2024 Prelims YIJC H2 Chem P3 (Ans)

©YIJC [Turn over YISHUN INNOVA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME TEACHERS’ COPY CG INDEX NO CHEMISTRY Paper 3 Free Response Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 12 September 2024 2 hours READ THESE INSTRUCTIONS FIRST This document consists of 30 printed pages and 2 blank pages. For Examiner’s Use Section A 1 / 21 2 / 18 3 / 21 Section B 4 or 5 / 20 Penalty units significant figures Overall / 80 Write your name, class and index number on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all the questions. Section B Answer one question. 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.

2 ©YIJC [Turn over Section A Answer all the questions in this section. 1 (a) The tripeptide A is made by reacting amino acids B, C and D together. Table 1.1 shows the pKa values of the different functional groups present on each amino acid. Table 1.1 amino acid pKa of –carbonyl group pKa of –amino group pKa of side chain B 2.09 9.10 − C 1.88 9.60 3.65 D 2.02 8.80 − (i) Explain the term -amino acid. [1] -amino acid is one with both the –NH2 group and the –CO2H group bonded to the same carbon atom. (ii) One of the amino acids, B, present in the tripeptide A contains more than one chiral carbon atom. Draw the displayed formula of B. [1]

3 ©YIJC [Turn over (iii) Draw the predominant species of C at pH 4.0. [1] (iv) Suggest a pH at which the predominant species of C is a zwitterion using the information in Table 1.1. [1] pH 2.00 (accept any value between 1.88 and 3.65) (v) Explain why the side-chain of D does not have a Ka value. [1] The amide side chain is neutral because the lone pair of electrons on the N atom in –CONH2 group is not available to accept H+ as it is delocalised into the (electron-withdrawing) C=O group.

4 ©YIJC [Turn over (b) 1,2-diaminopropane can be synthesised from propene by the following route. (i) Outline the mechanism for step 1 to show the formation of 1-bromo-2-chloropropane. Show all charges, relevant lone pairs and show the movement of electron pairs using curly arrows. [2] Electrophilic Addition Step 1: Step 2: (ii) Use your mechanism to explain the preferential production of 1-bromo-2-chloropropane over 2-bromo-1-chloropropane. [1] The additional electron-donating alkyl group in the secondary carbocation disperse the positive charge on the C atom [√] more effectively. 1-bromo-2-chloropropane is preferentially formed as it came from a more stable [√] secondary carbocation as compared to the primary carbocation of 2-bromo-1- chloropropane. (iii) State the reagents and conditions for step 2. [1] NH3 (excess) in ethanol (or alcohol) or concentrated NH3, heat in sealed tube.

5 ©YIJC [Turn over (iv) 1,2-diaminopropane can react with bromoethane to form compound E, C7H18N2. Suggest the structure for E. [1] or or (c) Alkanes are very unreactive. However, alkanes such as butane, C 4H10, do react with chlorine. There are two possible monochloroalkanes that can be formed from butane. One of the monochloroalkanes exhibits stereoisomerism. (i) Suggest two reasons why alkanes are generally unreactive. [2] The C-H bond is non-polar. The C-H bond does not break heterolytically and only homolytically. The C-C and C-H bonds are strong. (ii) Predict the relative proportions of 1-chlorobutane and 2-chlorobutane that are likely to be produced from C4H10. Explain your answer. [2] Ratio of CH3CH2CH2CH2Cl and CH3CHClCH2CH3 = 6 : 4 = 3 : 2 Based on the different types of hydrogen, there are 6 possible H atoms which can be substituted to form 1-chlorobutane, while only 4 H atoms can be substituted to form 2-chlorobutane. (iii) Draw three-dimensional structures for the two stereoisomers of the monochloroalkane formed from the chlorination of butane. [1]

6 ©YIJC [Turn over (d) Use the data in Table 1.2 , together with relevant data from the Data Booklet, to calculate the average bond energy of the Si-Cl bond in SiCl4. Show your working. Table 1.2 enthalpy change of vapourisation of SiCl4 molecules, SiCl4(l) → SiCl4(g) = +29 kJ mol−1 standard enthalpy change of atomisation of Si(s) = +338 kJ mol−1 standard enthalpy change of formation of SiCl4(l) = −687 kJ mol−1 [3] average bond energy of the Si-Cl bond = [ -(+29) –(−687) + (+338) + 2(244) ]  4 = 371 kJ mol−1 (e) Describe the reaction of SiCl4 with water, stating the pH of the resulting mixture. [1] SiCl4(l) + 2H2O(l) → SiO2(s) + 4HCl(g) pH = 1 or 2 [] 4 × BE (Si-Cl)] +338 −687 energy / kJ mol−1 SiCl4(l) Si(s) + 2Cl2(g) Si(g) + 2Cl2(g) Si(g) + 4Cl(g) 2 × BE (Cl2) = +2(244) SiCl4(g) +29

7 ©YIJC [Turn over SiCl4 which is covalent undergoes complete hydrolysis in water vigorously [ ]to give strongly acidic solution. (f) Elements in Group 17 are known as the halogens. State how the reactivity of the halogens as oxidising agents varies down the group, and relate this variation to relevant Eo values. [2] Cl2 + 2e− ⇌ 2Cl− Eo = +1.36 V Br2 + 2e− ⇌ 2Br− Eo = +1.07 V I2 + 2e− ⇌ 2I− Eo = +0.54 V Down the group, the reactivity of the haloge