2024 MI PU3 MYE P2 (Answers) with marker's comment

Class Adm No Candidate Name: This question paper consists of 16 printed pages and 2 blank pages. 2024 Mid-Year Exams Pre-University 3 H2 CHEMISTRY 9729/02 Paper 2 Structured Questions 28 June 2024 2 hours Candidates answer on the Question paper. Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so. Write your name, class and admission number on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Question 1 2 3 4 5 Total Marks 15 16 15 16 13 75

2 Answer all questions. 1 Ibuprofen, aspirin, and paracetamol are commonly sold as over -the-counter painkillers that offer versatile relief for different forms of pain. Table 1.1 shows the physical properties of the painkillers and some organic compounds. For Examiners’ Use Table 1.1 compound Mr melting point / °C solubility in water / g dm–3 solubility in ethanol / g dm–3 density / g cm–3 ibuprofen (C13H18O2) 206.0 75 0.021 1.2 1.03 aspirin (C9H8O4) 180.0 135 3.0 80 1.40 paracetamol (C8H9NO2) 151.0 169 12.8 130 1.26 phenol (C6H6O) 94.0 40.5 84.2 highly soluble 1.07 ethanoyl chloride (C2H3OCl) 78.5 –112 – – 1.10

3 [Turn over A student was provided with three unlabelled tablets, one from each of the three different painkillers. To identify them, he crushed each of the tablets and dissolved them in ethanol. To a small portion of each solution, he conducted two simple chemical tests to distinguish between the painkillers. For Examiners’ Use (a) (i) Define the term relative molecular mass, Mr. [1] The weighted average mass of one molecule relative to the 1/12 the mass of a 12C atom. Marker’s comment: - Poorly done, many students wrote C12 (ii) Suggest, briefly, two reasons for the difference in solubilities between ibuprofen and paracetamol in water. [2] Stronger instantaneous dipole -induced dipole forces of attraction between ibuprofen molecules due to the larger electron cloud size (Mr = 202.0) than paracetamol molecules (Mr = 151.0) OR longer non-polar alkyl chain ; Paracetamol molecules form more extensive hydrogen bonding with water molecules (due to greater number of electronegative O, N atoms and lone pairs available) than ibuprofen molecules. ; Marker’s comment: - Very poorly done, with many students only giving one reason - Many students did not make mention to main keyword of electron cloud size / no. of electrons - Students who counted the no. of H-bonds did not correctly identify the number (iii) Suggest the reagents and conditions, stating the observations, for the two simple chemical tests used to distinguish between ibuprofen, aspirin, and paracetamol. [4] To 1 cm3 of each solution, add neutral FeCl3(aq). ; Paracetamol will give a dark / violet colouration. For aspirin and ibuprofen, no dark / violet colouration OR neutral FeC l3(aq) remains yellow / orange. ; OR To 1 cm3 of each solution, add Br2(aq). ; For paracetamol, orange Br2(aq) decolourises (and white ppt forms). For aspirin and ibuprofen, Br2(aq) remains orange. ; OR To 1 cm3 of each solution, add Na2CO3(s). ; For aspirin and ibuprofen, effervescence of a gas that forms white ppt in Ca(OH)2(aq). For paracetamol, no effervescence is observed. ; To another 1 cm3 of each solution, add KMnO4(aq) in H2SO4(aq), heat/warm. ; For ibuprofen, purple KMnO 4(aq) decolourised (and effervescence which produces white ppt in limewater). For aspirin and paracetamol KMnO4(aq) remains purple. ; Marker’s comment: - Many students were still unable to identify the functional groups successfully (e.g. confused between ketone/aldehyde/ester/acid).

4 (b) The student planned to synthesise aspirin from phenol and ethanoyl chloride (in 1 : 2 molar ratio) via an acylation reaction to form an intermediate A, before turning it into aspirin under controlled conditions in step 2. The liquid ethanoyl chloride was measured out using a graduated gas syringe and added to phenol in an inert solvent. For Examiners’ Use (i) Suggest a catalyst for step 1. [1] step 1: AlCl3 or FeCl3 catalyst (can accept AlBr3 or FeBr3) ; (ii) Draw the structure of the intermediate A. [1] Suggested Intermediate A: ; Marker’s comment: - Some students could not appreciate that the question asked for a “reaction intermediate” (stable structure) and not a “mechanism intermediate” (not so stable structure). (iii) State the reagents and conditions for step 2. [1] step 2: I2 NaOH, warm, followed by dilute acid ; (iv) Calculate the volume of ethanoyl chloride that is needed to completely react with 4.00 g of phenol to form aspirin using the suggested synthetic pathway in (b)(i). [3] amount of phenol used = 4.00 ÷ 94.0 = 0.04255 mol ; mass of ethanoyl chloride required = 0.04255 × 2 × 78.5 = 6.68 g ; volume of ethanoyl chloride required = 6.68 ÷ 1.10 = 6.07 cm3 (3 s.f.) ; Marker’s comment: - Many students did not identify the data that was relevant from the table (e.g. Mr, notice units of density was g cm-3). - A significant number of students still multiplied amount by 24.0, not recognising that the formula only applies for gases since the volume of 1 mol of gas at rtp is 24.0 dm3.

5 [Turn over (v) The resultant aspirin product was washed, dried, and weighed. Given that 5.57 g of aspirin was formed, calculate the percentage yield of the reaction. [1] For Examiners’ Use Theoretical mass of aspirin = 0.04255 × 180.0 = 7.66 g [no marks, LO tested in (b)(ii)] % yield = (5.57 ÷ 7.66) × 100% = 72.7% ; Marker’s comment: - Generally well done. (vi) Suggest why there are no solubility values for ethanoyl chloride. [1] Ethanoyl chloride reacts readily with both water and ethanol ; (in a nucleophilic acyl substitution reaction to form ethanoic acid and ethyl ethanoate respectively, and HCl). Marker’s comment: - Many students did not recall that ethanoyl chloride reacts vigorously with water, choosing to talk about how it is insoluble instead (ethanoyl chloride is able to form H - bonds with water). An insoluble substance would still have a value for solubility, either a very very small number, or at the ext reme end, “0”. A “ –” implies something else entirely. Students should also note that liquids and gases have solubility values associated to them as well. - Some students recognised the above, but did not talk about ethanol, which is required by the question (“values”). [Total: 15]

6 2 (a) Arrange the following molecules in increasing order of gas phase relative basic strength, explaining your answer. [4] For Examiners’ Use (weakest base) CH3CONH2 < C6H5-NH2 < CH3CH2NH2 (strongest base) ; CH3CONH2 is neutral (not basic) as the lone pair on N is delocalised due to p-orbital overlap with the π electron cloud across the C=O. As O is highly electronegative, the lone pair on N is not available for donation (to H+ usually). ; C6H5-NH2 (phenylamine): Lone pair on N is delocalised due to partial p-orbital overlap of N with π electron cloud of the benzene ring , hence less available for dona