SJI 2021 Y4 Prelim Chemistry 6092 P1 & 2 Answers

1 St. Joseph’s Institution 2021 Year 4 OP Preliminary Exam Answer Scheme Paper 1 1 2 3 4 5 6 7 8 9 10 A C C D B A D D C B 11 12 13 14 15 16 17 18 19 20 B A D C D C B C D C 21 22 23 24 25 26 27 28 29 30 C A B D A B B C D C 31 32 33 34 35 36 37 38 39 40 A D B C C B C C C D Paper 2 Section A Qn Suggested answers Mark A1a 1 A1b To react with the colourless spots to make them visible. 1 A1ci X and Y 1 A1cii L, M, N and P 1 A1d Show measurements on diagram (W and Z) Rf value = W / Z = (answer to 2 d.p) 1 1 TOTAL 6 S W Z

2 Qn Suggested answers Mark A2ai 400 – 450 °C, 200 – 250 atm, finely divided iron catalyst 1 A2aii Calcium hydroxide reacts with ammonium-based fertilisers to form ammonia gas, which escapes into the atmosphere. This results in the loss of nitrogen from the fertilizer which is an important element for plant growth. 1 1 A2bi When aqueous ammonia is added dropwise then in excess, green precipitate formed which is insoluble in excess aqueous ammonia. 1 A2bii Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s) [1m – ionic equation, 1m – state symbols] 2 A2ci Ammonium chloride salt is thermally unstable and decomposes on heating to form hydrogen chloride and ammonia gas. 1 A2cii crystallisation 1 A2d calcium oxide / quicklime 1 TOTAL 9 A3ai An acid is a substance or compound that dissolves and dissociates in water to form hydrogen ions (as the only positive ions). 1 A3aii CH3COOH(aq) ⇌ CH3COO- (aq) + H+ (aq) [1m – equation with reversible sign, 1m – state symbols] 2 A3b Similarity: In both experiments, the colourless solution turns blue / solid dissolves to form a blue solution. Difference: In experiment 1, effervescence is observed which is not observed in experiment 2. 1 1 A3ci 4KI + 2CuCl2 → 2CuI + I2 + 4KCl 1 A3cii The oxidation number of copper decreases from +2 in CuCl2 to +1 in CuI. This is reduction / CuCl2 is reduced. The oxidation number of iodine increases from -1 in KI to 0 in I2. This is oxidation / KI is oxidised. Since oxidation and reduction occurs simultaneously, this is a redox reaction. 1 1 A3d step 1: Add excess zinc oxide to dilute hydrochloric acid. step 2: Filter the mixture. The filtrate is zinc chloride solution. step 3: Heat the filtrate to saturation, leave the solution to cool. Collect the crystals by filtration. step 4: Rinse the crystals with distilled water and dry between pieces of filter paper. 1 1 1 1 TOTAL 12

3 Qn Suggested answers Mark A4a Student A is wrong, Student B is correct. [must state but no mark] If concentration is decreased, the speed of reaction will decrease, however the volume of carbon dioxide formed will also decrease as dilute hydrochloric acid is the limiting reactant / zinc carbonate is in excess. If zinc carbonate granules are used, the total surface area will decrease, hence speed of reaction will decrease. 1 1 A4bi temperature 1 A4bii When temperature decreases, the reactant particles lose kinetic energy and move slower. The number of particles with energy equal or greater than the activation energy decreases. There are lesser collisions, resulting in a lower frequency of effective collisions between reactant particles, hence, the speed of reaction decreases. 1 1 TOTAL 5 A5a Magnesium decreases in size / grey solid is formed around magnesium. Magnesium is a more reactive metal than zinc hence will displace zinc from zinc nitrate solution. 1 1 A5b Iron solid decreases in size. Iron is more reactive than copper hence displaces copper from copper(II) sulfate solution. OR Colour of solution turns from blue to green. Iron displaces copper from blue copper(II) sulfate solution to form green iron(II) sulfate solution. 1 1 A5ci Effervescence of colourless gas is observed on the iron rod. Magnesium loses electrons which flow to the iron attracting hydrogen ions which are selectively discharged as hydrogen gas. 1 1 A5cii The aluminium has a layer of oxide coated on the surface hence it cannot transfer electrons to the iron / cannot conduct electricity 1 1 TOTAL 8 A6ai 2Cl-(aq) → Cl2(g) + 2e 1 A6aii Chloride ions are preferentially discharged because there is a high concentration of chloride ions. 1 A6aiii 12.5 cm3 1 A6bi sulfur dioxide 1 A6bii Sulfur dioxide dissolves in moisture to form sulfurous acid. Sulfurous acid is further oxidized to form sulfuric acid which dissolves in the rainwater as acid rain. SO2 + H2O → H2SO3 2H2SO3 + O2 → 2H2SO4 1 1

