0.025 mol/ dm3 × 2 = 0.05 mol/ dm3 [M1] pH = 1.3 (taken from table 9.2) [A1] A: 1.30 (3sf)

6 (ii) pH = 𝑘 [𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐻+] ; [1] (A: if embedded in working) students use any 2 sets of data to find 2 k values [1] conclude that since the two k values are different, students claim is false [1] Data 1 Data 2 Data 3 Data 4 pH 1.3 1.6 1.9 2.0 1.3 = 𝑘 0.05 1.6 = 𝑘 0.025 1.9 = 𝑘 0.0125 2.0 = 𝑘 0.010 K value K = 0.065 K = 0.04 K =0.02375 K = 0.02 (c) (i) when solution is pH 3, Thymol blue appears yellow + Methyl red appears red + Bromothymol blue appears yellow + Phenolphthalein appears colourless Overall is mixture of yellow and red which makes it orange ; [1] (ii) Criteria Claim Evidence + reasoning has a narrow pH range in which it changes colour Does not fulfil pH range in which it changes colour is from 1.2 to 10.5 (wide range; typically the range is about 1.6 to 2.5) has a distinctive colour at lower pH and a different colour at higher pH Does not fulfil Lower pH can have colours such as red and orange and higher pH can have colours such as blue or purple (state more than 1 colour for lower pH and more than 1 colour for higher pH) / More than 2 colours or has multiple colours at lower pH / higher pH changes colour only once during the titration Does not fulfil Can change from orange to yellow (at pH 6) to green (at pH 7) (quote at least 2 colour change) / Changes colour twice before equivalence point pH range of indicator in which indicator changes colour to fall within the definite ‘step’ of the reaction Does not fulfil pH range of indicator which is from pH 1.2 to 10.5 and pH change of the ‘step’ is between pH 3 to 11 1m

7 B10 (a) Catalytic properties; Form coloured compounds; High melting/boiling points; High d