2021 RVHS Prelim Paper 3 QP

River Valley High School 9729/03/PRELIMS/21 2021 Preliminary Examination [Turn over RIVER VALLEY HIGH SCHOOL JC 2 PRELIMINARY EXAMINATION CANDIDATE NAME CLASS 2 0 J CENTRE NUMBER S INDEX NUMBER H2 CHEMISTRY 9729/03 Paper 3 Free Response 21 September 2021 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and index number in the spaces at the top of this page. 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 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. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Question Number 1 2 3 4 5 s.f. units Total Marks 20 22 18 20 20 80 This document consists of 32 printed pages and 0 blank page.

2 River Valley High School 9729/03/PRELIMS/21 2021 Preliminary Examination Section A Answer all the questions in this section. 1 In a 2014 paper published in the Journal of Agricultural and Food Chemistry, Hendon and Colonna-Dashwood discovered the effect of water hardness on coffee flavour. C ompounds in hard water tend to attach to the flavo urful elements in roasted coffee beans during brewing. Water with higher levels of magnesium will likely extract more flavour from a coffee bean. Water described as "hard" is high in concentration of Total Dissolved Solids (TDS) , specifically calcium and m agnesium. The hardness of water may be reported in parts per million (ppm). The solute concentration of a dilute aqueous solution in units of mg dm3 is called parts per million, or ppm. Classification ppm Soft 0  17.1 Slightly hard 17.1  60.0 Moderately hard 60.0  120 Hard 120  180 Very hard > 180 (a) In a sample of Singapore’s tap water, the concentration of magnesium and calcium ions present are found to be 5.97  105 mol dm 3 and 5.49  104 mol dm3 respectively. These two ions can be separated by selective precipitation with potassium hydroxide. The numerical values of solubility product of magnesium hydroxi de and calcium hydroxide at 25 ° C are 1.50  1011 and 5.50  106 respectively. (i) Calculate the total concentration of magnesium and calcium ions in ppm, and hence classify the hardness of water in this sample of tap water. [2] (ii) Calculate the minimum pH of the solution at which the magnesium ion precipitates as magnesium hydroxide. [2] (iii) The magnesium hydroxide continues to precipitate out of the solution as potassium hydroxide is being added continuously. Eventually, the concentration of the hydroxide becomes high enough to precipitate the calcium ions as well. What is the concentration of magnesium ions when calcium ions begin to precipitate? [2]

3 River Valley High School 9729/03/PRELIMS/21 2021 Preliminary Examination [Turn over …………………………………………………………………………………….............. …………………………………………………………………………………………....... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… …………………………………………………………………………………….............. …………………………………………………………………………………………....... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… …………………………………………………………………………………….............. …………………………………………………………………………………………....... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… …………………………………………………………………………………….............. …………………………………………………………………………………………....... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… …………………………………………………………………………………….............. …………………………………………………………………………………………....... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………..............

4 River Valley High School 9729/03/PRELIMS/21 2021 Preliminary Examination …………………………………………………………………………………………...... (b) A balanced extraction is a well -brewed cup of coffee that is aromatic and rich in flavours. Eugenol is a flavour note with a “woodsy” taste found in coffee, wine and whisky. Eugenol Like other alkenes, it undergoes hydrohalogenation when treated with hy drogen halides. (i) Draw a label led diagram showing the orbital overlap between the carbon atoms C1 and C2 and state the hybridisation involved. Do not include other atoms. [2] (ii) Hydrohalogenation of unsymmetrical alkenes results in a mixture of products. In such cases, the major product can be predicted using Markovnikov’s rule. Describe the mechanism of the reaction between eugenol and hydrogen chloride. You may represent eugenol using . [2] (iii) With reference to your mechanism in (b)(ii), explain why the major product is formed. [2] …………………………………………………………………………………….............. …………………………………………………………………………………………....... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… …………………………………………………………………………………….............. …………………………………………………………………………………………....... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ………………………………………………………………………………………………

5 River Valley High School 9729/03/PRELIMS/21 2021 Preliminary Examination [Turn over …………………………………………………………………………………….............. …………………………………………………………………………………………....... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… …………………………………………………………………………………….............. …………………………………………………………………………………………....... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… …………………………………………………………………………………….............. …………………………………………………………………………………………....... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… (c) Hydration of alkenes via hydroboration favours formation of the anti –Markovnikov product. The hydrobora tion reaction involves 2 stages: first with limited borane, BH 3, followed by treatment with alkaline hydrogen peroxide. R = alkyl/aryl group It is suggested that the mechanism goes through the formation of the intermediate below. C C BH3

6 River Valley High School 9729/03/PRELIMS/21 2021 Preliminary Examination (i) The initial reaction between the alkene and borane can be considered to occur in a similar fashion as the hydrohalogenation reaction in (b)(ii). Suggest the mechanism showing the formation of the intermediate given above when eugenol reacts with borane. In dicate clearly the polarity of the B–H bond in borane by drawing + and – on the appropriate atoms. [2] (ii) By determining the change in the oxidation number of the reactive carbon , suggest the role of hydrogen peroxide in Step 2 in the reaction. [1] …………………………………………………………………………………….............. …………………………………………………………………………………………....... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… …………………………………………………………………………………….............. …………………………………………………………………………………………....... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… …………………………………………………………………………………….............. ……………………………………………………………………