2023 SAJC H2 Chem Prelim P1 (Ans)

ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS HIGHER 2 CANDIDATE NAME CLASS 2 2 S CHEMISTRY Paper 1 Multiple Choice Additional Materials: Multiple Choice Answer Sheet Data Booklet 9729/01 15 September 2023 1 hour READ THESE INSTRUCTIONS FIRST Write in soft pencil. Do not use staples, paper clips, glue or correction fluid. Write your name and class on the Answer Sheet in the spaces provided. 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 XX printed pages (including this cover page) and 1 blank page.

1 The relative atomic mass of boron, which consists of the isotopes 10 5 B and 11 5 B is 10.8. What is the percentage of 11 5 B atoms in the isotopic mixture? A 0.2 % B 0.8% C 20 % D 80 % Ans: D Let x be the relative abundance of 11 5 B 11 x + 10 (1−x) = 10.8 X = 0.8 Hence % abundance = 0.8 x 100 = 80% 2 A mixture of sodium iodide, 23Na127I, and sodium bromide, 23Na79Br, were vapourised and the ions passed through an electric field. Which statements about the results are correct? 1 There would be 4 angles of deflections observed. 2 The particle with the largest angle of deflection will be detected at the negative plate. 3 The positive plate will detect more particle types than the negative plate. A 1 only B 2 and 3 only C 1 and 2 only D 1,2 and 3 Ans: B When the mixture is vapourised, there are only 3 different particle types, Na +, I− and Br−. Hence, only 3 angle of deflections will be obtained. Statement 1 is wrong. The (charge/mass) of the 3 particles are Na+= 0.0435, I−= 0.00787 and Br−= 0.0127. Therefore, Na+ will give the largest angle of deflection, which will be detected at the negative place. Hence statement 2 is correct. Since there are 2 anions and 1 cation, the positive plate will detect one more particle type as compared to the positive place. Statement 3 is correct.

3 [TURN OVER 3 Which of the following statements about ice and its structure is correct? A The bond angle around O in ice is 109.5°. B Ice is more dense than water. C 2 electrons on O in ice are involved in hydrogen bonding. D Hydrogen bonds formed in ice are as strong as the O-H covalent bonds. Ans: A Ice has an open structure where each O is covalently bonded to 2 H atoms and the 2 lone pairs (4 electrons) each form a hydrogen bond with 2 other neighboring water molecules. (option C is thus incorrect). This leads to O being tetrahedrally bonded to 4 H atoms and thus have a bond angle of 109.5°. (option A is correct). This open structure of ice leads to it being less dense than water (option B is wrong) and thus float on water. Hydrogen bond is an intermolecular force which is weaker than a O -H covalent bond. (option D is wrong) 4 Benzylamine is commonly used in the industrial production of many pharmaceutical products and has the following structure. NH2 Which bond angle is not present in benzylamine? A 105° B 107° C 109.5° D 120° Ans: A 107° is present about the N as it has 3 bond pairs and 1 lone pair. 109.5° is present on the side chain C as it has 4 bond pairs and 0 lone pair 120° is present as the C on the benzene only has 3 bond pairs and 0 lone pair.

5 In a vessel, 5.0 dm3 of nitrogen gas at 20 °C and pressure of 200 kPa was compressed by a piston to 2.5 dm3 and heated to 60 °C. What is the pressure in the heated vessel? A 90 kPa B 455 kPa C 715 kPa D 1200 kPa Ans: B P1V1 / T1 = P2V2 / T2 (200)(5)/(20+273) = P2(2.5) / (60+273) P2 = 454.6 = 455 kPa (3sf) 6 Magnesium, aluminium, silicon and phosphorus are consecutive elements in Period 3 of the Periodic Table. Which of the following properties generally decreases from magnesium to phosphorus? 1 Electrical conductivity 2 Ionic radius 3 Melting point of their oxides 4 pH of their chlorides in water A 1 and 2 only B 1 and 3 only C 2 and 4 only D 3 and 4 only Ans: D Electrical conductivity increases from Mg to Al due to more delocalised electrons in Al in its sea of delocalized electron. Hence incorrect. The ionic radius of P, being an anion, is larger than the other 3 which are cations. Hence incorrect. Their structure of their oxides transit from ionic (for Mg) to ionic with covalent character (for Al) to giant covalent (for Si) to simple covalent (for P). Hence their melting point decreases and correct. The pH of their chlorides starts from around 6 for Mg to 3 for Al to 2 for Si and P . Hence also correct.

5 [TURN OVER 7 Which of the following properties of a salt will make it the most soluble in water? size of the ions magnitude of their lattice energy A small small B small large C large small D large large Ans: A ∆Hsoln = ∑(∆Hhyd) – LE For a salt to be most soluble, ∆Hsoln should be a large negative/exothermic value. Given that both ∆H hyd and LE are negative values, in order for ∆H soln to be a large negative value, the magnitude of ∆H hyd should be large while the magnitude of LE should be small. ∆Hhyd ∝ charge density = charge radius Hence, for magnitude of ∆Hhyd to be large, the size of the ions should be small. Note for review: the interplay between L.E. and hydration energy can be quite tricky – e.g. solubility trend of group 2 sulfates down grp.

8 A reaction involving 3 species, F, G and H have the following rate equation. rate = k[G][H]2 Which of the following graphs will be obtained? A When [G] and [H] are kept constant, B When [G] and [H] are kept constant, C When [F] and [G] are kept constant, D When [F] and [H] are kept constant, Ans: C The order of reaction w.r.t [F] is zero. Hence the rate vs conc graph will be rate / mol dm–3 s–1 [F] / mol dm–3 [F] / mol dm–3 time / s rate / mol dm–3 s–1 [H]2 / mol2 dm–6 [G] / mol dm–3 time / s

7 [TURN OVER The graph for the [F] vs time graph will be The order of reaction w.r.t [G] is first order. Hence the [G] vs time will be The order of reaction w.r.t [H] is second order. Hence the rate vs [H]2 is

9 Use of the Data Booklet is relevant to this question. Hydrogen iodide decomposes according to the equation shown. 2HI H2 + I2 The density of pure hydrogen iodide at 298 K is 2.85 g cm–3. Which expression gives the concentration of hydrogen gas that are present in 1.00 dm3 of pure hydrogen iodide at 298 K? A Kc√ 2.85 127.9 B 2.85 127.9 √Kc C √2850 127.9 Kc D 2850 127.9 √Kc Ans: D The Kc = [H2][ I2] / [HI]2 Therefore, [H2] = √(Kc[HI]2) = [HI]√Kc In 1 dm3 of pure HI, [HI] = (2.85 x 1000)/Mr of HI = 2850 / 127.9

9 [TURN OVER 10 A weak monoacidic base, J, has a pKb value of 3.5. 25.0 cm3 of 0.08 mol dm-3 HCl is titrated against 0.10 mol dm-3 of J. Which statements about the titration curve obtained are correct? 1 A suitable indicator for this titration is methyl orange. 2 The pH at maximum buffer capacity is 3.5. 3 The region of rapid pH change occurs at 20 cm3. 4 The volume at maximum buffer capacity is 10 cm3. A 1 and 3 only B 2 and 4 only C 1,2 and 3 only D 2,3 and 4 only Ans: A This is a weak base -strong acid titration. Hence the salt is acidic. pH at equivalence point will be lower than 7. A suitable indicator is therefore one that has the working range below 7 and methyl orange has working range below 7. Statement 1 is correct. After the equivalence point, it is a basic buffer. Hence at MBC, pOH = pKb =3.5 Therefore pH = 14