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

Yr6/4/CHEM/HP3/Sep2020 Turn over - 4 - From 1960 to 2010, the change in ESC over the two polar regions are fairly similar. However the change in the concentration of stratospheric ozone over the two polar regions are very different. Suggest a reason that may have caused this difference. [1] Temperature difference/ Antarctic is colder/ different climate (e) The change in the amount of UV-B radiation (280nm-320nm) reaching Earth’s surface in 2008 compared to 1978 is shown below. Comparing 1978 to 2008, the amount of UV-B radiation reaching the Earth’s surface at higher latitudes and polar regions show a significant increase. However the amount of UV-B radiation that reaches the equatorial region is fairly constant. Comment on this observation. [1] Ozone concentration at equatorial region is not depleted thus blocking UV radiation./ OWTTE Percentage change

Yr6/4/CHEM/HP3/Sep2020 Turn over - 5 - (f) HFCs (hydrofluorocarbons) and gaseous alkanes are described as “ozone-friendly” refrigerant gases compared to CFCs. The Ozone Depleting Potential (ODP) and Global Warming Potential (GWP)* of CFCs, HFCs and gaseous alkanes are given in the table below. Ozone Depleting Potential (ODP) Global Warming Potential (GWP)* CFCs CFC-11 1 5160 CFC-113 0.81 6080 HFCs HFC-134a 0 1360 HFC-23 0 12500 HFC-143a 0 5080 HFC-125 0 3450 Alkanes Ethane 0 3.0 Propane 0 3.0 Butane 0 3.0 *GWP refers to the amount of heat a gas can trap in the atmosphere compare to an equivalent amount of carbon dioxide. (i) Explain why HFCs do not deplete the stratospheric ozone like the CFCs. [1] The C-F bonds do not break as they are stronger/ No <<halogen>> radicals are formed as the C-F bonds are stronger. Accept comparison using BE

Yr6/4/CHEM/HP3/Sep2020 Turn over - 6 - (ii) List one advantage and disadvantage in using gaseous alkanes as refrigerant gases over HFCs and CFCs. [2] Advantage: Will not contribute much/ lesser effect on global warming. Do not accept it is not a Greenhouse gas or it does not deplete ozone (as HFC also have this advantage). Disadvantage: Flammable gas/ fire hazard All data and information are cited from Hannah Ritchie (2018) - "Ozone Layer". Published online at OurWorldInData.org. Retrieved from: 'https://ourworldindata.org/ozone-layer' [Online Resource]

Yr6/4/CHEM/HP3/Sep2020 Turn over - 7 - 2. Conductometric titration can be used to determine the concentration of acids or alkalis. In such titration, the electrical conductivity of the analyte is recorded as the titre is added from the burette. In an investigation, 25.00 cm3 of sulfuric acid is pipetted into a beaker and titrated with aqueous barium hydroxide. The electrical conductivity of the aqueous sulfuric acid, as aqueous barium hydroxide is added to it, is recorded. Source: https://www.qsstudy.com/chemistry The data collected is presented in the graph below.

Yr6/4/CHEM/HP3/Sep2020 Turn over - 8 - (a) The chemical equation for the reaction between the sulfuric acid and aqueous barium hydroxide is, H2SO4(aq) + Ba(OH)2 (aq) → BaSO4 (s) + 2 H2O (l) . Explain the trend observed for the electrical conductivity of the sulfuric acid as aqueous barium hydroxide is added to it. [2] Barium sulfate formed is insoluble in water thus conductivity decreases as concentration of ions (H+ and SO42- ) decrease; After equivalence/ endpoint, conductivity increases as excess barium hydroxide increases/ restores the conc. of ions. (b) By drawing appropriate straight lines on the graph, deduce the volume of aqueous barium hydroxide needed to reach the equivalence point. [2] Two intersecting linear lines; Accept range 22.00 – 23.00 cm3 Accept 1 or 2 decimal place. (c) The concentration of the aqueous barium hydroxide used is 0.100 mol dm-3. Using your answer to part (b), determine the concentration of the sulfuric acid. [1] [H2SO4] = ".$"" × "."''("."'(" = 0.0900 mol dm-3 Accept range from 0.088 – 0.092 mol dm-3 Accept 0.09 or 0.090. (d) List one advantage of conductometric titration. [1] Can be used for non acid-base/ precipitation/ weak acid-base/ redox/ complex ion titrations Or Can be used in titrations that where there is no suitable indicators/ end-point cannot be easily detected due to colours/ turbidity of reactions. Or More accurate reading than use of indicators.

Yr6/4/CHEM/HP3/Sep2020 Turn over - 9 - Section B Answer all of the questions from one of the options. Write your answers in the boxes provided. Option D --- Medicinal Chemistry 3. The properties of three analgesics are summarized below. Drug Strength Acts Addictive A mild at the site of pain no B strong on brain