2021 NJC Prelim P2 Ans

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 xx printed pages and xx blank pages.

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. Element A S No. of moles/ mol 83.2 y 10083.2 32.1 or 0.523 [1] (ii) Hence, find the value of y. Mole ratio of A : S = 2 : 1 = 83.2 𝑦 : 100 −83.2 32.1 No. of moles of A atoms = 2  no. of moles of S atoms 83.2 𝑦 = 2  100−83.2 32.1 y = 79.5 [1] (iii) Element A is from Group 17. Hence, identify Element A using your answer in a(ii). Bromine [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. 100%  50%  25%  12.5%  6.25% Time taken = 85 x 4 = 340 days [2]

(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 𝑯𝒆𝟐 𝟔 (conservation of protons and mass number) Left side Right side Mass number 40 + 1 = 41 35 + x = 41 x = 6 No. of protons 18 + 0 =18 16 + y = 18 y = 2 Hence the product’s mass number is 6 and number of protons is 2. From the Periodic Table, the element that has 2 protons is He. [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 = 104.5o [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 Both CCl4 and S2Cl2 exists as simple molecular structure Less energy is required to overcome the weaker instantaneous dipole-induced dipole between CC l4 molecules than permanent dipole -permanent dipole between S2Cl2 molecules. [2] [Total: 10] 2 (a) In the Haber Process, nitrogen reacts with hydrogen to form ammonia in the presence of iron catalyst. This reaction is reversible and exothermic. Write a balanced equation for the Haber Process N2 + 3H2 2NH3 [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. Kc = [𝑁𝐻3]2 [𝑁2][𝐻2]3 mol2dm6 [2] (ii) Use the information given, calculate the value of Kc. N2 + 3H2 2NH3 Initial amt 0.833 3.33 0 Change  1 20.298 = 0.149  3 20.298 = 0.447 +0.298 Equilibrium amt 0.684 2.883 0.298 Equilibrium conc 0.684 0.1 = 6.84 2.883 0.1 = 28.83 0.298 0.1 = 2.98 Kc = (2.98)2 (6.84)(28.83)3 = 5.42  105 [2]

(iii) By using La Chatelier’s Principle, explain why the Haber Process needs to be conduced at an optimal temperature of 500 C. A low temperature will favour the exothermic reaction to produce some heat. The equilibrium position will shift to the right to produce more ammonia. However, at lower temperatures, rate of reaction is slower. [1] (c) State and explain the effect , if any, on how a change to the following will affect the Kc value. I: Increase in temperature Increase in temperature will favour endothermic reaction/ backward rxn/ position of equilibrium shift left. Kc will decrease. II: Increase in concentration of N2 Increase in [N2] will not affect Kc as there is no change in temperature. [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]

(d) (ii) Explain how iron catalyst can increase the rate of formation of ammonia. The catalyst provides an alternative reaction pathway with lower activation energy (Ea2). The fraction of molecules with K.E. ≥ Ea increases. The frequency of effective collisions increases hence rate of reaction increases. [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. Energy evolved when 1 mole of water is formed when an acid is completely neutralised by alkali under standard conditions. [1]

(ii) Write a balanced equation for the neutralisation of methanoic acid with potassium hydroxide. [1] HCOOH + KOH  HCOOK+ + H2O (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] The enthalpy change of neutralisation in a(ii) will be less exothermic. Some of the heat evolved is absorbed to fully dissociate the methanoic acid. Hence less heat is evolved. (iv) Suggest a suitable indicator for the titration between methanoic acid and potassium hydroxide. [1] Phenolphthalein / thymol blue (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 Na2O(s) + 2HCl(aq) 2NaCl(aq) + H2O(l) Amount of Na2O = 7.0 2(23.0)+16.0 = 0.1129 mol Amount of HCl = 200 1000  1.5 = 0.3 mol Limiting reagent is Na2O Hence amount of water formed = 0.1129 mol Heat evolved = (200)(4.18)(15) = 12540J H =  12540 0.1129 = 111 k