2023 TJC H2 Chem Prelim P1 (Ans)

9729 / TJC PRELIM / 2023 TEMASEK JUNIOR COLLEGE 2023 JC2 PRELIMINARY EXAMINATION Higher 2 [Worked Solutions] CHEMISTRY 9729/01 Paper 1 Multiple Choice 18 September 2023 1 hour Additional Materials: Multiple Choice Answer Sheet Data Booklet READ THESE INSTRUCTIONS FIRST Write in soft pencil. Do not use staples, paper clips, glue or correction fluid. There are thirty questions on this paper. Answer all questions. For each question there are four possible answers A, B, C, D. Choose the one you consider correct and record your choice in soft pencil on the separate Answer sheet. Read the instructions on the Answer sheet very carefully. Write your name & Civics Group on the Answer sheet. Shade your index number in the appropriate boxes. Each correct answer will score one mark. A mark will not be deducted for a wrong answer. Any rough working should be done in this booklet. The use of an approved scientific calculator is expected, where appropriate. This document consists of 15 printed pages and 1 blank page. Write your name and Civics Group Last 4 digits of entry proof index number

2 2 MCQ ANSWERS (A:8, B:8, C:8, D:6) 1 2 3 4 5 6 7 8 9 10 C C C C A B C A B D 11 12 13 14 15 16 17 18 19 20 B B A B D C A D C B 21 22 23 24 25 26 27 28 29 30 A D A D A B C A B D 1 Magnesium thiocyanate, Mg(SCN) 2, has been found to support heart health by promoting healthy blood circulation and maintaining normal blood pressure levels. What are the numbers of electrons in the magnesium ion and the thiocyanate ion? Magnesium ion Thiocyanate ion A 12 29 B 10 29 C 10 30 D 10 31 Answer: C Worked Solution: No. of electrons in Mg2+ ion = 12 – 2 = 10 No. of electrons in SCN− ion = 16 + 6 + 7 + 1 = 30 2 In which pair of species do both species have only one unpaired p electron? 1 Ar− and F 2 Ar+ and Si+ 3 Ga and C−

3 9729 / TJC Prelim / 2023 [Turn over A 1 and 2 only B 2 and 3 only C 2 only D 3 only Answer: C (2 only) Worked Solution: Atoms and ions of electronic configuration of Group 13 and 17 elements will have only one unpaired electron. For reference Atom Group Electronic configuration of atom Electronic configuration of ion Ga 13 [Ar] 3d104s24p1 C/Si 14 s2p2 C−: 1s22s22p3 Si+: [Ne] 3s23p1 F 17 Is22s22p5 Ar 18 [Ne] 3s23p6 Ar+: [Ne] 3s23p5 Ar−: [Ne] 3s23p64s1 3 The 68Ga isotope is used in positron emission tomography (PET) scan to detect tumors. The radioactive decay of 68Ga isotope has a half-life of 68 days and produces the neutral particle, 68X. This transformation of 68Ga occurs with a proton changing into a neutron. Which of the following statement is incorrect? A 68X has 38 neutrons. B 68X is an isotope of zinc. C There are 3 electrons in the valence shell of 68X. D 2.42 % of the original 68Ga isotope remains after a year. Answer: C Worked Solution: 68Ga 68X No. of protons 31 30 (Zinc) No. of neutrons 68 – 31 = 37 38 No. of electrons 31 30 Options A and B are correct.

4 4 Option C: Incorrect. Electronic configuration of Zn: 1s 22s22p63s23p63d104s2. There are only 2 electrons in the valence shell of n=4. Option D : Correct. No. of half-lives = 365/68 = 5.37 % 68Ga remaining = 100 x ( 1 2) 5.37 = 2.42% (D is correct) 4 Which of the following species contain at least one unpaired electron? 1 NO 2 NO2 3 NO2− A 1 only B 2 only C 1 and 2 only D 1, 2 and 3 Answer: C (1 and 2 only) Worked Solution: 1 2 3 5 Adrenaline is a hormone that is produced by the body during times of stress. What are the bond angles w, x, y and z in adrenaline, from the smallest to the largest? Smallest bond angle ⎯⎯⎯⎯⎯⎯⎯→ Largest bond angle A x z y w

