2024 Prelims NYJC H2 Chem P2 (Ans)

[Turn over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS CHEMISTRY 9729/02 Paper 2 Structured 9 September 2024 2 hours Candidates answer on the Question Paper Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class in the spaces at the top of this page. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 /12 2 /7 3 /6 4 /6 5 /11 6 /11 7 /22 Total /75 This document consists of 21 printed pages and 2 blank pages. H

2 H2 Chemistry 9729/02 NYJC J2/24 PX [Turn Over For Examiner's Use Answer all questions in the space provided. 1(a) The Periodic Table usually shows hydrogen on its own and not a member of a group. (i) Suggest why the element hydrogen could be placed at the top of Group 1. Hydrogen atom has one valence electron and it has a smaller number of protons than lithium ..................................................................................................... [1] (ii) Suggest why the element hydrogen is not placed at the top of Group 1, in terms of • its structure • one physical property related to its structure Hydrogen has a simple molecular structure with weak instantaneous dipole – induced dipole interaction between the molecules, while Group I elements have a giant metallic structure with strong metallic bonds between the metallic cations and sea of delocalised valence electrons. [1] (Difference in structure of hydrogen and Group I elements) The sea of delocalised valence electrons in Group I elements act as mobile charge carriers to conduct electricity , while hydrogen does not contain mobile charged carriers /electrons are localized in covalent bonds. Hence does not conduct electricity. [1] (Difference in Physical property for hydrogen and Group I elements, accept other physical properties like mpt/bpt etc) ..................................................................................................................... [2] The trihydrogen complex cation, H 3+, was first discovered in 1911 when J.J . Thomson was performing his cathode ray tube experiments. In 1989, H 3+ was detected in the ionosphere of Jupiter. H3+ is mainly found in interstellar space where there is high cosmic radiation. In the presence of high energy gamma radiation, hydrogen gas can lose an electron to form H 2+, which then reacts with molecular hydrogen to form H3+. H2 → H2+ + e− H2+ + H2 → H3+ + H (b) Describe and explain how the behaviour of beams of the ions H2+, H3+ and electrons differ in an electric field. ion H2+ H3+ angle of deflection ∝ charge mass 1/2 1/3 Simplest ratio 3/2 1 Both H2+, H 3+ are deflected towards the negative plate. Angle of deflection of H2+ will be greater than that of H3+ (3/2 times). [1] An electron will be deflected towards positive plate by a much larger angle of deflection compared to H2+ and H3+ due to the much smaller mass of an electron compared to a proton. [1] [2] (c) Given that hydrogen can exist as isotopes 1H, 2H and 3H, complete Table 1.1.

3 H2 Chemistry 9729/02 NYJC J2/24 PX [Turn Over For Examiner's Use Table 1.1 number of protons number of neutrons number of electrons Explanation (not required in answer) 2H 1 1 1 each 2H atom has 1 electron, 1 proton and 1 neutron 1H2+ 2 0 1 each 1H atom has 1 electron, 1 proton and 0 neutrons 3H3+ 3 3 atoms x 2 = 6 2 each 3H atom has 1 electron, 1 proton and 2 neutrons [3] [1] for any 3 boxes correct (d) Describe the reactions, if any, of the chlorides NaCl, AlCl3 and PCl5 with water. Write equations for all the reactions that occur, and suggest the pH of the resulting solutions. Relate the reactivity of these chlorides to their structure and bonding. 1. NaCl has a giant ionic lattice structure where the Na+ and Cl− ions are held together by strong ionic bonds. 2. Hence the ions are able to form ion – dipole interactions with water, thus NaCl undergoes hydration to form a neutral solution of hydrated ions. 3. pH = 7 4. NaCl(s) + aq → Na+(aq) + Cl‾(aq) [state symbols are required] 5. AlCl3 has simple molecular structure with weak instantaneous dipole – induced dipole interactions between the molecules. 6. AlCl3 has a slight ionic character, allowing it to undergo hydration. 7. AlCl3(s) + 6H2O(l) → [Al(H2O)6]3+(aq) + 3Cl‾(aq) [Al(H2O)6]3+ then undergoes hydrolysis to give [Al(H2O)5OH]2+ (aq) (part of marking pt 6) 8. [Al(H2O)6]3+ (aq) + H2O (l) Ý [Al(H2O)5OH]2+ (aq) + H3O+ (aq) Al3+ in [Al(H2O)6]3+ has a small size and high charge, thus has a high charge density and is highly polarising. Thus it is able to polarise the O−H bond in H2O, causing H+ to be released to give an acidic solution . 9. pH  2−3 10. PCl5 has simple molecular structure with weak instantaneous dipole – induced dipole interactions between the molecules. 11. PCl5 undergoes hydrolysis to give a highly acidic solution and white fumes of HCl gas. When PCl5 dissolves, the slightly positive P atom attracts the slightly negative O atom from water, breaking the O−H bond and releasing a H+ to make the solution acidic. 12. pH  1− 2 13. PCl5(s) + 4H2O(l) → H3PO4(aq) + 5HCl(g) 13 marking points in total. 3-4 marking pt – 1 mark, 5-8 marking pt – 2 marks, 9-11 marking pt – 3 marks, 12-13 marking pt – 4 marks (also subjected to no major conceptual error) ................................................................................................................................ [4] [Total: 12]

4 H2 Chemistry 9729/02 NYJC J2/24 PX [Turn Over For Examiner's Use 2 Ozone, O 3, is a non -linear molecule present in the Earth’s upper atmosphere. It absorbs ultraviolet radiation from the Sun and so protects living organisms from this type of radiation. (a) (i) Draw the dot-and-cross diagram for the ozone molecule. O O Oxx xx xx [1] [1] (ii) State one harmful consequence to a person of increased exposure to ultraviolet radiation. uv radiation can penetrate the skin and reach the cells, resulting in increased risk of skin cancer/sun burn. [1] Nitrogen oxides in aircraft emissions results in the depletion of the ozone layer. The reaction of nitrogen monoxide with ozone is given below. NO(g) + O3(g) → NO2(g) + O2(g) (b) (i) Suggest and explain the following differences between NO2 and O3. . • Bond angle around central atom in O3 is smaller than that in NO2. There are 2 bond pairs and 1 lone pair around central O in O 3 while there is 1 lone electron and 2 bond pairs around N in NO2. There is less repulsion between the lone electron and bond pairs hence bond angle is >120 o (and <180o since lone e exerts some repulsion vs 0lp) while there is more repulsion between lone pair and bond pairs hence bond angle is <120o. [1] FYI structure of NO2: N O OH2 NH2 • Boiling point of O3 is lower than that of NO2. Both have simple molecular structure and contains permanent dipole -permanent dipole interactions between polar molecule. There is a larger difference in electronegativity between N and O res