Copy of MGS Chemistry P2 (Answers)

1 METHODIST GIRLS’ SCHOOL SEC 4 CHEMISTRY PRELIM EXAM 2024 Answer Scheme Section A A1 (a)(i) F 1 (a)(ii) A 1 (a)(iii) E 1 (b) + 2 A2 (a)(i) Carbon dioxide / CO2 1 (a)(ii) Redox reaction or reduction 1 (a)(iii) CuO + C → Cu + CO2 1 (b)(i) At mass 0g, there is no theoretical yield of copper or If no copper carbonate is used, no copper is made Or No reaction 1 (b)(ii) 80g The theoretical yield is directly proportional to the mass of copper at the start / the graph has a positive gradient or; linearly related or; as mass of copper(II) carbonate doubles, mass of copper doubles so at 40g, the starting mass of copper will be 80g. or; when the yield is 20g, the mass of CuCO 3 is 40g. hence by proportion, when the yield is 40g the mass of CuCO3 is 80g. 1 1 H H N Key: x : electron of hydrogen • : electron of nitrogen H H

2 (b)(iii) Tick (✓) two boxes. She did not use enough copper carbonate. She did not dry the copper at the end. She did not heat the copper oxide for long enough. Her copper contains solid impurities. 2 (c) Students need to calculate out the % yield of both Nina and Kai and make a comparison. Kai’s % yield is higher than that of Nina’s, so he has extracted more copper than Nina. % yield for Kai = 4.8/5 x 100% = 96% % yield for Nina = 18 / 26 x 100% = 69.2% 1 1 A3 (a) Cathode: Mg2+ (l) + 2 e- → Mg (l) Anode: 2 Cl - (l) →Cl2 (g) + 2e- State symbols not required 2 (b) To prevent molten magnesium from (coming into contact and hence) reacting with oxygen in the air 1 (c) Magnesium chloride has a very high melting point. Lowering its melting point will reduce energy cost / less energy needed./ lower cost of electricity 1 (d) Fe (s) → Fe2+ (l) + 2 e- or Fe (s) → Fe3+ (l) + 3 e- The iron will react with the chlorine evolved to form iron(II)/ iron(III) chloride OR The iron anode, being a reactive anode, will be oxidized and shrink in size / become smaller / dissolve into the molten electrolyte, and no bubbles will be formed. 1 1

3 A4 (a) Propanone: In experiment 1 and 2, increases (rate)/doubling concentration doubles rate iodine: In experiment 2 and 3, no effect (on rate) Must state the experiments 1 1 (b) particles move faster/particles have more energy / KE more particles have (energy greater than) the activation energy/sufficient energy to overcome the energy barrier and leads to higher frequency of effective collisions 1 1 (c) 1 (d) isotope number of electrons number of neutrons Iodine−127 53 74 Iodine−130 54 77 2 A5 (a)(i) The reverse reaction is favoured ; preferred , backward and forward reaction is different. many molecules formed and few ions or partially ionised 1 (a)(ii) It produces hydroxide ions in water 1 (b) less than 12 more than 7 smaller concentration of hydroxide ions or partially dissociated 1 1 (c)(i) CH3NH2 + HCl → CH3NH3Cl methylammonium chloride 1 1 (c)(ii) Fe3+ + 3OH- → Fe(OH)3 1 Mass of colourless product formed /g Time/s 2 3

4 A6 (a)(i) Diesel and petrol are mixtures of hydrocarbons / contains different hydrocarbons but bio-ethanol is made up of only one compound 1 (a)(ii ) Yeast, absence of oxygen and temperature of 35-40oC 3 conditions – 2m 2 conditions – 1m 1 condition – 0m 2 (a)(ii i) Bioethanol comes from plants which is a sustainable /renewable resource / infinite resource while petrol comes from crude oil/ fossil fuels which is non-renewable resource / finite 1 (a)(i v) Molar mass of C2H5OH = 46g/mol Enthalpy change of combustion in kJ/mol = -30.5 x 46 = 1403 kJ/mol 1 (b) LPG consists of smaller molecules than diesel with smaller surface area, allow smaller chains LPG has weaker intermolecular forces of attraction than diesel less energy is required to break/overcome the weak intermolecular forces of attraction between the molecules in LPG assume answer refers to LPG if no reference allow LPG has smaller chains ignore all references to few carbon atoms in LPG / is a short chain hydrocarbon unless there is a direct comparison with petrol allow weaker forces. 1 1 1 A7 (a)(i) Depolymerisation is a process in which polymers are broken down into their monomers. 1

5 (a)(ii) Simpler molecules formed Structural Formula Hexamethylendiamine Adipic acid 2 (b)(i) H H H H | | | | — C — C — C — C — | | | | H CN H CN 1 mark for 2 repeat units 1 mark for both the side bonds present 2 (b)(ii) Addition polymerisation results in a single product formed, the polymer, while condensation polymerisation results in the formation of a polymer and small molecules eg. water. The monomers for addition polymerisation must be unsaturated alkenes with carbon to carbon double bonds while the monomers for condensation polymerisation can be saturated. Condensation polymerisation forms 2 types of linkages. They are ester linkages from reactions between hydroxyl and carboxyl groups or amide linkages from amine and carboxyl groups. Addition polymerisation forms long carbon chains without ester or amide linkages. In addition polymerisation, only one type of functional group, carbon to carbon double bonds, are required in the monomers. In condensation polymerisation, two types of functional groups, amine and carboxyl groups or hydroxyl and carboxyl groups, are required in the monomers. 2

6 8(a) W (respiration) – organic compounds in both plants and animals to CO2 in the atmosphere X (photosynthesis) – CO2 in atmosphere to organic compounds in plants only 1 1 8(b) Z (combustion) – coal and petroleum to CO2 in the atmosphere 1 8(c)(i) Petroleum 1 8(c)(ii) Decomposition of plants and animals that died and buried in the ground millions of years ago. 1 8(d) Carbon dioxide is slightly soluble in water to form carbonic acid. or CO 2 from atmosphere dissolves in water in the ocean/sea/river to form carbonate ions 1 8(e) Burning of hydrogen produces only water, does not increase or decrease atmospheric cycle. 2H2 + O2 → 2H2O 1 1 Total 8 9(a) (i) Silica gel Adsorption – weak van der waals’/ intermolecular forces of attraction between surface atoms of silicon dioxide/ silica gel and water molecules (ii) Calcium chloride Absorption – weak forces of attraction between ions of calcium chloride and water molecules when it becomes hydrated crystals (iii) Ionic bonds in calcium chloride broken when it dissolves in the moisture 1 1 1 9(b) Moles of CaCl2 = (32.5 – 20) ÷ (40 + 71) = 0.11261 Moles of water = (36.5 – 32.5) ÷ (2+16) = 0.22222 Mole ratio of CaCl 2 to H2O = 0.11261 : 0.2222 = 1:2 Formula is CaCl2.2H2O mole ratio calculation/ working 1m Mole ratio 1m and Formula 1m 3 9(c) Mass of 1 mole of SiO2 = 32 + 32 = 64g Mass of water removed by SiO2 = 0.4 x 64 = 25.6 g Mass of 1 mole of CaCl2 = 40 + 71 = 111 g 1

7 Mass of water removed by 1 mole CaCl2 = 3 x 111 = 333 g 1 9(d) 2 9(e)(i) Evaporation to dryness 1 9(e)(ii) Boiling point of water is 100 0C, lower than that of calcium chloride / calcium chloride is stable