2020 SOTA Yr6_Chem_HL_Prelim_P3_MS

Yr6/4/CHEM/HP3/Sep2020 12pages © School of the Arts, Singapore CHEMISTRY Higher level Paper 3 Preliminary Examinations Wednesday 2 September 2020 1 hour 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. • 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 [45 marks]. Section A Questions Answer all the questions. 1 –- 2 Section B Questions Answer all of the questions from one of the options Option D -– Medicinal chemistry 3 –- 8 Name: Class: Index: 45

Yr6/4/CHEM/HP3/Sep2020 Turn over - 2 - Section A Answer all questions. Write your answers in the boxes provided. 1. The Earth’s atmosphere is divided into five main layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere. The oxygen and ozone in the stratosphere absorbs most of the Ultraviolet (UV) radiation and protects Earth from the harmful radiation. (a) Oxygen molecule dissociates as shown when it absorbs UV radiation. O2 → O• + O• Formulate the chemical equation that shows the dissociation of an ozone molecule upon absorbing UV radiation. [1] O3 → O2 + O• (b) The information on the type of UV radiation absorb by oxygen and ozone are shown below. Type of UV radiation Wavelength/ nm [2] Oxygen UV-C 200 – 280 Ozone UV-B 280 – 315 Comment on the difference in the type of UV radiation absorbed by oxygen and ozone. Stronger O-O bond in O2 than in ozone; hence O2 absorbs higher energy UV Accept converse argument. Max 1m if only difference bond strength is mentioned.

Yr6/4/CHEM/HP3/Sep2020 Turn over - 3 - Some halogenated compounds will dissociate in the stratosphere to form halogen radicals. In the study of stratospheric ozone, Equivalent Stratospheric Chlorine (ESC) is used as the measure for the amount of halogens (chlorine and bromine) in the stratosphere. The relative values of the Equivalent Stratospheric Chlorine (ESC) and the concentration of stratospheric ozone, over Antarctic and Artic from 1960 to 2040 (projected) is shown below. (c) Using this information, briefly describe the relationship between the concentration of stratospheric ozone and ESC. [1] Ozone concentration decreases with increase in ESC/ Negative correlation Do Not accept inverse relationship. (d) The average daily temperatures of Antarctic and Artic in summer and winter are shown in the below. Season Average daily temperature / oC Antarctic Artic Summer -28.2 0 Winter -60 -40

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