2020 SNGS Prelim Answers

2020 SNGS Sec 4 OP Prelim 2020 Sec 4 Prelim P1 Answers 1 D 21 C 2 A 22 B 3 D 23 B 4 D 24 C 5 C 25 B 6 C 26 A 7 B 27 C 8 A 28 B 9 D 29 B 10 D 30 B 11 B 31 D 12 C 32 C 13 C 33 A 14 D 34 C 15 D 35 C 16 B 36 D 17 B 37 B 18 B 38 D 19 C 39 C 20 A 40 C

2 2020 Sec 4 Prelim P2 Answers Section A A1 (a) (i) brass [1] (ii) nitrogen monoxide [1] (iii) Aluminium/ Ammonium chloride [1] (iv) Ammonium chloride [1] (v) water [1] (b) Mix aqueous sodium carbonate and aqueous silver nitrate. [1] Filter the mixture [0.5] to obtain residue.[0.5] Wash with plenty of water [0.5] and dry between sheets of filter paper. [0.5] Wrong reagent max 1m [3] A2 (a) Closely together , disorderly [0.5] Slide over each other [0.5] Unacceptable : closely packed [1] (b) R : Group II [1] Q : Group VI [1] [2] (c) RCl2 has a giant ionic structure [0.5] The elec trostatic forces of attraction between oppositely chargely io ns are weakened [1] The ions are mobile for the conduction of electricity.[0.5] QCl2 has a simple covalent/molecular structure [0.5m] consisting discrete molecules [0.5m] which are electrically neutral. [0.5m] There are no mobile electrons/ions for the conduction of electricity. [4] (d) Dilute/aqueous hydrochloric acid [1] [1] (e) Any macromolcules [1]

3 A3 (a) name of halogen Melting point /oC Boiling point /oC bromine -7.2 58.8 chlorine -100.9 -34.7 Iodine 113.8 184.5 [1] (b) Reactivity decreases down the group [0.5] atomic size / electron shell /radius increases down the Group [0.5] OR distance between nucleus and valence shell increases [0.5] electrostatic forces of attraction between nucleus and electron decreases [0.5] gains electron less easily[0.5] [2] (c) (i) seawater turns reddish brown/orange[1] Chlorine is more reactive than bromine. [0.5] Chlorine displace bromine from its solution [0.5] [2] (ii) Add dilute nitric acid followed by aqueous barium nitrate [1] white precipitate observed.[1] [2] (d) Chlorine is oxidised [0.5] as the oxidation state of chlorine increases from 0 in Cl2 to +3 in ClF3. [1m] reducing agent [0.5] [2] A4 (a) carbon dioxide / CO2 [1] (b) Zinc ion [0.5] and copper(II) ion[0.5] When aqueous ammonia is added, precipitate is formed and dissolves in excess aqueous ammonia.[1] [2] (c) ZnCO3→ ZnO + CO2 Or CuCO3 → CuO +CO2 [1] (d) Ag+ +Cl- → AgCl [1] [1] (e) All the chloride ion have been precipiated/reacted[1] [1] A5 (a) Concentrated [0.5] copper(II) bromide [0.5]/ Concentrated CuBr2 [1] (b) (i) Electrode C : 2H+ +2e → H2 [1] Electrode D : 4OH- → O2 + 2H2O + 4e [1] [2] (ii) Haber process/ rocket fuel/fuel cell [1] (c) Mole of H2 = 0.084/24 = 0.0035 mol [0.5] Mol of copper = 0.0035 mol[0.5] Mass of copper = 0.0035x64= 0.224g [1] [2]

