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