5 9729 / TJC Prelim / 2023 [Turn over B x z w y C y x z w D w y z x Answer: A Worked Solution: 6 Organic liquids P and Q have identical boiling points. When equal volumes of P and Q are mixed together, there is no reaction and liquid mixture R is obtained. The vapour pressure of liquid mixture R is less than that of liquid P or liquid Q at the same temperature. Which of the following statement is incorrect? A R is less volatile that P. B P and Q are constitutional isomers. C The mixing of P and Q is exothermic. D The average intermolecular forces in R is stronger than that in P or in Q. Answer: B Worked Solution: Option B: Constitutional isomers have different physical properties like boiling points. P and Q form stronger intermolecular forces with each other upon mixing to give mixture R (higher boiling point and less volatile). The amount of energy released is more than that required to overcome the attractions between particles of P and between particles of Q (exothermic).

6 6 7 The following redox reactions have been observed to occur spontaneously. 2VO2+ + H2O2 ⎯⎯ → 2VO2+ + 2H+ S2O82– + H2O2 ⎯⎯ → O2 + 2SO42– + 2H+ Which of the following shows an increasing order of oxidising power? A H2O2 < VO2+ < S2O82– B S2O82– < H2O2 < VO2+ C VO2+ < H2O2 < S2O82– D VO2+ < S2O82– < H2O2 Answer: C Worked Solution: 2VO2+ + 2H2O + H2O2 ⎯⎯ → 2VO2+ + 2H2O + 2H+ +4 –1 +5 –2 VO2+ is oxidised by H2O2 (H2O2 is reduced, it is the Oxidising Agent.)  oxidising power of VO2+ < H2O2 S2O82– + H2O2 ⎯⎯ → O2 + 2SO42– + 2H+ +7 –1 0 +6 H2O2 is oxidised by S2O82– (S2O82– is reduced, it is the Oxidising Agent.)  oxidising power of H2O2 < S2O82– Strength of oxidising power: VO2+ < H2O2 < S2O82– 8 Use of the Data Booklet is relevant for this question. A 20 dm3 vessel contains 0.25 mol ammonia gas and a 30 dm3 vessel contains 0.42 mol hydrogen chloride gas . When the two vessels were connected at 500 K, the following chemical reaction occurs : NH3(g) + HCl(g) ⎯⎯ → NH4Cl(s) What is the resultant pressure in Pa in the combined vessel? A 14127 B 20775 C 34902 D 55677 Answer: A NH3(g) + HCl(g) → NH4Cl(s) Limiting reagent is ammonia gas . Thus, the resultant pressure of the combined vessel is due to the remaining hydrogen chloride gas.

7 9729 / TJC Prelim / 2023 [Turn over No of moles of hydrogen chloride gas remaining = 0.42-0.25 = 0.17 mol Resultant volume of combined vessel = 50 dm3 = 50 x 10-3 m3 P = nRT V P = − 3 0.17( )(8.31 500) 50 10 x = 14127 Pa 9 Which property of E, F and G will give the trend shown below? Property E F G A boiling point SiCl4 PCl5 BCl3 B volatility HCl HBr HF C solubility in water P4O10 SiO2 MgO D reducing power Mg Ca Ba Answer: B Worked Solution: Option A : Incorrect. All three are simple molecular (non-polar).Electron cloud size hence extent of distortion of electron cloud of PCl5 > SiCl4 > BCl3 . Strength of id-id of PCl5 > SiCl4 > BCl3 hence bp of PCl5 > SiCl4 > BCl3. Option B : Correct. All three are simple molecular. Highest bp for HF as largest amount of energy required to overcome stronger hydrogen bonding. Electron cloud E F G property

8 8 size hence extent of distortion of electron cloud of HBr > HC l. Strength of id -id of HBr > HCl hence bp of HBr > HCl. Volatility of HCl > HBr > HF. Option C: Reaction : P4O10 + 6H2O 4H3PO4 Soluble Reaction: SiO2 does not react with water. In