2025 TKGS Chemistry Prelim Paper 2 MS

TANJONG KATONG GIRLS’ SCHOOL PRELIMINARY EXAMINATION SECONDARY FOUR EXPRESS CHEMISTRY CANDIDATE NAME ANSWERS CLASS 4 INDEX NUMBER CHEMISTRY 6092/02 Paper 2 15 August 2025 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 24. The use of an approved scientific calculator is expected, where appropriate. For Examiner’s Use Section A Section B Total / 80 This document consists of 24 printed pages.

2 Tanjong Katong Girls’ School Sec 4 Preliminary Examination 2025 Section A Answer all questions. 1 The electron arrangement in the outer shells of five elements, A, B, C, D and E are shown in Fig.1.1. All elements are from Period 3 of the Periodic Table. Fig. 1.1 Use the letters A, B, C, D and E to answer the following questions. Minus [1] from Q1 if they use name of elements. Each letter may be used once, more than once or not at all. (a) Which two elements are most likely to be metals? C and D [1] (b) Which element has an atomic number of 16? A [1] (c) Which element will form double covalent bonds with an oxygen atom? A [1] (d) Which two elements will form an ionic compound with the formula type XY? C and E OR D & A [1] (e) Which element will form an oxide that reacts with both acids and bases? C [1] [Total: 5]

3 Tanjong Katong Girls’ School Sec 4 Preliminary Examination 2025 2 (a) Table 2.1 shows information of atoms. Complete Table 2.1. Table 2.1 atoms number of protons number of neutrons number of electrons K19 39 19 20 19 Cr24 52 24 28 24 [2] [1] for each correct row (b) The strength of interactions between particles determines whether the substance is a solid, liquid or gas at room temperature. Potassium oxide, K2O, is a solid while sulfur trioxide, SO3, is a liquid at room temperature. (i) Explain in terms of structure and bonding, why potassium oxide is a solid while sulfur trioxide is a liquid at room temperature. K2O has a giant ionic structure with strong electrostatic forces of attraction between oppositely charged ions . This requires large amount of energy to weaken, hence, K2O exists as a solid. accept if students wrote “ionic bonds” instead of “electrostatic forces of attraction between oppositely charged ions” SO3 has a simple molecular / simple covalent structure with weak intermolecular forces of attraction. This requires small amount of energy to weaken, hence, SO3 exists as a liquid. [1] correct structure for both K2O and SO3 [1] correct bonding for both K2O and SO3 [1] compare strength / energy required OR [1] correct structure + bonding for either K2O and SO3 Candidates who did not gain full credits due to the following - Failure to mention the “giant ionic structure” and “simple covalent structure”. - Give contradicting statements such as “strong electrostatic forces of attraction between atoms” for K2O and “weak intermolecular forces of attraction between atoms/ions”

4 Tanjong Katong Girls’ School Sec 4 Preliminary Examination 2025 (ii) State one other difference in the physical properties of potassium oxide and sulfur trioxide. K2O conducts electricity in the molten and aqueous state while SO3 does not conduct electricity.[1] Candidates did not gain credit miss out the point that ionic compound only conduct electricity “in molten and in aqueous” state. Some candidates give melting and boiling point as an answer despite the question indicating to state other difference. (iii) Draw a ‘dot-and-cross’ diagram to show the bonding in K2O. Show all outer shell electrons. K x x x x x x x x + O 2- x x 2 [1] for cation and [1] for anion Candidates need to follow instructions and show only the outer shells electrons. [2] (c) Potassium oxide, aluminium oxide and sulfur trioxide are each added to separate samples of dilute sulfuric acid and aqueous sodium hydroxide. Complete Table 2.2 to show whether a reaction occurs in each case. Put a tick (✓) in the box if a reaction occurs. Table 2.2 dilute sulfuric acid dilute sodium hydroxide potassium oxide ✓ aluminium oxide ✓ ✓ sulfur trioxide ✓ [2] [2] for all correct oxides [1] for two correct oxides [Total: 10]

5 Tanjong Katong Girls’ School Sec 4 Preliminary Examination 2025 3 A student performs three experiments. In the first experiment, she adds large pieces of copper(II) carbonate to dilute hydrochloric acid. Copper(II) carbonate added is in excess. (a) Fig. 3.1 shows how the mass of the reaction mixture changes with time as the reaction proceeds. Fig. 3.1 (i) In the second experiment, powdered copper(II) carbonate is used instead of large pieces of copper(II) carbonate. All other conditions and the mass of copper(II) carbonate stay the same. Draw a line on the grid in Fig. 3.1 to show how the mass of the reaction mixture changes with time when powdered copper(II) carbonate is used. [1] [1] initial gradient steeper and line starts at 200.0 + line is curved and ends at the same final mass + line end at earlier time. (ii) In the third experiment, the student uses large pieces of copper(II) carbonate and hydrochloric acid of a higher concentration. All other conditions stay the same. Describe and explain the difference in the rate of reaction when hydrochloric acid of a higher concentration is used. time / min mass of reaction mixture /g

6 Tanjong Katong Girls’ School Sec 4 Preliminary Examination 2025 When hydrochloric acid of a higher concentration is used, the rate of reaction increases. [no marks] There are more reactant particles per unit volume .[1] Hence, frequency of effective collision increases [1] resulting in increased reaction rate. Common mistakes • Candidates mentioned “HCl molecules” , showing that they have forgotten acids dissociates in aqueous solutions to form H+ ions. • “Rate of effective collisions”, “more effective collisions”, “chances of effective collisions” are not accepted. Better candidates recognise that H + ions are responsible for acidic reactions and wrote “more H+ ions per unit volume” to explain the increase in concentration of acid. (b) After the copper( II) carbonate has reacted completely with hydrochloric acid, the student places a clean strip of zinc metal into the resulting solution. Suggest and explain the observations. A reddish-brown / brown solid is formed. / Blue solution fades. [1] Zinc displaces copper from copper(II) ions as zinc is more reactive.[1] Accept observations of CuCO3 reacting with HCl: Effervescence [1] Acid + metal carbonate forms CO2 [1] Reject “zinc displace Cu2+”, “Cu2+ is discharged” Common mistakes: • Some candidates assumed that the HCl is in excess. The HCl could be reacting in good ratio with the CuCO 3 and hence be used up as well. Suggestions of zinc reacting with exce