2023 SOTA Yr 6 Chem HL Prelim Exam P2

Yr6/4/CHEM/HP2/Aug2023 20 pages © School of the Arts, Singapore CHEMISTRY Higher level Paper 2 Preliminary Examinations Wednesday 30 August 2023 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/Aug2023 - 2 - 1. (a) An oxide of chromium contains 68.4% of chromium and 31.6% of oxygen. The mass spectrum of the oxide shows its molecular ion peak at m/z = 156. Determine the molecular formula of the oxide, showing your working. [3] ………..………………………………………………………………………………………… ………………………………………..………………………………………………………… ………..………………………………………………………………………………………… ………..………………………………………………………………………………………… ………………………………………..………………………………………………………… ………..…………………………………………………………………………………………………..………………………………………………………………………………………… ………………………………………..………………………………………………………… ………..………………………………………………………………………………………… (b) (i) When copper compounds are introduced into a gas flame, the flame turns green. Calcium compounds produce a brick red flame while chromium compounds produce a silver-white flame. Explain why different colours are observed in the gas flame. [2] ………..………………………………………………………………………………………… ………………………………………..………………………………………………………… ………..………………………………………………………………………………………… ………..………………………………………………………………………………………… ………………………………………..………………………………………………………… ………..………………………………………………………………………………………… (ii) Explain the convergence of lines in an emission spectrum. [1] ………..………………………………………………………………………………………… ………………………………………..………………………………………………………… ………..………………………………………………………………………………………… (iii) The frequency of convergence limit for the chromium atom is 1.64 × 1015 s−1. Calculate the ionization energy, in J, for a single atom of chromium using sections 1 and 2 of the data booklet. [1] ………..………………………………………………………………………………………… ………………………………………..………………………………………………………… ………..…………………………………………………………………………………………

Yr6/4/CHEM/HP2/Aug2023 Turn over - 3 - (c) (i) State the full electron configuration of the chromium atom. [1] ………..………………………………………………………………………………………… (ii) Predict and explain the magnetic properties of chromium atoms and chromium(III) ions. [2] ………..………………………………………………………………………………………… ………………………………………..………………………………………………………… ………..………………………………………………………………………………………… ………..………………………………………………………………………………………… (iii) Describe how the bonds between water ligands and chromium(III) ion are formed using Lewis acids and bases. [1] ………..………………………………………………………………………………………… ………………………………………..………………………………………………………… ………..………………………………………………………………………………………… ………..………………………………………………………………………………………… (iv) Explain why chromium(III) nitrate solutions are green, using section 17 of the data booklet. [3] ………..…………………………………………………………………...…………………... ………………………………………..………………………………..……………………… ………..…………………………………………………………………..…………………… ………..……………………………………………………………………..………………… ………..………………………………………………………………………..……………… ………………………………………..…………………………………………..…………… ………..……………………………………………………………………………..………… ………..………………………………………………………………………………..……… ………..…………………………………………………………………………………..…… ………………………………………..……………………………………………………..…

Yr6/4/CHEM/HP2/Aug2023 - 4 - 2. (a) Methane is the main component of natural gas and a common gaseous fuel. (i) Write the chemical equation for the combustion of methane under standard conditions. [1] ………..……………………………………………………………………………………..…… ………………………………………..……………………………………………...………...… (ii) Calculate the enthalpy change of combustion of methane, in kJ mol-1, using section 12 of the data booklet. [2] ………..…………………………………………………………………………………..……… ………………………………………..…………………………………………………..……… ………..…………………………………………………………………………..……………… ………..………………………………………………………………………..………………… (iii) The enthalpy change of combustion of methane calculated using average bond enthalpies is -808 kJ mol-1. Explain the difference between this value and your answer from (a)(ii). [1] ………..…………………………………………………………………………………..……… ………………………………………..…………………………………………………..……… ………..…………………………………………………………………………..……………… ………..………………………………………………………………………..…………………

Yr6/4/CHEM/HP2/Aug2023 Turn over - 5 - (b) Methane reacts with chlorine, Cl2, to form a range of substituted chloromethanes. The structural formulae of methane and two chloromethanes are shown below. Methane Dichloromethane Tetrachloromethane (i) Discuss the overall polarity of the three molecules. [3] ………..…………………………………………………………………………………..……… ………………………………………..…………………………………………………..……… ………..…………………………………………………………………………..……………… ………..………………………………………………………………………..………………… ………..…………………………………………………………………………..……………… ………..………………………………………………………………………..………………… ………..…………………………………………………………………………..……………… ………..………………………………………………………………………..………………… ………..…………………………………………………………………………..……………… ………..………………………………………………………………………..………………… (ii) Explain why methane is a gas while tetrachloromethane is a liquid at SATP conditions (298 K, 100 kPa). [1] ………..…………………………………………………………………………………..……… ………………………………………..…………………………………………………..……… ………..…………………………………………………………………………..……………… ………..………………………………………………………………………..………………… (iii) Outline why methane is insoluble in water. [1] ………..…………………………………………………………………………………..……… ………………………………………..…………………………………………………..……… ………..…………………………………………………………………………..……………… ………..………………………………………………………………………..………………… C H HH H C Cl HH Cl C Cl ClCl Cl

Yr6/4/CHEM/HP2/Aug2023 - 6 - (c) At sea level, ozone is commonly formed during the combustion of methane in excess oxygen. The chemical equation for the formation of ozone is shown below. 3O2 (g) → 2O3 (g) ΔH¡ = + 285.4 kJ (i) Calculate the average bond enthalpy, in kJ, of the oxygen-oxygen bond in ozone using section 11 of the data booklet. [2] ………..…………………………………………………………………………………..……… ………………………………………..…………………………………………………..……… ………..…………………………………………………………………………..……………… ………..………………………………………………………………………..………………… ………..…………………………………………………………………………..……………… ………..………………………………………………………………………..………………… (ii) Draw the Lewis structure of an ozone molecule. Label the formal charge of each oxygen atom in the ozone molecule. [2] (iii) Using your answer to (c)(i) and section 11 of the data booklet, comment on the bond order and bond length of the oxygen-oxygen bond in ozone. [2] ………..……………………………………..…………………………………………………… ………………………………………..………..………………………………………………… ………..…………………………………………..………………………………………………

Yr6/4/CHEM/HP2/Aug2023 Turn over - 7 - (d) The Born-Haber cycle of magnesium oxide is shown below. (i) State the processes for ΔH1¡ and ΔH2¡. [2] ΔH1¡ : ……..…………………...………………………………………………………………… ΔH2¡: ………………………………….……..………………………….……………………….. (ii) Using section 8 of the data booklet, calculate the enthalpy change of the 2nd ionization energy of magnesium. [2] ………..……………………………………..…………………………………………………… ………………………………………..………..………………………………………………… ………..…………………………………………..……………………………………………… ………..…………………………………………..……………………………………………… Mg2+ (g) + 2e- + O (g) Mg2+ (g) + O2- (g) MgO (s) Mg+ (g) + e- + O (g) Mg (g) + O (g) Mg (s) + ½ O2 (g) ΔHat¡(Mg) = + 148 kJ mol-1 and ΔHat¡(