TKGS 2024 Chem Prelim P2 MS

TANJONG KATONG GIRLS’ SCHOOL PRELIMINARY EXAMINATION SECONDARY FOUR EXPRESS CANDIDATE NAME MARK SCHEME CLASS 4 INDEX NUMBER CHEMISTRY 6092/02 Paper 2 12 August 2024 1 hour 45 minutes Candidates answer on the Question Paper. No Additional Materials are required. READ THESE INSTRUCTIONS FIRST Write your Centre number, index number and name on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. DO NOT WRITE ON ANY BARCODES. Section A Answer all questions. Write your answers in the spaces provided. Section B Answer one question. Write your answers in the spaces provided. The number of marks is given in brackets [ ] at the end of each question or part question. A copy of the Periodic Table is printed on page 26. The use of an approved scientific calculator is expected, where appropriate. For Examiner’s Use Section A Setter : Ms Vina Ng Markers : Mrs Lee Suan Ching, Ms Esther Foo, Mrs Chan Liying, Ms Vina Ng Section B Total / 80 This document consists of 26 printed pages.

2 Tanjong Katong Girls’ School Sec 4 Preliminary Examination 2024 Section A Answer all questions. 1 (a) Fig. 1.1 shows part of the Periodic Table. H e C N O F Ne S Cl Ar Br Kr Fig. 1.1 Select elements from Fig. 1.1 to answer the following questions. You may use each element once, more than once or not at all. Write down the symbol for an element which (some students gave more than one element): (i) exists as diatomic molecules, N / O / F / Cl / Br [1] No marks awarded if student gives mo re than one answer and there is a wrong answer given among the correct ones. (ii) is a noble gas, He / Ne / Ar / Kr [1] (iii) has seven electrons in its outer shell. F / Cl / Br [1]

3 Tanjong Katong Girls’ School Sec 4 Preliminary Examination 2024 (b) Some versions of the Periodic Table place hydrogen in Group 1 (Fig. 1.2). Other versions place hydrogen alone and not with any other group (Fig. 1.3). Fig. 1.2 Fig. 1.3 (i) Give one similarity between hydrogen and the elements in Group 1. Any one point [1]: They have one electron in the outer shell. They have one valence electron. They form ions with the 1+ charge. They lose one electron to form ions. They react with non-metals. They act as reducing agents. They form charge of +1/1+ They have valency 1 Reject: They have one valence electron shell They have high reactivity The y have oxidation state of +1 Accept: Valance (BOD for spelling error) (ii) Give one property of hydrogen that does not fit with the properties of the elements in Group 1. Any one point [1]: It is a non-metal/ not a metal. It does not conduct electricity. It is a gas. It has lower boiling and melting point. It has lower density. It exists as a simple covalent compound. It does not react with water. It does not react with acids. It does not form basic oxide. It can form acidic compounds. It is a diatomic molecule.

4 Tanjong Katong Girls’ School Sec 4 Preliminary Examination 2024 It gains one electron. Reject: It is not shin y / dull. It has low reactivity (without specifying the reactant). (iii) Hydrogen is in Period 1 of the Periodic Table. Explain wh y Period 1 only contains 2 elements. The first electron shell can hold up to two electrons. [1] OR The two elements in Period 1 have two or less electrons. (WTTE) (c) Hydrogen is used to make margarine from vegetable oil. Describe, in terms of bonds, what happens when hydrogen reacts with vegetable oil. ……………………………………………………………………………………………. ………………………………………………………………………………………….[2] [Total: 8] The carbon-carbon double bond/ C=C [1] in vegetable oil undergoes an addition/hydrogenation [1] reaction with hydrogen/hydrogen is added. Reject: Double carbon/ double carbon-carbon bond Accept: Double covalent bonds between carbon atoms

5 Tanjong Katong Girls’ School Sec 4 Preliminary Examination 2024 2 (a) There is a universal way for describing at oms across all scientific disciplines. In this system, the nuclide of an element is represent ed by the atomic symbol, atomic number and the nucleon number. Naturally occurring lithium contains the isotopes Li 3 6 and Li3 7 . (i) What is meant by the term, nucleon number? Nucleon number is the total number of protons and neutrons in the nucleus. [1] (ii) The isotopes Li3 6 and Li3 7 have accurate isotopic masses of 6.015 and 7.016 respectively. The Ar of lithium is 6.942. Calculate the relative percentage abundances of the two isotopes, to two decimal places. [2] Let the relative percentage abundance of Li 3 6 be x% ( x 100)(6.015) + ( 100ିx 100 )(7.016) = 6.942 [1] (x)(6.015) + (100 − x)(7.016) = 694.2 6.015x + 701.6 − 7.016x = 694.2 −1.001x = − 7.4 x = 7.3926% relative percentage abundance of Li 3 6 is 7.39% relative percentage abundance of Li3 7 is 92.61% [1] for both

6 Tanjong Katong Girls’ School Sec 4 Preliminary Examination 2024 (iii) A nuclear reaction is a reaction in which there is a change to an atomic nucleus. An experimental nuclear reactor uses Li 3 6 and deuterium, H1 2 , as a fuel. Three nuclear reactions between thes e two atoms are described below (p+ is a proton; n is a neutron). The number of protons and neutrons must be conserved in each reaction. Li 3 6 + H1 2 → 2H e 2 4 Li3 6 + H1 2 → He 2 4 + X + n Li3 6 + H1 2 → Y + p+ Suggest the identities of X and Y. Li3 6 + H1 2 → He 2 4 + X + n Li3 6 H 1 2 He 2 4 n no. of protons 3 1 2 no. of neutrons 3 1 2 1 No. of protons on reactants side = 4 No. of neutrons on reactants side = 8 − 4 = 4 No. of protons and neutrons mu st be conserved. Hence, X must have 2 protons and 1 neutron. X must have proton number of 2 and a nucleon number of 2 + 1 = 3 . Hence, X = He 2 3 [1] no need to show working Li3 6 + H1 2 → Y + p+ Li3 6 H 1 2 p+ no. of protons 3 1 1 no. of neutrons 3 1 No. of protons on reactants side = 4 No. of neutrons on reactants side = 8 − 4 = 4 No. of protons and neutrons mu st be conserved. Hence, X must have 3 protons and 4 neutrons.

7 Tanjong Katong Girls’ School Sec 4 Preliminary Examination 2024 Y must have proton number of 3 and a nucleon number of 3 + 4 = 7 Hence, Y = Li3 7 [1] no need to show working Since the question asks for the identity of X and Y, the identities, i.e. He and Li should be written. This is in addition to the proton numbers and nucleon numbers. Reject: Isotopes of He/ Li Lithium and Helium (b) Aqueous copper(II) sulfate was electrolysed in two cells using different electrodes as shown in cells 1 and 2. (i) Write ionic equations for the reactions at the positive electrodes. Cell 1: 4OH –(aq) → 2H2O(l) + O2(g) + 4e– [1] Cell 2: Cu(s) → Cu2+(aq) + 2e– [1] Reject: e+ Marks will be deducted if no state symbols are written (ii) Describe the change that would be s een at the negative electrode in both cells. Red-brown/ brown/ pink solid / copper is deposited/coated onto the electrode.[1] OR The cathode increases in mass/ increase in size. Reject: carbon electrode aqueous copper(II) sulfate copper electrode aqueous copper(II) sulfate