Dunman (2024) 4E 6092 Prelim P1+P2 MS

1 DUNMAN SECONDARY SCHOOL 6092 CHEMISTRY 4E Preliminary Examination 2024 Marking Scheme PAPER 1 1 2 3 4 5 6 7 8 9 10 A B A A C C C D C A 11 12 13 14 15 16 17 18 19 20 B C C C C D A C C D 21 22 23 24 25 26 27 28 29 30 D C D C B D B A B D 31 32 33 34 35 36 37 38 39 40 A A D D D B A B C B PAPER 2 Section A Qn Answer Mark (max) 1(a) A, B, E 1 1(b) C 1 1(c) D 1 1(d) E 1 1(e) D and B 1 2(a) Acidic oxide : SiO2 / P2O5 / SO2 Basic oxide : Na2O / MgO Amphoteric oxide : Al2O3 [1] any 1-2 correct; [2] all correct 1 2(b) Correct sharing of electrons [1] Correct outermost electrons for each atom [1] 2

2 Qn Answer Mark (max) 2(c) • Silicon dioxide has a giant covalent structure. • The extensive network of silicon and oxygen atoms • are held by many strong covalent bonds, • which require a lot of energy to overcome it, hence high melting point. • Sulfur dioxide has a simple covalent structure. • The molecules are held by weak intermolecular forces of attraction which require a little amount of energy to overcome it, hence low melting point. 6 points [3], 4-5 points [2], 2-3 points [1] 3 2(d) movement : from slide past each other and exchanging position within liquid to move randomly at high speeds in all direction. arrangement : from closely packed and disorderly arranged to far apart and disorderly arranged. 2 2(e) • Dichlorine molecule exists as neutral molecules . They do not have mobile ions or electrons to carry charge. • Hypochlorous acids have mobile ions which are free to move around to carry charged. 2 3 formula of salt Formulae of reagents used method used MgCl2 (aq) Mg / MgCO 3 / MgO HCl (aq) addition of excess solid to acid filtration evaporation and crystallisation (NH4)2SO4 NH3 (aq) H2SO4 (aq) Titration between alkali and acid Evaporation and crystallisation BaCO3 (s) Ba(NO3)2 (aq) Na2CO3 (aq) / K2CO3 (aq) / (NH4)2CO3 (aq) Mixing of the two salt solutions to form precipitate Filtration to collect residue 5

3 Qn Answer Mark (max) 4(a) The total energy absorbed to break 1 mol of O-O bond and 2 mol of O -H bond in H2O2 is less than the total energy released to form 4 mol of O-H in H2O and 1 mol of O=O in O2. Since there is net release of energy to the surrounding, it is an exothermic reaction. [1] total energy absorbed to break bond is less than total energy released to form bond [1] correct number and type of bonds 2 4(b)(i) catalyst 1 4(b)(ii) [1] for 2 correct [2] for 3-4 correct [3] for all correct - reactants and products - activation energy with catalyst - activation energy without catalyst - enthalpy change - shape of graph 3 5(a) Insert a filter paper soaked in acidified potassium manganate(VII) [1] If purple KMnO4 turns colourless, SO2 is still present. OR If purple KMnO4 remains purple, SO2 is not present [1] 2 5(b)(i) No. of moles of iodine = 12/ 1000 x 0.0250 = 0.000300 mol [1] 2 2H2O2 2H2O + O2

4 Qn Answer Mark (max) I2 : SO2 1 : 1 0.0003 : 0.0003 Volume of SO2 produced = 0.000300 x 24 = 0.00720 dm3 [1] 5(b)(ii) SO2 is oxidised as the oxidation state of sulfur increases from +4 in SO2 to +6 in SO42-. [1] I2 is reduced as the oxidation state of iodine decreases from 0 in I2 to -1 in I-. [1] 2 6(a) pollutant gas how it is formed carbon monoxide Incomplete combustion of carbon- containing fuel nitrogen monoxide Nitrogen reacts with oxygen at high temperature in car engines 2 6(b) In lean burn engine, the amount of carbon monoxide formed is lower as more air mixed with petrol means more oxygen provided for complete combustion of fuel to occur. In lean burn engine, the amount of nitrogen monoxide formed is lower as lower temperature lower the rate of reaction between nitrogen and oxygen molecules in the car engine. [1] for describing the effect [1] for each reason 3 6(c) Carbon dioxide produced is a green house gas. [1] It causes global warming which melts polar ice caps, causing sea levels to rise and permanently flooding coastal areas. [1] 2 7(a) Reddish brown aqueous bromine turns colourless. [1] Bromination / addition of bromine [1] 2 7(b)(i) Carboxyl group [1] 1 7(b)(ii) 1 7(c)(i) 1