4 (d) Universal indicator change from green to blue/purple [1] Chloride ions are selectively discharged [0.5] Potassium hydroxide is formed which is alkaline [0.5] OR Universal indicator change from green to colourless. [1] Chloride ions are selectively discharged [0.5] Chlorine gas is formed [0.5] at the anode [2] A6 (a) It consists of atoms [0.5] of different sizes. [0.5] The orderly arrangement of atoms is disrupted. [0.5] The layers of atoms cannot slide over each other easily when a force is applied. [0.5] [2] (b) Iron in steel is more reactive than copper [1] Iron displace copper from its solution [1] [2] (c) blue [0.5] solution turns green [0.5] reddish brown/pink solid formed. [1] [2] (d) Calcium reacts with water in copper (II) sulfate solution [0.5] to form hydrogen gas [0.5] Ca + 2H2O→ Ca(OH)2 +H2 [1] [2] (e) Write the ionic equations at anode and cathode. Anode : Fe → Fe2+ +2e [1] cathode : 2H++2e → H2 [1] [2] Section B B7 (a) Zinc is more reactive than iron. [0.5] Zinc will lose electron more easily than iron [1] Thus Zinc corrodes in place of iron. [0.5] [2] (b) Carbon dioxide is an acidic oxide / acidic gas [1], and thus zinc oxide reacts with it as a base. [1]

5 (c) High cost of electricity [1] Zinc cyanide is toxic/harmful [1] [2] (d) Correct electrodes and terminal of battery [1] Correct electrolyte [0.5] Fully submerged [0.5] [2] (e) For hot dip galvanisation, the hot zinc reacts more easily with oxygen to form the oxide layer[1]. The zinc layer coated using electro galvanisation is at room temperature thus may not react easily with oxygen to form oxides. [1] [2] (f) Chemical reduction of metal oxide using CO/C. Unacceptable : electrolysis [1] B8 (a) Nitrogen [1] [1] (b) Catalytic converter [1] [1] (c) The temperature of lean engine is lower than normal engine.[0.5] Nitrogen reacts with oxygen in air to form nitrogen oxides slowly at lower temperature. [0.5] [1] (d) Form a very stable/process is irreversible[1] compound carboxyhaemoglobin[0.5] Prevent the transport of oxygen around the human body[0.5] [2]

6 e) Lightning/ forest fire [1] (f) (i) Mol of CO2 = 137500/5500 x 8 =200mol [1] Volume of CO2 = 200 x 24 =4800dm3[1] [2] (ii) Correct reactant and product and exo [1] Label Ea and arrow[0.5] Label ΔH and arrow[0.5] Unacceptable : endo [2] [Total: 10 marks] E B9 (a) PbCO3 + 2HNO3 → CO2 + H2O + Pb(NO3)2 [1] [1] (b) Mole of CO2 = 0.1/24 = 0.004167 mol [0.5] Mol of PbCO3 = 0.004167 mol [0.5] Mass of PbCO3 = 0.004167 x 267 =1.11g [1] [2] (c) An insoluble layer [0.5] of lead(II) sulfate [0.5]will form /coat around lead(II) carbonate, preventing further reaction. [1] [2]

7 (d) Initial mass for both experiments [0.5] Final mass for both experiments [0.5] Rate for both experiments [1] Wrong shape –1 m Ethanoic acid is a weak acid [0.5] there are less H+ ions per unit volume/concentration [0.5m] leading to lesser frequency of collision between H+ ions and Mg [0.5m] resulting in lesser frequency of effective collision. [0.5m] [4] (e) carbonate used time taken for white precipitate to form in limewater/ s Iron(II) carbonate 50 Lead(II) carbonate 30 [1] O B9 (a) So that the concentration of thiosulfate is proportional to volume of thiosulfate solution [1] [1] (b) (i) 40[0.5], 10[0.5], 10[0.5] 14 [0.5] [2] (ii) The speed increases [0.5] concentration of thiosulfate increases[0.5] increase in the frequency of collisions between reactant particles [0.5] increase in frequency of effective collisions [0.5] [2] I III

8 (c) The time taken for the cross to disappear in a smaller beaker will be shorter [1] as a smaller amount of sulfur is needed to cover the cross [1] as its base is smaller [2] (d) When the temperature is increased, the particles gain kinetic energy/ move faster. [0.5] There is also an increase in the proportion of particles having energy equal or more than the activation energy.[0.5] Frequency of collision increases [0.5] Hence chances of effective collisions increase [0.5] [2] (e) Acidified potassium manganate (VII) turns from purple [0.5] to colourless.[0.5] [1]