SAJC Prelim P1 Answers

1 [Please Turn Over NAME Class ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION Chemistry (9729) Paper 1 Multiple Choice 17 September 2018 1 hour Additional Materials: Multiple Choice Answer Sheet, Data Booklet READ THESE INSTRUCTIONS: 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 and D. Choose the one you consider correct and record your choice in soft pencil on the separate Answer sheet. 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 14 printed pages.

2 Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 D C C B A B A D D C Q11 Q12 Q13 Q14 Q15 Q16 Q17 Q18 Q19 Q20 D D D C A B C C A A Q21 Q22 Q23 Q24 Q25 Q26 Q27 Q28 Q29 Q30 A D B B A B B A C A 1 To identify an oxide of nitrogen, the oxide is mixed with an excess of hydrogen and passed over a catalyst at a suitable temperature. The water collected weighs 0.133 g. The ammonia produced is neutralised by 18.50 cm3 of 0.2 mol dm-3 HCl. Which of the following could be the oxide? 1 N 2O 2 NO 2 3 NO 3 4 N 2O4 A 3 only B 1 and 2 only C 2 and 3 only D 2 and 4 only Answer: D No. of moles of water = 0.133/18 = 0.00739 No. of moles of NH3 = 18.50/1000 x 0.2 = 0.0037 Ratio of NH3 to water is 1: 2 Therefore, x : y (i.e. N to O) of oxide is also 1:2 2 This question is about sparingly soluble salts. AgCl Ag I Hg I2 Ksp 1.0 × 10−10 8.3 × 10−16 1.1 × 10−21

3 [Please Turn Over From the above information, which statements are not true? A For a solution containing 1 mol dm −3 of NaC l and NaI, the concentration of chloride ions when AgI just starts to precipitate is 1 mol dm−3. B The solubility of HgI2 is 6.50 × 10−8 mol dm−3. C The solubility of HgI2 is the lowest. D The units for Ksp for HgI2 is mol3 dm−9. Answer: C Option A: Since silver chloride and silver iodide has the same number of ions, the solubility is directly proportional to the Ksp. Hence silver iodide is precipitated before silver chloride. When silver iodide precipitates, chloride ions are still in solution and have not precipitated yet. Hence, the concentration of chloride ions is still 1 mol dm -3. Option B: Let solubility of HgI2 be x, hence Ksp = 4x3. x = 6.50 × 10−8 mol dm−3 Option C: The solubility of AgCl = 1 x 10-5 mol dm-3, AgI = 2.88 x 10-8 mol dm-3. AgI is less soluble than HgI. Option D: For HgI2 Ksp = [Hg2+][I]2, hence units is (mol dm-3)3 = mol3dm-9 3 The following tests were performed on an aqueous solution containing bromide and iodide ions. Step Test Observations 1 Add excess AgNO 3(aq). A mixture of cream and yellow precipitate were formed. 2 To the mixture from step 1, add concentrated NH3. The cream precipitate dissolved to form a colourless solution. The yellow precipitate was insoluble. 3 Filter the mixture from step 2. Filtrate was a colourless solution. Residue was a yellow solid. 4 Add Cl2(aq) to filtrate. Brown solution is formed. Which of the following statements is correct? A In step 2, the cream precipitate dissolved because the addition of concentrated NH3 results in formation of a complex which decreases the Ksp of AgBr. B In step 2, the yellow precipitate was insoluble because the Ksp of AgI is higher than the Ksp of AgBr.

4 C In step 2, the yellow precipitate was insoluble because the ionic product of Ag I is higher than the Ksp of AgI. D In step 4, a disproportionation reaction has taken place. Answer: C The yellow precipitate remained in step 2 because IP of AgI > Ksp of AgI. 4 A stable ion of Q has the following properties: • has a noble gas configuration • has more protons than electrons • was obtained by removing electrons from the same orbital Which of the following could be Q? A Al B Ca C Cu D S Answer: B From the first 2 properties, Q has to be a main group metal. Hence, option C and D are incorrect. Since it was obtained by removing electrons from the same orbital, Q has to be either a metal in Group 1 or 2. 5 Chromium and its compounds undergo the following reactions. N has the following composition by mass: Cr, 15.5%; S, 38.1%; N, 29.2%. Which of the following can be deduced from the above reaction scheme? A The values of x and y in N are 4 and 2 respectively. B The types of reactions that occur are ligand exchange and redox only. C The formula of K is Cr(OH)2(H2O)6. D H2O2 is a reducing agent in the conversion of K to L. Answer: A Option A: The empirical formula: Cr, 15.5%; S, 38.1%; N, 29.2% Cr : S : N = 0.298 : 1.19 : 2.09 = 1: 4 : 7 Cr2(SO4)3 K green solution L yellow NaOH (aq) in excess H2O2 M orange H+ NH4SCN N NH4[Cr(SCN)x(NH3)y]

5 [Please Turn Over From K to N, there should be no change in oxidation state and merely a ligand exchange. In N, the coordination number is 6 and the oxidation number of Cr = +3. Given that there is one cation NH4+, hence the overall charge is -1. Thus, There are 4SCN- ligands to make the overall charge -1. Option B: The conversion of L to M is also an acid base reaction where L is CrO 42- and M is Cr2O72-. Option C: K is Na3[Cr(OH)6] (aq). K is not a solid unlike the formula given in option C. Option D: H2O2 is an oxidising agent in the conversion of K to L as the oxidation number of Cr increases from +3 to +6. 6 Which one of the following statements about Group 2 elements and their compounds is correct? A Beryllium chloride has a higher boiling point than magnesium chloride as it has a more exothermic lattice energy than magnesium chloride. B The reducing power of Group 2 elements increases down the group. C 1 mole of barium carbonate, on heating over a short period of time, decomposes to give more carbon dioxide gas than 1 mole of magnesium carbonate. D Barium has a higher melting point than magnesium due to more electrons. Answer: B Option A: Beryllium chloride is a simple covalent molecule while magnesium chloride is an ionic compound. Hence beryllium chloride should have a lower boiling point as it has weaker id-id interactions than the strong ionic bonds in magnesium chloride. Option B: The E 0 value for M2+ / M is more negative down the group, indicating M is more easily oxidised hence its reducing power increases down the group. Option C: Ba2+ has a smaller charge density than Mg2+ and distorts electron cloud of CO32− to smaller extent, hence C – O bond is weakened to a smaller extent and more energy is required to decompose BaCO 3. Hence, less CO2 is given off when BaCO3 is decomposed. Option D: Ba and Mg are both metallic with strong electrostatic forces of attraction between metal cation and sea of delocalised electrons. The number of electrons in the sea of delocalised electrons would be the same as both lose 2 valence electrons per atom. Ba 2+ has a smaller charge density so the metallic bonds are weaker and less energy required to break them. Barium should have a lower melting point. 7 Liquefaction can be defined as a process that generates a liquid from a gas and this can be done by altering pressure. In the liquefaction of CH4 and NH3, the pressure needed for NH3 is less than CH4.

6 Which of the following reasons best explains this observation? A NH3 has stronger intermolecular forces of attraction than CH4. B NH3 has weaker bond energy than CH4. C NH3 molecules are bigger than CH4 molecules. D NH3 molecules possess less kinetic energy than CH4 molecules. Answer: A The process of liquefaction is making the molecules of gas come closer together via pressurisation. If the intermolecular forces between the molecule