2023 TJC H2 Chem Prelim P2 (Ans)

9729 / TJC PRELIM / 2023 DO NOT WRITE IN THIS MARGIN [Turn over TEMASEK JUNIOR COLLEGE 2022 JC2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME ANSWERS CENTRE NUMBER S INDEX NUMBER Chemistry 9729/02 Paper 2 Structured Questions 23 August 2023 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your Centre number, index number, name and CG in the spaces at the top of this page. 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. Answer all the questions in the spaces provided on the Question Paper. 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 17 printed pages and 3 blank page. For Examiner’s Use Paper 1 /30 Paper 2 Q1 Q2 Q3 Q4 Q5 Q6 Total /75 Paper 3 /80 TOTAL (%) /100

2 DO NOT WRITE IN THIS MARGIN 9729 / TJC PRELIM / 2023 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN Answer all the questions in the space provided. 1 Atoms and ions of elements are made up of the three subatomic particles, protons, electrons and neutrons, in varying amounts. (a) Complete the following table to show the number of each particle in 7Li+ and 14C2–. Ion protons electrons neutrons [1] 14C2– 6 8 8 [1] 7Li+ 3 2 4 [2] (b) Separate beams of 14C2– and 7Li+ ions are passed through an electric field in the set- up below. The angle of deflection of the 14C2– beam is 7.0o. Sketch on the diagram below, the paths taken by beams of 7Li+ and 14C2– ions in the presence of an electric field. Label your sketch clearly. [2] [Total: 4] 14C2– 7Li+ 7o 7o [1] direction [1] equal angle

3 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC PRELIM / 2023 DO NOT WRITE IN THIS MARGIN [Turn over 2 (a) (i) Explain what is meant by the term ‘electronegativity’, and how it relates to the concept of bond polarity. [2] [1] Electronegativity refers to the ability of an atom to attract the shared electron pair in a covalent molecule/bond [1] Bond polarity is a measure of how equally the electrons are shared between the two bonded atoms. Bonded atoms with different electronegativities will result in a polar bond. (ii) State and explain the change in electronegativity across a period in the Periodic Table. [2] Across a Period ✓ Nuclear charge increases as number of protons increases. ✓ Screening effect remains approximately constant as electrons are added to the same electronic shell. ✓ Increase in the attractive force between the nucleus and the bonding electrons in the outer shell and atomic radius decreases. ✓ Trend: Electronegativity increases across the period. 2✓ : 1m (iii) Explain, in terms of electronegativity why the bonding in NaC l is different from the bonding in PCl5. [1] [1] Difference in electronegativity between Na and Cl is greater than between P and Cl. Hence transfer of electrons occur between Na and C l resulting in an ionic compound whereas sharing of electrons occur between P and C l resulting in a covalent compound. (b) Period 3 elements react with chlorine gas, C l2 (g), to form chlorides. The chlorides of the elements in the third period behave in different ways when added to water, depending on their structure and bonding. The table shows the differences in observations which occur when three Period 3 chlorides are added to water. Period 3 chloride Observations when added to excess water pH of solution formed with water NaCl White solid disappears. Colourless solution obtained. 7 PCl5 Colourless solution obtained. 1 Liquid Q Vigorous reaction with cold water. White solid and pale yellow solution obtained. 1 (i) Explain in terms of bonding, why NaC l and PCl5 behave differently when added to water. Write equation for any reaction that occurs. [2] [1] NaCl is ionic and it undergoes hydration in water. PCl5 is covalent and it undergoes hydrolysis in water to form an acidic solution. [1] PCl5(s) + 4H2O(l) → H3PO4(aq) + 5HCl(aq)

4 DO NOT WRITE IN THIS MARGIN 9729 / TJC PRELIM / 2023 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN (ii) PCl5 also reacts with limited amount of water. Write the equation for the reaction. [1] [1] PCl5(s) + H2O(l) → POCl3(l) + 2HCl(g) (iii) State the identity of Q. [1] [1] Q is SiCl4 (c) Phosphorus(V) chloride, PC l5, is involved in the following reaction scheme. Suggest the reagents and conditions for steps I, II and IV and the products W to Z in the synthetic scheme below. [7] Z W X Y Answer PCl5 [Total: 16] IV I II III PCl5 V [1] Ethanolic NaOH, heat [1] aq H2SO4, aq KMnO4, heat [1] aq NaOH, heat [1] each for W, X, Y, Z

5 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC PRELIM / 2023 DO NOT WRITE IN THIS MARGIN [Turn over 3 Group 2 elements of the Periodic Table are also known as the alkaline earth metals. The metals are named after their oxides, the alkaline earths. "Earth" was a term applied by early chemists to non-metallic substances that are insoluble in water and resistant to heating. Fig. 3.1 shows some reactions of the Group 2 element magnesium and its compounds. Fig. 3.1 (a) MgCO3(s) undergoes thermal decomposition in Reaction 1. Describe and explain the trend in thermal stability of Group 2 carbonates in terms of the charge density of the cation and polarizability of the large anion. [2] [2] 3 = 1 mark Down the Group, charge of the cation remains constant but ionic radius of cation increases. Hence  charge density of the cation decreases.  Polarising power of cation decreases resulting in  less distortion of the electron cloud of the large anion CO32– ,  weakening the C–O covalent bond less.  More energy (or higher temperature) needed to decompose the carbonate.  Carbonates become more stable to heat down the Group. (b) Write an equation for Reaction 3. Explain the relative reactivity of the Group 2 elements (magnesium, calcium and barium) with water, and relate this reactivity to relevant Eo values. [3] [1] Mg(s) + 2H2O(l) → Mg(OH)2(aq) + H2(g) EH2O/H2 o = −0.83 V  EMg2+/Mgo = −2.38 V ECa2+/Cao = −2.87 V EBa2+/Bao = −2.90 V Down the Group,  atomic radius i