2021 NJC Prelim P2 Qns

1 NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 1 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 8873/02 Tuesday 24 August 2021 2 hours READ THESE INSTRUCTIONS FIRST Write your name, index number, form class, tutorial class and subject tutor’s name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. The use of an approved scientific calculator is expected, where appropriate. Section A Answer all the questions. Section B Answer one question. 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. For Examiner's Use Question no. Marks Section A B7 B8 TOTAL /80 This document consists of 28 printed pages and 3 blank pages.

2 Section A Answer all the questions in this section in the spaces provided. 1 (a) Sulfur reacts with an element A to form a compound with empirical formula S A2. The percentage by mass of A in SA2 is 83.2%. (i) Assuming the relative atomic mass of A to be y. Find the number of moles of A and S respectively in terms of y. [1] (ii) Hence, find the value of y. [1] (iii) Element A is from Group 17. Hence, identify Element A using your answer in a(ii). …………………………………………………………………………………………..[1] Sulfur has 21 unstable isotopes. Among these isotopes, the one with the longe st half-life of 85 days is 35S isotope. (b) (i) Suggest the time taken for 35S to decrease to 6.25% of its initial amount. [2]

3 1 (b) (ii) When an atom of 40Ar collides with a high-speed neutron, 35S and another product is produced. State the identity of this product, including its mass and proton number. Ar_ 18 40 + nA_ 0 1 S + _____16 35 ……………………………………………………………………………..…………..[1] (c) Disulfur dichloride, S 2Cl2, is an important precursor of the extremely toxic chemical warfare agent sulfur mustard. It was widely used during the First World War and the Iran–Iraq conflict. S2Cl2 can be synthesized at 200 C as shown in the reaction below. CS2 (g) + 3 Cl2 (g)  CCl4 (g) + S2Cl2 (g) (i) Given the structural formula of disulfur dichloride, S2Cl2 is ClꟷSꟷSꟷCl. Draw the ‘dot-and-cross’ diagram of S2Cl2 and state the ClꟷSꟷS bond angle. ClꟷSꟷS bond angle = ……………………………. [2] (ii) In terms of structure and bonding, predict the difference in boiling point between CCl4 and S2Cl2. Compound Mr Boiling Point/ oC CCl4 154.0 77 S2Cl2 135.2 138 …………………………………………………………………………………………….. . ………………..…………………………………………………………………………… …………………………………………………………………………………………….. ……………………………………………………………………………...…………..[2] [Total: 10]

4 2 In the Haber Process, nitrogen reacts with hydrogen to form ammonia in the presence of iron catalyst. This reaction is reversible and exothermic. (a) Write a balanced equation for the Haber Process ………...……………………………………………………………………………..…………..[1] (b) A 20 cm 3 vessel containing 0.833 mol of N 2 was connected with another 80 cm 3 vessel containing 3.33 mol of H2. The reaction mixture was heated at a constant temperature of 500 C. At equilibrium, 0.298 mol of NH3 was produced. (i) Write the Kc expression for the Haber Process. State its units. [2] (ii) Use the information given, calculate the value of Kc. [2] (iii) By using Le Chatelier’s Principle, explain why the Haber Process needs to be conduced at an optimal temperature of 500 C. ……………………………………………………………………………………………… ………………………..…………………………………………………………………….. ……………………………………………………………………………...…………..[1]

5 2 (c) State and explain the effect , if any, on how a change to the following will affect the Kc value. I: Increase in temperature ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ………………………………..…………………………………………………………………….. ……………………………………………………………………………………...………………. II: Increase in concentration of N2 ……………………………………………………………………………………………………… ………………………………..…………………………………………………………………….. ……………………………………………………………………………………….……………... [3] (d) (i) The graph below shows how the percentage yield of ammonia varies over time by using iron as the catalyst. A new catalyst Z was introduced and results showed that it can increase the rate of reaction better than that of iron. Sketch on the graph for the new catalyst Z on the same axes shown above. [2]

6 2 (d) (ii) Explain how iron catalyst can increase the rate of formation of ammonia. [3] [Total: 14] 3 (a) Methanoic acid, HCOOH, being a weak acid, is the simplest carboxylic acid and is an important intermediate in chemical synthesis. The energy content of methanoic acid can be determined by means of calorimetric experiments. These experiments are usually carried out using polystyrene cup in a normal school laboratory. The standard enthalpy change of neutralisation can also be determined. (i) Define what is meant by the standard enthalpy change of neutralisation. ………………...……………………………………………………………………………. ………………...……………………………………………………………………………. ………………...……………………………………………………………………………. ………………...………………………………………………………………………… [1] (ii) Write a balanced equation for the neutralisation of methanoic acid with potassium hydroxide. ………………...………………………………………………………………………… [1]

7 3 (a) (iii) How would you expect the enthalpy change of neutralisation in (a)(ii) to compare with the enthalpy change of neutralisation of nitric acid with potassium hydroxide? Explain your answer. ………………...……………………………………………………………………………. ………………...……………………………………………………………………………. ………………...………………………………………………………………………… [1] (iv) Suggest a suitable indicator for the titration between methanoic acid and potassium hydroxide. ………………...………………………………………………………………………… [1] (b) Study the energy cycle and answer the questions that follow. (i) When 7.0 g of Na2O(s) is dissolved in 200 cm3 of 1.5 mol dm3 HCl(aq), the temperature of the solution increased by 15 C. Calculate H2 [3] (ii) By making use of the energy cycle and your answer to b(i), calculate H1. Given ethalpy change of neutralisation between NaOH(aq) and HCl(aq) is 57.0 kJ mol1. [2] [Total: 9] H3 H2 Na2O(s) + 2HCl(aq) +H2O(l) 2NaOH(aq) + 2HCl(aq) 2NaCl(aq) + 2H2O(l) H1

8 4 The following reaction scheme involves a series of reactions involving different organic compounds.. (a) State the name of compound A. ………………...………………….…………………………………………………………… [1] (b) State the reagents and conditions required for Steps 1 and 2. Step 1: ………………...………………………………………………………………….………. Step 2: ……………………………................................................................………………. [2] (c) In addition to compound A that is formed at the end of Step 1, compound D, which is an isomer of compound A, is also produced. Fill in the expected ratio of isomer D. Structure Compound A Compound D Ratio 1 [1] (d) By considering the molecular formula, draw the skeletal formula of two more isomers that are constitutional isomers of compound A and D.