4 Qn Suggested answers Mark OR Sulfur dioxide reacts with moisture and oxygen in air to form sulfuric acid, which dissolves in rainwater to form acid rain. 2SO2 + 2H2O + O2 → 2H2SO4 A6biii Flue gas desulfurization 1 A6ci When hydrogen is burned, only water is produced which is not an air pollutant. 1 A6cii When petrol is burned, it produces carbon dioxide which is a greenhouse gas which traps excess heat, causing global warming, leading to melting of ice caps at polar regions causing rise in sea level then flooding of lowland areas (or any other effects) 1 1 TOTAL 10 Paper 2 Section B Qn Suggested answers Mark B7ai The more negative the E° value, the more reactive the metal / The more positive the E° value, the less reactive the metal. 1 B7aii copper, iron, nickel, zinc, magnesium, sodium 1 B7bi +0.80 V [must state but no mark] Silver is less reactive than copper, hence the E° value must be more positive / larger. 1 B7bii Ag+(aq) + e ⇌ Ag(s) 1 B7c The salt bridge is used to close the circuit / maintain electrical neutrality / allow ions to move from one electrolyte to another 1 B7d The decomposition of sodium carbonate produces no products but the decomposition of sodium nitrate produces sodium nitrite and oxygen. The decomposition of zinc carbonate and copper(II) carbonate produces the metal oxide and carbon dioxide / one gas only but the decomposition of zinc nitrate and copper( II) nitrate produces the metal oxide, nitrogen dioxide and oxygen / two gases. 1 1 B7e Potassium nitrite and oxygen Since potassium is more reactive than sodium, and sodium decomposes into sodium nitrite and oxygen, then potassium nitrate should also decompose similarly. 1 1 B7fi any value from 86 to 152 s 1 B7fii Yes. [must state but no mark] The more reactive the metal, the more thermally stable its compound, hence the longer the time taken to collect 50 cm3 of gas. 1

5 Qn Suggested answers Mark Since nickel is more reactive than iron and less reactive than zinc, the time taken to collect 50 cm3 of gas should be between 85 s to 153 s. / Since the time taken for copper(II) carbonate is 44 s and time taken for magnesium carbonate is 221 s, this shows that the time taken for the decomposition of carbonates of more reactive metals is longer. 1 TOTAL 12 B8ai The atomic radius of the Group I element is larger than that of the Group VII element. 1 B8aii For the same period, there are more protons in an atom of a Group VII element than a Group I element hence the electrostatic forces of attraction between the protons and valence electrons are stronger, the valence shell is attracted closer to the nucleus making the atomic radius smaller for Group VII element. 1 1 B8b Reactivity increases down Group I. The atomic radius increases from 152 to 244 pm down Group I, hence the valence electron is further away from the nucleus , hence the electrostatic forces of attraction becomes weaker hence the valence electron is more easily lost. 1 2 B8c The melting point increases down Group VII. As the molecular size / relative molecular mass increa