2021 SNGS Prelim Answers

2021 SNGS Prelim MARKING SCHEME PAPER 1 1 B 11 D 21 A 31 C 2 D 12 D 22 B 32 C 3 B 13 D 23 D 33 D 4 B 14 D 24 C 34 A 5 C 15 A 25 C 35 D 6 D 16 A 26 C 36 C 7 C 17 B 27 A 37 B 8 A 18 B 28 C 38 A 9 B 19 B 29 C 39 B 10 C 20 B 30 B 40 C PAPER 2 A1 (a) (i) B has Weak intermolecular force of attraction [0.5] between simple molecules[0.5] Little amount of energy required to overcome forces of attraction[0.5] A has strong covalent bonds between the atoms [0.5]which extends throughout the vast network[0.5] A lot of energy required[0.5] [3] (ii) Giant metallic structure [0.5] E has strong electrostatic forces of attraction between cations and sea of delocalised electrons.[0.5] Mobile electrons[0.5] Giant ionic structure [0.5] solid D has opposite charged ions held fixed in position in solid state[0.5] no mobile ions.[0.5] [3] (b) Any group I [1] (c) iron [1]

2 A2 (a) Filter the mixture [0.5] to obtain residue[0.5] Wash residue with distilled water[0.5], completely dry the residue with sheets of filter paper[0.5] [2] (b) The universal indicator will turn blue[1] [1] (c) Silver ion has lower Ar/Mr than dichromate ion Silver ion diffuse faster than dichromate ion [1] Red solid formed nearer to ammonium dichromate[1] 2Ag+ + Cr2O72- → Ag2Cr2O7 [1] [3] (d) White solid [0.5] , silver chloride [0.5] occurs near to silver nitrate than ammonium chloride[0.5] Silver ion has higher Ar than chloride ion [0.5] [2] A3 (a) liquid hydroxypropanoic acid does not dissociate [0.5] to form mobile ions.[0.5] OR liquid hydroxypropanoic acid exists as simple molecules. Molecules are electrically neutral [0.5] No mobile ion[0.5] [1] (b) hydroxypropanoic acid : Universal indicator turns from green to orange/yellow.[0.5] hydroxypropanoic acid is weak acid. hydroxypropanoic acid partially dissociate to form H+[1] sulphuric acid : Universal indicator turns from green to red. [0.5] sulfuric acid is strong acid. sulfuric acid completely dissociate to form H+[1] [3] (c) Mole of NaOH= Mole of acid = 0.0232 x 0.1 = 0.00232 mol [1] Mr of acid = 0.172/0.00232 = 74.1 [1] [2] A4 (a) carbon dioxide[0.5], carbon monoxide[0.5]. [1] (b) Iron more reactive than hydrogen[1] hydrogen ions are preferentially discharge/reduced than iron (III) ion[1] [2]

3 (c) Anode : 2O2- → O2 + 4e [1] Cathode : Fe3+ +3e →Fe [1] [2] (d) 2Fe2O3 → 4Fe + 3O2 Mole of Fe2O3 = 10x 106/160= 62500 mol [1] Mole of Fe = 62500 x 2 =125000 mol[1] Mass of Fe = 125000x 56 = 7 tonnes.[1] [3] (e) Aluminium oxide is very stable.[1] Alumium oxide cannot be reduced by carbon/hydrogen/carbon monoxide[1] [2] A5 (a) Beaker Y as the electrodes are metal of different reactivity[1] Metals are connected in an electrolyte[1] [2] (b) [2] (c) Electrode B : Effervescence, colourless and odourless gas at the anode.[1] Electrode A: Cathode become bigger/ reddish brown solid coated on the cathode[1] [2] (d) The electrode B become smaller/no gas evolved Electrode B oxidise to form copper (II)ions [1]as it is a reactive electrode. OR the electrolyte remain blue. [1] concentration of Cu2+ remain the same.[1] [2] A6 (a) A,C,D,B [1] (b) Carbonate of Metal A. A is the least reactive metal. [1] Its carbonate has the lowest thermal stability.[1] [2]