5 Qn Answer Mark (max) 7(c)(ii) Compound T has lower melting/boiling point than the polymer / Compound T has lower density than the polymer 1 7(d) • Condensation polymerisation is formed from monomers with two reactive functional groups in each molecule i.e. diamines and dicarboxylic acids but addition polymerisation is formed from unsaturated monomers with carbon- carbon double bonds. • Condensation polymerisation involves removal of a small molecule such as H2O but addition polymerisation does not involve removal of a small molecule. • Condensation polymerisation occurs at high temperature and pressure in the presence of a catalyst while addition polymerisation can take place at room conditions. (any 1 point) 2 7(e) Physical method - Poly(ethene) are melted, cooled, pulled into long, thin strands, and cut into pellets. Chemical method - Poly(ethene) waste can undergo cracking to form short chains of alkanes and alkenes, that can be used as fuel and chemical feedstock 2 8(a) In experiment 1, effervescence observed. Hydroxide ions are preferentially discharged as it is lower in the electrochemical series. 4OH- (aq) → O2 (g) + 2H2O (l) + 4e In experiment 2, silver rod decreases in size. Silver is the reactive anode, it will oxidise and dissolve to form silver ions. Ag (s) → Ag+ (aq) + e [1] for ionic equation [1] observation [1] reason 3 8(b)(i) In experiment 1, the silver ions in solution are depleted by discharge and there is no replenishment. In experiment 2, the silver ions in solution are replenished from the dissolving of the silver rod anode. [1] 1

6 Qn Answer Mark (max) 8(b)(ii) Collect a small sample of electrolyte from each experiment and add 2 drops of sodium chloride into the samples separately. [1] White precipitate (insoluble silver chloride) would be observed from experiment 2 but not from experiment 1. [1] 2 8(c) Mg(s) → Mg2+ (aq) + 2e [1] Magnesium is more reactive than iron, hence lose electrons more readily and corrode in place of iron. [1] 2 9(a)(i) Isotopes have the same number of protons but different number of neutrons. In Fig. 9.1, there are two ions detected which have different m/z values of 6 and 7. This indicates that these two ions have different number of neutrons. 2 9(a)(ii) Average atomic mass of lithium = (3.75/100 x 6) + (96.25/100 x 7) = 6.9625 ≈6.96 (3 s.f.) 1 9(b)(i) x = 72, y = 74 2 9(b)(ii) Chlorine exists as diatomic molecules. There are 3 possible combinations: 2 atoms of chlorine-35, 1 atom of chlorine-35 and 1 atom of chlorine-37, 2 atoms of chlorine-37. 1 9(c)(i) Agree with the student. [0] The largest m/z is 44, which belongs to the ion formed by the largest molecule. Since propane has a Mr of 44, the unknown hydrocarbon is propane. 1 9(c)(ii) CH2+ 1 9(c)(iii) The highest m/z value would be 58. There would be more peaks. 2 9(d) It is much harder / takes more energy to remove an electron from a positively charged ion. 1 Section B Qn Answer Mark (max) 10(a) ZnSO4 1 10(b) • Comparing 1&2, when the concentration of acid is increases from 0.5 mol / dm3 to 1.0 mol / dm3, • there are more hydrogen ions per unit volume.

7 Qn Answer Mark (max) • Frequency of collisions and effective collisions between the hydrogen ions and zinc increases, speed of reaction increases. • Hence, the time taken to collect 15 cm3 of gas decreases. • Comparing 1&3, when the temperature is increased from 20˚C to 40˚C, • the particles gain kinetic energy / move faster. • There will be more particles having energy equal to or gre