2019 NJC Prelim P2 QP

[Turn over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/02 Monday 16 September 2019 2 hours READ THE INSTRUCTIONS FIRST Write your subject class, registration number and name on all the work you hand in. Write in dark blue or black pen. You may use a soft 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. 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 1 /17 2 /9 3 /17 4 /18 5 /14 P2 Total /75 Marks Weightings Paper 1 /30 15 % Paper 2 /75 30 % Overall Percentage Paper 3 /80 35 % Paper 4 /55 20 % Grade This document consists of 20 printed pages.

NJC/H2 Chem Prelim/02/2019 2 For Examiner’s Use 1 (a) Brass is a mixture of copper and zinc. When a piece of brass is placed in dilute hydrochloric acid, only one of the metals present dissolves. Explain the observation and write an equation for the reaction that occurs. ………………...…………………………………………………………………….……………… ………………...…………………………………………………………………….……………… ………………...…………………………………………………………………….……………… ………………...…………………………………………………………………….……………… ………………...…………………………………………………………………….……………… ………………...…………………………………………………………………….……………… ………………...…………………………………………………………………….……………[3] (b) Copper and zinc are the two electrodes in a Daniell cell. The cell is set up as shown in Fig 1.1. Fig 1.1 (i) Indicate the direction of electron flow on Fig 1.1. [1] (ii) Write the equation for the reaction taking place and calculate ΔGo for the reaction. ………………...…………………………………………………………………….…… [3]

NJC/H2 Chem Prelim/02/2019 3 For Examiner’s Use [Turn over (iii) Discuss the effect of the following changes on the cell potential of the above cell. (I) adding excess amount of aqueous ammonia into the Zn2+/Zn half-cell ………………...…………………………………………………………………….……….. ………………...…………………………………………………………………….……….. ………………...…………………………………………………………………….……….. ………………...…………………………………………………………………….……….. ………………...…………………………………………………………………….…… [2] (II) using a smaller copper electrode ………………...…………………………………………………………………….……….. ………………...…………………………………………………………………….……….. ………………...…………………………………………………………………….……….. ………………...…………………………………………………………………….……….. ………………...…………………………………………………………………….…… [2] (iv) How many hours will it take for t his galvanic cell to plate 15.0 g of metallic copper from 1 mol dm–3 solutions of Cu2+(aq) and Zn2+(aq) using a current of 5.0 A? [2]

NJC/H2 Chem Prelim/02/2019 4 For Examiner’s Use (v) The Nernst equation is an equation that relates the cell potential, E cell, of an electrochemical reaction to the standard cell potential, Eocell. It can be simplified and written as Nernst equation : Ecell = Eocell – RT zF lnQ where R is the ideal gas constant T is temperature in Kelvin z is the number of electron transferred F is Faraday's constant Q is [Zn2+] [Cu2+] for the Daniell cell Using the above equation, calculate Q for the overall redox reaction of the Daniell cell when it reaches equilibrium at 25 oC. [2] (vi) In another set-up conducted at 25 oC, the concentration of Cu2+ ions and Zn2+ ions have been adjusted to 0.125 mol dm–3 and 0.002 mol dm–3 respectively. Using the Nernst equation, calculate the e.m.f. of this cell. [2] [Total: 17]

NJC/H2 Chem Prelim/02/2019 5 For Examiner’s Use [Turn over Question 2 starts on the next page

NJC/H2 Chem Prelim/02/2019 6 For Examiner’s Use 2 Saccharin is an artificial sweeting agent used in some soft drinks and produced in various ways. Fig 2.1 shows t he original route by Remsen and Fahlberg , starting with methylbenzene undergoing electrophilic substitution reaction using chlorosulfonic acid, C lSO3H. This is also known as aromatic sulfonation process. Fig 2.1 (a) Draw a dot-and-cross diagram of chlorosulfonic acid, ClSO3H and hence state the bond angle for Cl−S−O. Cl−S−O bond angle : ……………. [2]

NJC/H2 Chem Prelim/02/2019 7 For Examiner’s Use [Turn over (b) (i) Reaction I of the aromatic sulfonation process involves the formation of an electrophile. Identify the electrophile. Electrophile : ………………….. [1] (ii) Hence, describe the mechanism for reaction I of the reaction. Show all charges and relevant lone pairs and show the movement of electron pairs by using curly arrows. [2] (iii) State the types of reaction that occur during reactions II and III. reaction II : …………………………………… reaction III : …………………………………... [2] (iv) Suggest reagents and conditions for reactions II and III. reaction II : ……………………………………………….. reaction III : ………………………………….................... [2] [Total : 9]

NJC/H2 Chem Prelim/02/2019 8 For Examiner’s Use 3 Perchlorate (ClO4–) compounds are used as oxidisers in some fireworks to aid the combustion reaction. These perchlorates can contaminate bodies of water near fireworks displays. Elevated concentrations of perchlorate in water can affect wildlife and it may also affect human health if it contaminates drinking water. (a) One common perchlorate present in fireworks is ammonium perchlorate, NH4ClO4. Given that the solubility of ammonium perchlorate is 30.6 g per 100 cm 3, express its solubility in mol dm−3 and hence, calculate the solubility product, Ksp, for NH4ClO4. [2] (b) A group of NJC students conducted a study to determine the perchlorate content of the water in the Singapore River before and after the fireworks display in the National Day Parade Rehearsal. They followed the methodology described below one week before the rehearsal:  Collect a 100.0 cm3 sample of the river water and acidify it with 10 cm3 of 1 mol dm3 sulfuric acid.  The resulting solution is added to an excess of 50 cm 3 of 5 × 105 mol dm3 iron(II) sulfate solution. The mixture is then made up to 250 cm3. This is solution X.  A 25.0 cm3 sample of solution X is pipetted into a conical flask. The 25.0 cm3 solution is titrated against 1 × 106 mol dm3 potassium manganate(VII) to determine the amount of remaining iron(II) ions in the sample. (i) Write the half equation for the conversion of perchlorate ions to chlorine in the acidic medium and hence the overall equation for the reaction between perchlorate and iron(II) ions. Half equation : ……………………………………………………………………………… Overall equation : …………………………………………..……………………………… [2] (ii) Describe how you would recognise the endpoint of the titration. ……………………………………………………………………………………………. [1]

NJC/H2 Chem Prelim/02/2019 9 For Examiner’s Use [Turn over (iii) The 25.0 cm3 sample of solution X required 30.00 cm3 of potassium manganate(VII) for complete reaction. Calculate the concentration of perchlorate ions in the river water. [3] (iv) Given that the perchlorate concentration in the Singapore River increased by 20% in the 24 hours after the fireworks display for the National Day Parade rehearsal, calculate the concentration of perchlorate ions in the