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