2024 TMJC H1 Chem Prelim P2 (Ans)

CANDIDATE NAME SUGGESTED ANSWERS CIVICS GROUP ______________________________________________________________________________ H1 CHEMISTRY 8873/02 Paper 2 Structured Questions 12 September 2024 2 hours Candidates answer on the Question Paper. Additional materials: Data Booklet ______________________________________________________________________________ READ THESE INSTRUCTIONS FIRST Write your name and civics group in the spaces provided at the top of this page. Write in dark blue or black pen on both sides of the paper. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all the questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. ________________________________________________________________________________________ This document consists of 25 printed pages and 1 blank page. Examiner’s Use Paper 1 MCQ / 30 Paper 2 Section A Q1 / 9 Q2 / 6 Q3 / 14 Q4 / 8 Q5 / 10 Q6 / 13 Section B Q7 / 20 Q8 / 20 / 80 Total / 100 Grade TAMPINES MERIDIAN JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION

2 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H1 Chemistry Section A Answer all the questions in this section in the spaces provided. 1 (a) Aluminium and boron are two elements of Group 13. (i) Complete the electronic configuration for Al and Al3+. Al: 1s2 ………………………………. Al3+: 1s2 ………………………………. [1] (ii) Atoms of elements are made up of the three subatomic particles: protons, electrons and neutrons, in varying numbers. Complete Table 1.1 for each subatomic particle of an atom of Al13 26 . Table 1.1 [1] (b) The compounds of aluminium and boron have many uses. Alumin ium oxide, Al2O3, is commonly used as a refractory material for lining furnaces while aluminium chloride, AlCl3, is an antiperspirant agent. Boron trichloride, BCl3, is used in the refining process of aluminium. (i) Draw the dot-and-cross diagrams for Al2O3 and AlCl3. [1] [1] [2] (ii) Briefly describe the bonding in AlCl3 in terms of the overlap of orbitals. ………………………………………………………………………………………………. …………………………………………………………………………………………… [1] atom number of protons number of neutrons number of unpaired electrons Al13 26 13 13 1 There are 3 single Al–Cl covalent bonds where the orbitals overlap head-on to form () bonds. [1] 2s22p63s23p1 2s22p6 [1] for both [1] for all three entries

3 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H1 Chemistry [Turn over (iii) With reference to the Valence Shell Electron Pair Repulsion Theory , explain the shape and bond angle with respect to the B atom in BCl3. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. …………………………………………………………………………………………… [2] (c) Table 1.2 shows the melting points of two different boron compounds, BCl3 and NaBH4. Table 1.2 compound melting point / °C BCl3 – 107 NaBH4 400 With reference to structure and bonding, explain the difference in melting points of the two compounds. ……………………………………………………………………………………………….…… ………………………………………………………………………………………….………… …………………………………………………………………………………….……………… ……………………………………………………………………………….…………………… ………………………………………………………………………………………………… [2] [Total: 9] There are 3 bond pairs and 0 lone pairs around B atom. To minimise repulsion and maximise stability, it is trigonal planar [1]. There are equal bond pair – bond pair repulsions, bond angle is 120o. [1] NaBH4 has a giant ionic lattice structure, while BCl3 is simple molecular . More energy is required to overcome the stronger electrostatic attraction between oppositely charged ions in NaBH4 than the weaker instantaneous dipole – induced dipole attractions between BCl3 molecules, hence NaBH4 has a higher melting point than BCl3. [2]

4 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H1 Chemistry 2 This question is about ionisation energies of elements. (a) (i) Define the term first ionisation energy. ………………………………………………………………………………………………. ………………………………………………………………………………………………. …………………………………………………………………………………………… [1] (ii) Explain the general trend in first ionisation energy of elements down a Group in the Periodic Table. ………………………………………………………………………………………………. ……..……………………………………………………………………………………….. ………………………………………………………………………………………..…….. ………………..…………………………………………………………………………….. …….…………………………………………………………………………………….. [2] (b) The first six ionisation energies for an unknown element X is given in Table 2.1. Table 2.1 1st 2nd 3rd 4th 5th 6th ionisation energy of X / kJ mol–1 1050 1900 2910 4960 6270 21300 (i) Deduce the Group number of element X, giving your reasoning. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. …………………………………………………………………………………………... [2] The first ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of singly - charged gaseous cations. [1] First ionisation energy decreases down the Group as the number of protons increases, hence nuclear charge increases. However, the number of electron shells and shielding effect also increases. Hence, the valence electrons experience weaker electrostatic forces of attraction to the nucleus. Less energy is required to remove the valence electron. [2] The first big jump in ionisation energy takes place between the 5 th and 6 th electron. This means that the 6th electron is from an inner quantum shell . [1] Hence there are 5 valence electrons and thus element X is in Group 15. [1]

5 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H1 Chemistry [Turn over (ii) The graph in Fig . 2.1 shows a sketch of the first ionisation energies of six consecutive elements within Periods 2 and Period 3. The letters do not represent the symbols of elements. Fig. 2.1 Identify the alphabet letter in Fig. 2.1 that belongs to element X. ………………………………………………………………………………………...… [1] [Total: 6] X corresponds to letter F. [1] / kJ mol–1

6 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H1 Chemistry 3 (a) Ethanoic acid is a weak acid. CH3COOH CH3COO– + H+ A sample of ethanoic acid can be mixed with aqueous sodium ethanoate to make a buffer solution. (i) Define the term pH. …………………………………………………………………………………………….… …………………………………………………………………………………………… [1] (ii) Explain with aid of equations, how the sample mixture of ethanoic acid and sodium ethanoate behaves as a buffer. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. …………………………………………………………………………………………… [3] (iii) The sample of ethanoic acid with a pH value of 3.0 has a molar concentration of 0.0750 mol dm–3. The degree of dissociation, , is the percentage of acid molecules that dissociate into ions:  = [acid]dissociated [acid]initial  100% Using this information, determine  for the sample of ethanoic acid.  = ……………………..% [2] It is defined as the negative logarithm to base 10 of the concentration of H+ in solution in mol dm–3 OR pH = –log10[H+] [1] When small amount of acid, H+, is added, CH3COO− + H+ → CH3COOH [1] The added H+ is removed as CH3COOH, [H+] is slightly changed hence pH remains fairly constant. When a small amount of base, OH− is added, CH3COOH +