2024 Prelims YIJC H2 Chem P1 (Ans)

©YIJC [Turn over YISHUN INNOVA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUGGESTED SOLUTIONS CG INDEX NO CHEMISTRY Paper 1 Multiple Choice Additional Materials: Multiple Choice Answer Sheet Data Booklet 9729/01 13 September 2024 1 hour READ THESE INSTRUCTIONS FIRST Write in soft pencil. Do not use staples, paper clips, highlighters, glue or correction fluid/tape. Write your name and class on the Answer Sheet in the spaces provided unless this has been done for you. 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. Read the instructions on the Answer Sheet very carefully. 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.

©YIJC [Turn over 2 1 A sample of calcium contains three naturally occurring isotopes, 40Ca, 42Ca and 44Ca. The sample is made up of 96.64% of 40Ca and the relative atomic mass of calcium in this sample is 40.11. What is the percentage of the isotope 42Ca in the sample? A 0.44% B 0.67% C 1.22% D 2.09% Answer: C 40Ca 96.64% 42Ca x% 44Ca (3.36 – x)% 40.11 = (40.0 × 96.64) + (42.0x) + [(3.36 − x) × 44.0] 100 x = 1.22% 2 Aluminium chloride dimerises to give Al2Cl6. Which statement about aluminium chloride and its dimer is correct? A AlCl3 has a high melting and boiling point due to strong electrostatic forces of attraction between Al3+ and Cl−. B Each aluminium atom is surrounded by four chlorine atoms in the dimer, Al2Cl6. C Some of the bond angles decrease while some remains the same when AlCl3 dimerises to form Al2Cl6. D The geometry around each aluminium atom in Al2Cl6 is trigonal planar. Answer: B A: Aluminium chloride has simple molecular structure due to the high polarising powe of A l3+ and the polarisability of the Cl− ions. B: From the above diagram, each aluminium atom is surrounded by 4 chlorine atoms. C and D: As seen in the diagram, the bond angles will all decrease when aluminium chloride dimerises as the shape around the central aluminium atom changes from trigonal planar (120°) to tetrahedral (approximately 109.5°.) Al Cl Cl Cl xx xxxx xx xx xx xxxx xx + Al Cl Cl Cl xx xxxx xx xx xx xxxx xx Al Cl Cl Cl Al ClCl Clxx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx

©YIJC [Turn over 3 3 The graph shows the behaviour of one mole of an ideal gas. Which of the following will produce the graph shown? y-axis x-axis condition A pV p at constant temperature B p V at constant temperature C p 1/V at constant temperature D T (in K) V at constant pressure Answer: B For option A: If temperature is constant for 1 mol of an ideal gas, • pV = nRT = constant • The pV against p plot will be a horizontal line that cuts the y-axis at a value of RT. For Option B: If temperature is constant for 1 mol of an ideal gas, • pV = nRT  p = nRT V • The p against V plot will be a curve as shown. For option C: If temperature is constant for 1 mol of an ideal gas, • pV = nRT  p = nRT V • The p against 1 V will be a straight line that passes through the origin. For option D: If pressure is constant for 1 mol of an ideal gas, • pV = nRT ⇒ T = p nR V • The T against V will be a straight line that passes through the origin. p pV RT p V x y p V

©YIJC [Turn over 4 4 Sodium, magnesium, aluminium, silicon and phosphorus are all elements in Period 3 of the Periodic Table. Three statements about the oxides and chlorides of these elements are given. Which statements are correct? 1 The ionically bonded oxides all react with dilute hydrochloric acid. aldehyde 2 All metal chlorides produced neutral solutions when added to water. alkene 3 The two most electronegative elements both form covalently bonded chlorides. amide A 1, 2 and 3 B 1 and 2 only C 1 and 3 only D 2 and 3 only Answer: C 1 Correct. Ionic oxides are basic in nature and hence will react with dilute hydrochloric acid. 2 Incorrect. Only NaCl gives a neutral solution. MgCl2 and AlCl3 give an acidic solution. 3 Correct. The two most electronegative elements are Si and P. The chlorides of both elements are covalent chlorides.

©YIJC [Turn over 5 5 The table shows the trend in the stated properties from magnesium to barium in Group 2. Which row is correct? decomposition temperature of the carbonate reducing power A increases increases B increases decreases C decreases increases D decreases decreases Answer: A The thermal stabilities of the Group 2 carbonates increase down the group. This is because going down the group, while the charge of the cations (+2) remains the same, the size of cation (or ionic radius) increases. This causes the charge density and hence the polarising power of the cations to decrease. As such, the cations are less able to polarise the electron cloud of the carbonate ion and the C -O covalent bonds within the carbonate ion are weakened to a smaller extent. Consequently, more energy and a higher temperature is required for the decomposition of the Group 2 carbonates. The reducing power of Group 2 elements increases down the group. Eө / V Mg2+ + 2e- ⇌ Mg –2.38 Ca2+ + 2e- ⇌ Ca –2.87 Ba2+ + 2e- ⇌ Ba –2.91 Down Group 2, Eө values become more negative. Equilibrium position lies more to the left, hence the higher tendency for oxidation process.

©YIJC [Turn over 6 6 Use of the Data Booklet is relevant to this question. The element astatine, At, is in Group 17 of the Periodic Table. 1 It is a coloured solid at room temperature. The ionically bonded oxides all react with dilute hydrochloric acid. aldehyde 2 Its hydride is thermally more stable than the hydride of iodine. All metal chlorides produced neutral solutions when added to water. alkene 3 It is more electronegative than iodine. The two most electronegative elements both form covalently bonded chlorides. amide Which statements about At will be true? A 1, 2 and 3 B 1 and 2 only C 2 and 3 only D 1 only Answer: D Astatine is below iodine in Group 7 of the Periodic Table. Since iodine is a black solid, At is expected to be a coloured solid. The hydride of astatine, HAt, is expected to have a weaker H-At bond than H-I bond. Hence the H-At bond can be overcome by heat more easily and HAt is not thermally stable. Electronegativity decreases down the group and thus At is less electronegative than iodine.

©YIJC [Turn over 7 7 When 30 cm3 of a hydrocarbon was burnt in excess oxygen, there was a contraction of 105 cm3. On treating the cooled resulting mixture with excess potassium hydroxide, a further contraction of 150 cm3 occurred. All volumes were measured at r.t.p. What is the formula of the hydrocarbon? A C3H6 B C3H8 C C5H10 D C5H12 Answer: D First contraction: volume of CxHy reacted + volume of O2 reacted – volume of CO2 formed = 105 cm3 volume of O2 reacted = 105 – 30 + 150 = 225 cm3 Second contraction: volume of CO2 produced = 150 cm3 CxHy + (x + y 4) O2 ⎯⎯ → xCO2 + ( y 2) H2O volume of gas used or formed / cm3 30 225 150 - volume ratio 1 7.5 5 stoichiometry mole ratio 1 x +