2020 SAJC H2 Prelim Paper 3 (Answers)

1 ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examinations Higher 2 CANDIDATE NAME CLASS 1 9 S CHEMISTRY Paper 3 Free Response Candidate answer on the Question Paper. Additional Materials: Data Booklet 9729/03 16 September 2020 2 hours READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen. You may use a 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. This document consists of ?? printed pages and ? blank page. For Examiner’s Use Q1 19 Q2 19 Q3 22 Q4/5 20 Total 80

2 Section A Answer all the questions in this section. 1 Elements in Group 17 are known as halogens. The term “halogen” means “salt-former” in Greek because these elements will readily react with alkali metal and alkaline earth metals to form important halide salts such as sodium chloride. (a) Describe and explain how the volatilities of the halogens vary down Group 17. [2] The volatility of the halogens decreases down the group. The halogens are non-polar simple covalent molecules held by instantaneous dipole-induced dipole interactions between the molecules . Due to the increasing size of the electron cloud of the halogens down the group , the id-id is stronger , which requires more energy to overcome. ………………………………………………………………………………………………………. (b) The table below shows the results of experiments in which the halogens, X2, Y2 and Z2 were added to separate aqueous solutions containing X–, Y– and Z– ions. The possible identities of the halogens are chlorine, bromine and iodine. X– (aq) Y– (aq) Z– (aq) X2 - no reaction Z2 formed Y2 X2 formed - Z2 formed Z2 no reaction no reaction - (i) Based on the results in the given table, identify X2, Y2 and Z2. Explain your answer. [1] X2 is bromine, Y2 is chlorine, Z2 is iodine, Y2 is the strongest oxidising agent since it can oxidised both X- and Z-. X2 is a stronger oxidising agent that Z2, since it can oxidised Z-. (ii) Describe and explain how the solubility of the solid silver halides, AgC l, AgBr and AgI, is affected by adding concentrated NH3. [3] Upon addition of conc NH3, the solubility increases due to the formation of soluble complex [Ag(NH3)2]2+, which decreases the [Ag+] and ionic product. White solid AgCl is soluble and cream solid AgBr is (partially) soluble, as IP<Ksp. Yellow solid AgI is insoluble because it has the lowest Ksp, such that IP>Ksp. ………………………………………………………………………………………………………. (c) A student was presented the following set-up, which involved sodium chloride solution in one half-cell.

3 The student predicted the following:  Hydrogen is produced at the cathode of cell 1 when concentrated or dilute sodium chloride solution is used.  The anode of cell 2 would increase in mass. (i) State which electrode, a to d, is the cathode of cell 1 and which is the anode of cell 2. [1] Cathode: b Anode: c (ii) Do you agree with each of his prediction? Explain your answer. You should use relevant Eo values from the Data Booklet for cell 1. [3] For cell 1, at cathode: Na+ + e−  Na Eo = -2.71 2H2O + 2e−  H2 + 2OH− Eo = -0.83 Water is preferentially reduced to hydrogen even with high concentration of Na+ due to its much less negative in Eo/large difference in Eo. Hence, I agree with this prediction. For cell 2, At anode: Cu  Cu2+ + 2e– Eo = +0.34 When an electric current is applied, copper at the anode is oxidised to Cu2+. Hence the mass of the electrode /anode decreases, not larger. I do not agree with his prediction ………………………………………………………………………………………………………. a b c d NaCl(aq) cell 1 cell 2

4 (d) Organohalogens are organic compounds that contain halogen atoms. The following compounds are examples of organohalogens. (i) Suggest and explain how the acidity of compound A might compare with that of compound B. [2] A is less acidic than B. The negative charge on the conjugate base of C is delocalised over two electronegative oxygen atoms of the O-C=O bond. Hence, the negative charge is dispersed and the carboxylate ion is more stable. OR The electron-donating alkyl group intensifies the negative charge on the conjugate base of A and the alkoxide ion is less stable. (ii) Describe and explain the relative reactivity of compounds A and C with respect to hydrolysis. [2] C undergoes hydrolysis faster than A. The carbon of the acyl bromide in C is more electron deficient as it is bonded to 2 highly electronegative atoms O and Br , whereas the carbon of the C-Br in A is bonded to only 1 electronegative Br . T hus in C, the carbon attracts nucleophiles most strongly. (iii) Suggest a simple chemical test to distinguish compounds B and C. State the observation and write equation(s) to explain the reactions that happened. [3] Test: AgNO3(aq) For B: no cream ppt formed For C: cream/pale yellow ppt formed RCOBr + H2O  RCOOH + HBr Ag+ (aq) + Br− (aq)  AgBr (s) OR

5 Test: K2Cr2O7 (aq), H2SO4 (aq), heat For B: orange solution remained For C: orange solution turned green Jones oxidation can be used to oxidise compound A to compound B. Jones reagent uses chromium trioxide dissolved in sulfuric acid, to form chromic (VI) acid, HCrO4−. During the reaction, HCrO4− is reduced to Cr3+. (iv) Determine the change in oxidation number for the carbon atom that is directly bonded to oxygen atom in compounds A and B. [1] For A: -1 For B: +3 Change in OS: +4 (v) Hence, using oxidation numbers, construct a balanced equation for this reaction. You should use RCH2OH and RCO2H to represent compounds A and B respectively. [1] Since C OS +4, while Cr OS -3, 4HCrO4− + 3RCH2OH  4Cr3+ + 3RCO2H

6 4HCrO4− + 3RCH2OH + 16H+  4Cr3+ + 3RCO2H + 13H2O ………………………………………………………………………………………………………. [Total: 19] 2 Ozone, O3, is a powerful oxidising agent that can destroy viruses and bacteria. Ozone is used to disinfect water and sterilise food. (a) To generate ozone for industrial use, electricity is passed briefly through oxygen gas. Some oxygen molecules decompose to form oxygen atoms, which collide with more oxygen gas to form ozone, until an equilibrium is reached. 3O2 (g) ⇌ 2O3 (g) (i) Draw a dot-and-cross diagram to show the bonding present within an ozone molecule and suggest a value for its bond angle. [2] Accept a bond angle between 110 to 119○ (inclusive) (ii) Write an expression for Kp for this equilibrium. [1] Kp = 3 2 2 3 () () O O P P (iii) The value of Kp under a particular set of industrial conditions is 5 x 10–8. Deduce the sign and magnitude for ΔG at these conditions. [2] Since Kp<<1, position of equilibrium lies largely to the left and there is much more reactants than products, ΔG is positive and has a high magnitude. (iv) Predict and explain the effect of compressing the volume of the reaction mixture on the yield of ozone. [2] When V is decreased, P is increased. Position of equilibrium shifts right to decrease the amount of gaseous particles so as to decrease the pressure . Hence, this is done to increase the yield of ozone. ……………………………………………………………………………………………………….

7 (b) The concentration of ozone in a