4 (c) (i) Metal A: No visible change/reaction [1] Metal D: pale green solution becomes colourless [0.5] and grey solid produced.[0.5] [2] (ii) Add excess iron/iron(II)oxide/iron(II) hydroxide /iron(II) carbonate to dilute sulfuric acid [1] Filter the mixture to obtain the filtrate[0.5] Heat to saturation and cool [0.5]Filter mixture to obtain residue[0.5] Wash residue with a small amount of distilled water[0.5] , completely dry the residue with sheets of filter paper[0.5] [3] (d) It will not rust. D is more reactive than iron/ D loses electron more readily than iron[1] It corrode in place of iron.[1] [2] Section B B7 (a) Structural formula of CF3CF3 [1] (b) HFCs do not contain chlorine atoms, which deplete the ozone layer [1]. [1] (c) Two fluorine atoms [1] (d) HCFC-141a HCFC-141b [2] (e) (i) UV light [1] (ii) CH4 + F2 → CH3F + HF[1] CH3F + F2 → CH2F2 + HF[1] [2] (iii) Fluorine has a smaller atomic radius than bromine/valence electrons are closer from the nucleus [0.5] Electrostatic forces of attraction between the nucleus and valence electrons are stronger [0.5]. valence electrons are more easily gain [0.5] Fluorine will react more vigorously than bromine.[0.5] [2]

5 B8 (a) Manganese(IV) oxide is a better catalyst because graph 1 has a steeper gradient [1] [1] (b) From the graph, volume of O2 gas = 120 cm3 No. of mol of O2 = 120/24000 = 0.005 mol [1] Hence, from the equation, no. of mol of H2O2 = 0.005X2 = 0.01 mol[0.5] Concentration = 0.01/ 50 X 1000 = 0.200 mol/dm3[0.5] [2] (c) [2] (d) Increase temperature of hydrogen peroxide OR used powdered catalyst [1] (e) The energy absorbed to break bonds in 2 mole of H2O2 is lesser than [1] the energy released to form bonds in 2 moles of water and 1 mole of oxygen.[1] [2] (f) Hydrogen peroxide is a reducing agent. Hydrogen peroxide is oxidised.[1] Oxidation state of oxygen increases from -1 in H2O2 to 0 in O2.[1] [2] E B9 (a) Reduction. [0.5]Nitrogen gain electron to form ammonium ion[0.5] [1] (b) 2 NO3- + 12 H+ + 10 e- → N2 +6 H2O [1] (c) Add aqueous sodium hydroxide and warm, test with moist red litmus paper. If red litmus paper turns blue, ammonium ion present.[1] If moist red litmus paper turns blue only when Al added, nitrate ion is present. [1] [2] (d) (i) The sudden increase in volume occurs because the number of moles of gas produced is 1.5 times that of the reactant. Volume goes up by 1.5 times.[1] Rate of combustion increases as oxygen is liberated by the process. [1] Concentration of oxygen increases. [1] [3] I

6 (ii) nitrogen dioxide dissolves in rain water and is oxidized by atmospheric oxygen to form acid rain [1] Acid rain will corrode buildings made of cement, reactive metals and limestone.[1] [2] (iii) 2CO + 2NO → N2 + 2CO2 [1] O B9 (a) (i) 4x410 + 2x 460 – 3x436 –y = 206 [1] y = 1640 +920 -1308 -206= 1046 kJ/mol[1] [2] (ii) Triple bond [1] (iii) Mole of H2 = 1000/24=41.667 mol [1] Energy change = 206/3 x 41.667 = + 2860kJ[1] [2] (b) (i) 2H2 → 4H+ + 4e [1] (ii) Hydrogen is oxidized in a redox reaction by losing electrons to oxygen. [1]The electrons given out by hydrogen flow through the external circuit [1], which constitutes the electric current. [2] (iii) [2]