2022 SASS A-Z Chem Prelims Ans Pt1

1 A – Z School’s Preliminary Papers 4 Express Chemistry ANSWERS: Part I A Anderson Secondary School B Beay Secondary School C Chung Cheng High School (Main) D Damai Secondary School E Edgefield Secondary School ANDERSON SECONDARY SCHOOL Paper 1 Paper 2 Qn Answer Marks Comments A1(a)(i) A and F OR B and E 1 (a)(ii) B and D OR C and D Or D and E 1 (a)(iii) A and F 1 (a)(iv) D 1 (b) true false Petroleum is separated by different density into different fractions. ✔ Alkanes used in diesel fuel have a lower boiling point than those used in motor cars. ✔ The fractions used as feedstock for making plastics is extracted from higher up the fractionating column than the fraction used for kerosene(paraffin). ✔ Each fraction contains a mixture of hydrocarbons. ✔ . 2 PRELIM 4E CHEM 2022 6092/01 20 1 2 3 4 5 6 7 8 9 10 A C B D B C C A B A 11 12 13 14 15 16 17 18 19 20 B B A B C D C A C A 21 22 23 24 25 26 27 28 29 30 B C B D B A C B B A 31 32 33 34 35 36 37 38 39 40 C B B A B B B A D B SASChemistry 2022edition

2 A2(ai) C2H4 + H2O → C2H5OH 1 (a)(ii) To increase the percentage yield of ethanol OR To conserve resources and cut down the cost of starting reactants 1 (a)(iii) Oxygen will oxidise ethanol to ethanoic acid / Ethanol formed may undergoes combustion hence reducing the yield of ethanol. 1 (b)(i) 200°C, 80 atm 1 (b)(ii) Optimum temperature is higher so that the reaction between steam and ethene is faster. Optimum pressure is lower because special equipment would be required to maintain the higher pressure which is not cost efficient. 1 1 A3(a) Graphite is made up of many layers of carbon atoms covalently bonded in a hexagonal lattice structure. The layers of carbon atoms are held loosely by weak intermolecular forces of attraction hence the layers can be easily separated using a sticky tape. 1 1 (b) Each carbon atom in graphene has one outer electron that is not used to form covalent bonds. These electrons can move freely and carry electric charges hence graphene has high electrical conductivity. Each carbon atom is covalently bonded to three other carbon atoms. A lot of energy is needed to break these strong covalent bonds hence graphene is strong. 1 1 1 1 (c) Graphene is transparent as it is made up of only a single layer of atoms which is very thin. 1 A4(a)(i) A strong acid completely ionized/ dissociates in aqueous solution/ when dissolved in water while a weak acid partially ionized/ dissociates in aqueous solution/ when dissolved in water. 1 (a)(ii) Tartaric acid is the weaker acid. Universal indicator turns orange in tartaric acid and red in sulfuric acid showing that tartaric acid has a higher pH/ pH of 4-6 while sulfuric acid has a lower pH/ pH of 1-3. Volume of gas collected 10s after reacting with Magnesium is lower with tartaric acid indicating that the speed of reaction is slower. Both tests show that the concentration of hydrogen ions is lower in tartaric acid as compared to sulfuric acid. 1 1 1

3 (b) no. of mol of NaOH produced = x 0.100 = 0.0022 mol by mol ratio, H2T : NaOH = 1 : 2 0.011: 0.0022 Concentration of tartaric acid = 0.0011 / = 0.044 mol/ dm3 1 1 1 A5(a) A polymer is a macromolecule formed through the joining of many small molecules / monomers. 1 (b) 1 1 1 (c) The component amino acids can be identified from its Rf value using chromatography with a suitable solvent and locating agent. 2 (d) Manufacture of proteins Manufacture of poly(ethene) monomers join together via condensation polymerisation monomers join together via addition polymerisation with the elimination of small molecules without loss of any small molecules. Monomers must have 2 different functional groups Monomers must have C = C bonds 2 Accept any other possible / reasonable difference(s).

4 A6(a)(i) 2 (a)(ii) It is described as substitution reaction because the hydrogen atoms in methane is replaced by halogen atoms. It is photochemical because it requires ultraviolet light to initiate the reaction. 1 1 (b) 1 (c) The OCl particle produced in step 1 reacts with an oxygen atom in step 2 to produce another Cl atom. This Cl atom is then able to start step 1 again. This is a chain reaction. 1 1 (d) Ozone filters out excessive ultraviolet radiation that could lead to increased incidence of skin cancer. 1 A7(a) CO is poisonous/toxic as it combines with haemoglobin in red blood cells to form carboxyhaemoglobin which reduces the transport of oxygen to other parts of the body, leading to headaches / dizziness / breathing difficulties / suffocation / death. SO2 irritates the eyes and lungs and cause breathing difficulties OR reacts with water to form acid rain which corrodes buildings/ harms aquatic life and plants. CO2 is a greenhouse gas that absorbs radiation and leads to global warming. 2 (b) The oxidation state of Zn, O, C remains unchanged as +2, −2 and +4 respectively. Both oxidation and reduction has not occurred hence the reaction is not a redox reaction. 1 1

5 (c)(i) • The mixture is heated and vapourises. • Cadmium vapour with the lowest boiling point exits the condenser and distilled over first at 766°C. • After cadmium is collected, Zinc with the second lowest boiling distilled over at 907°C. • Lead and iron with much higher boiling point remains in the distillation flask/ distilled out last. OR • The mixture is heated and vapourises. • Vapours rise into the fractionating column and begins to cool and condense. • Fractions with a lower boiling points are collected at the top of the fractionating column and fractions with a higher boiling points are collected at the lower sections of the fractionating column. • Zinc is collected at the second lowest section. 4 (c)(ii) Zinc is more reactive than iron hence provides sacrificial protection by corroding in place of iron. OR Zinc is used in galvanization of iron where It acts as a protective barrier for iron against water and oxygen. 1 1 1 1 B8(a) The greater the number of bonds between two atoms, the smaller the bond length. The bond length decreases from 154pm to 120pm when the number of bonds increase from C-C to C≡C. As the bond energy decreases, the bond length increases. As bond energy decreases from 431kJ/mol to 299kJ/mol from H–Cl to H–I, bond length increases from 127pm to 161pm. 2 . (b) As the number of bonds between two atoms increases, the number of shared electrons increases and the forces of attraction between the shared electrons and nuclei of atoms increases. OR The larger the bond energy, the stronger the covalent bond is and the stronger the forces of attraction between the shared electrons and nuclei of atoms. Hence, the atoms are pulled closer together causing a shorter bond length. 1 1

6 (c)(i) ΔH = Energy absorbed during bond breaking – Energy released during bond forming = (435 + 193) – 2 x 366 = –104 kJ 1 (c)(ii) 2 Energy Progress of reaction (d) Reaction of iodine with hydrogen is slower than the reaction of hydrogen with chlorine because chlorine is more reactive than iodine. Reaction of iodine with hydrogen is exothermic and the reaction of hydrogen with chlorine is endothermic because the energy absorbed for bond breaking is less than the energy released from bond forming when chlorine reacts with hydrogen/ the energy absorbed for bond breaking is more than the energy released from