Copy of ACSBR 6092 Sec 4 Prelims 2024 P2 ANS

1 Marker’s Report and Answers to 2024 Preliminary Examination 6092/2: No. Answers Marks A1ai Li+ Some candidates carelessly did not put the charge. [1] A1aii Ca2+ and Cl− Some candidates only wrote one ion and did not consider the rest. [1] A1aiii Ca2+ and O2− Many candidates did not realise about comparing the other possible ionic compound other than NaCl. [1] A1b Solution remains reddish-brown; ACCEPT no observable change OR solution turns reddish-brown (since aqueous bromine is added) Bromine is less reactive than chlorine hence bromine cannot displace chloride ions from its solution (recall the ionic equation!); Similar question to 2023 TYS A1. Common mistake is the usual one in bold above. [2] A2a methyl ethanoate This whole question is also from TYS. Some candidates could not spell the name of the ester. [2] A2b H2O ; HCl ; Some candidates did not read the bolded word in the question and gave the molecular formulae of the ester instead. [2] A3ai To slow down the reaction/decomposition of hydrogen peroxide; OWTTE REJECT prevent decomposition Many candidates mistakenly thought by putting substances at a lower temperature will stop decomposition entirely rather than slow the process down. [1]

2 A3aii Correct number of bonded and unbonded outer electrons with different symbols labelled with oxygen; Correct brackets and charge; Exactly same question as 2023 TYS A4. Many candidates could not obtain full marks as some drew hydrogen peroxide instead or did not notice the charges on the ion or complete the shells with gaining two electrons. [2] A3b S2O8 2- Some candidates forgot to include the charge while others could not do this typical exam question on formula writing. [1] A3ci Na2O2 + 2H2O ⟶ 2NaOH + H2O2 Many candidates obtained credit for this question except those who did not read the question and wrote sodium oxide instead. Some candidates did not balance the equation. [1] A3cii The reaction of sodium peroxide with water produces an alkaline solution of sodium hydroxide and hence Paul thought the indicator solution will turn blue; However hydrogen peroxide, which is also produced in the reaction, is a bleach and hence the solution will turn colourless; hence this does not support Paul’s statement. REJECT ‘basic’ as this includes insoluble oxides. Any contradiction results in no mark at all. Similar question to 2023 TYS A4. Many candidates did not read the entire question and hence only answered partially. [2] A3di C + 2O ⟶ CO2 Many candidates either did not notice the bolded word and wrote oxygen gas instead of oxygen atom. Some also did not prepare for this exam to realise the greenhouse gas in the question is not CO. [1] A3dii increased levels of greenhouse gases cause global warming which will cause extreme weather events / melting of polar ice OWTTE; Many candidates did not write the final effects of the global warming and just stopped at writing temperature increases. This answer above is straight from the syllabus and notes. [1]

3 A4a correct chemical equation on the correct lines; correct balanced chemical equation; Once shape of graph is correct, check for correctly labelled activation energy; and enthalpy change; REJECT enthalpy change and/or activation energy if no dotted lines are drawn or obviously drawn without a ruler. REJECT -H (based on O level markers report) If Endothermic graph is drawn, mark only for the balanced chemical equation (max 2 marks) Many candidates either did not balance the equation or got the formula of butane wrong. Most candidates remembered to use a ruler to draw the single-headed arrow and dotted line as they were advised during WA. Only a few candidates wrongly drew an endothermic graph for a combustion reaction. [4] A4b energy released per dm3 of gas = 120 kJ energy released per 24 dm3 of gas = 120 x 24 = 2880 kJ/mol or kJ. REJECT if wrong units are written (e.g. in small ‘j’ instead of J) Some candidates did not read that butane is a gas at r.t.p. and hence they need to use 24 dm3. [1] A4c Requires compression/liquification for storage/transportation OR butane (from crude oil) is non-renewable; REJECT flammable as all fuels are flammable. REJECT higher temperature or incomplete combustion Many candidates correctly wrote ‘non-renewable’ answer as they recalled butane is from crude oil. [1] A4d At a lower pressure, there are fewer molecules/particles per unit volume; hence butane and oxygen molecules/particles collide less often with lower frequency of effective collisions; Therefore rate of reaction decreases. REJECT if no ‘particles’ is mentioned (no mark for entire question) Many candidates did not study their ‘collision theory’ key phrases in the notes thoroughly to obtain full credit. [2] 2 C4H10 + 13 O2 8 CO2 + 10 H2O energy progress of reaction ΔEa 5760 kJ 2 C4H10 + 13 O2 8 CO2 + 10 H2O energy progress of reaction ΔEa 5760 kJ H Ea

4 A4e (Propane and butane exist as simple covalent molecules) Propane consists of smaller molecular size than butane; and hence has weaker intermolecular forces of attraction which require less energy to overcome; REJECT “smaller molecular mass”, weak intermolecular forces, ‘break forces’ Many candidates did not study the key phrases in the notes thoroughly to obtain full credit. Some mistakenly thought bonds are the same as forces of attraction. Some mistakenly thought one had a giant molecular structure. [2] A5a The percentage of ammonia present at equilibrium decreases with increasing temperature but percentage of ammonia present at equilibrium increases with increasing pressure. (Quote examples from table) Most candidates obtained full credit. [1] A5bi 100 °C and 1000 atm REJECT wrong units Most candidates obtained full credit except those who forgot the units. [1] A5bii At 100 °C, temperature is low causing the reaction to be too slow to proceed; At 1000 atm, the high pressure makes it costly to maintain at such a pressure OR costly using thick containers to withstand the high pressure of the reaction; OWTTE (reason for high cost must be mentioned) Some candidates did not read the entire data to notice 100 °C is a relatively low temperature whereas 1000 atm is a relatively high pressure. Generally the high pressure part was well explained. [2] A5c Moles of nitrogen = 1000 000 / 28 = 35714.29 Mole ratio of nitrogen: ammonia = 1:2 Moles of ammonia = 2 × 35714.29 = 71428.57; (OR 71429) Mass of ammonia = 71428.57 × 17 × 38.8% = 471 143 g = 471 000 g (to 3 sf); OR 0.471 tonnes (to 3 sf) Many candidates obtained full credit for this typical mole question except those who overlooked the mole ratio or miscalculated the Mr. Some carelessly rounded off wrongly or did not include units. [2] A5di Oxidation state of N increases from -3 in NH3 to 0 in N2; hence ammonia is oxidised; REJECT 3- which is the charge Many candidates obtained full credit except those who could not phrase this typical Redox answer straight from the notes. Many miscalculated the oxidation states. [2] A5dii There will be (unreacted or more or excess) nitrogen monoxide released into the atmosphere together with (less) nitrogen (and water vapour/steam) produced; REJECT not all NO reacted (with no mention of it in the gases released) Many candidates did not read to understand the question of ‘composition of gases’ from the equation. Some candidates who obtained credit carefully wrote about all the substances in the equation. [1]

5 A5diii NO will react with air/oxygen to form NO2 which reacts with moisture and air (or water and oxygen) to form nitric acid which reacts/dissolves in water; to form acid rain which corrodes limestone buildings/kills aquatic life OWTTE;