2020 SOTA Yr6_Chem_HL_Prelim_P2_MS

Yr6/4/CHEM/HP2/Sep2020 22pages © School of the Arts, Singapore CHEMISTRY Higher level Paper 2 Preliminary Examinations Tuesday 1 September 2020 2 hours 15 minutes INSTRUCTIONS TO CANDIDATES • Write your name, class and index number in the blanks below. • Do not open this examination paper until instructed to do so. • Answer all questions. • Answers must be written within the answer boxes provided. • A calculator is required for this paper. • A clean copy of the chemistry data booklet is required for this paper. • The maximum mark for this examination paper is [90 marks]. Name: Class: Index: 90

Yr6/4/CHEM/HP2/Sep2020 Turn over - 2 - 1. Ammonia and ammonium salts have a wide range of uses. (a) In an experiment, 20.0 cm3 of aqueous ammonia is titrated with dilute sulfuric acid. The titration curve for this experiment is shown below. The chemical equation for the reaction between ammonia and sulfuric acid is as follows. 2 NH3 (g) + H2SO4 (aq) ® 2 NH4+ (aq) + SO42- (aq) (i) Outline why ammonia is classified as a weak base. [1] It dissociates/ ionises partially << in water>>. (ii) Using the titration curve, estimate the pH at equivalence point of this titration. [1] pH at equivalence point: Accept range from 4.8 to 5.8. (iii) Using a relevant chemical equation, explain why pH is low at the equivalence point. [2] NH4+ + H2O à NH3 + H3O+; Ammonium is conjugate acid of weak base/ acts as weak acid. 01234567891011121314 0.005.0010.0015.0020.0025.0030.0035.0040.0045.0050.00 pH Volume of H2SO4/ cm3 Initial pH = 11.3

Yr6/4/CHEM/HP2/Sep2020 Turn over - 3 - (iv) Using information from section 22 of the data booklet, state a suitable indicator that can be used to detect the equivalence point of this titration. [1] Methyl orange/ bromophenol blue/ bromocresol green/ methyl red/ bromothymol blue (v) The concentration of the sulfuric acid used for this titration is 0.100 mol dm-3. Using information from the graph only, calculate the initial concentration of the aqueous ammonia. [1] [NH3] = !.#!! × !.!&&! × &!.!&!! = 0.220 mol dm-3 (vi) The initial pH of the ammonia solution is 11.3. Using your answer from part (a)(v) and the initial pH of the ammonia solution, calculate the base dissociation constant, Kb, for ammonia. [2] pOH = 2.7, [OH-] = 2.00 x 10-3 mol dm-3 ; Kb = (2.00 x10-3)2 / 0.220 = 1.82 x 10-5 mol dm-3 Do not penalise if unit is missing or incorrect. Allow ECF from (v). (b) Ammonia reacts with boron trifluoride, BF3, to form NH3BF3. The chemical equation for this reaction is as shown. NH3 + BF3 ® NH3BF3 (i) Using Lewis acid-base theory, explain the role of NH3 in this reaction. [1] Lewis base