2023 ACJC H2 Chem Prelim Paper 2 QP

1 © ACJC2023 9729/02/Prelim/2023 [Turn over Anglo-Chinese Junior College JC2 Preliminary Examinations Higher 2 CANDIDATE NAME FORM CLASS TUTORIAL CLASS INDEX NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/02 22 August 2023 2 hours READ THESE INSTRUCTIONS FIRST Write your index number and name 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, 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 Examiners’ use only 1 / 15 2 / 9 3 / 21 4 / 14 5 / 16 Total / 75 This document consists of 19 printed pages and 1 blank page.

2 © ACJC2023 9729/02/Prelim/2023 [Turn over 1 This question examines the chemistry of Group 17 elements and their halides. (a) A2 and B2 are halogens, and they are known to be more soluble in organic solvents. In an experiment, excess A2 was mixed with Na2S2O3(aq). When cyclohexane was added, two immiscible layers were observed which were later separated using a separatory funnel. An orange-red organic layer was obtained, and the aqueous layer was divided into two portions. To one portion of the aqueous layer, a solution of Ba(NO3)2 was added and a white precipitate formed which is insoluble in excess dilute nitric acid. To another portion of the aqueous layer, B2(aq) was added and shaken. When CHCl3 was added, a purple organic layer was obtained. (i) State the identities of A2 and B2. A2: ………………………… B2: ………………………… [1] (ii) Write a balanced ionic equation for the reaction between Na2S2O3 and A2. …………………………………………………………………………………………. [1] (iii) Explain why halogens are more soluble in organic solvents than in water, in terms of the energy changes involved. ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ………………………………………………………………………………………….. [2] (b) In another reaction, halogens can react with alkanes to form mono-substituted products. An example is the reaction of chlorine with 2-methylpentane. (i) Alkanes are generally unreactive. Explain why this is so. ………………………………………………………………………………………….. [1]

3 © ACJC2023 9729/02/Prelim/2023 [Turn over (ii) This reaction is seldom used for synthesis as there are many associated problems. Firstly, several isomeric products are formed. The relative ratio of the isomeric products may be more accurately determined if relative rates of abstraction of H atoms are considered. The relative rates of abstraction of H atoms are shown in Table 1.1. Table 1.1 type of H atoms relative rate of abstraction primary 1 secondary 4 tertiary 6 By examining the difference in stability of the intermediates formed when different types of H atom are abstracted, explain the trend in the relative rate of abstraction. ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ………………………………………………………………………………………….. [2] (iii) Predict the ratio of the following two products X and Y, from the reaction of chlorine with 2-methylpentane, taking into account the relative rates of abstraction given in Table 1.1. Explain your reasoning. [2]

4 © ACJC2023 9729/02/Prelim/2023 [Turn over (iv) Describe how you can distinguish between compounds X and Y in (b)(iii) by chemical means. ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ………………………………………………………………………………………….. [2] (v) Another problem of the reaction is poly-substitution. Suggest the condition that will give rise to formation of poly-substituted products. ………………………………………………………………………………………….. [1]

5 © ACJC2023 9729/02/Prelim/2023 [Turn over (c) Hydrogen halides are dissociated at high temperatures according to the following equation: 2HX(g) H2(g) + X2(g) The approximate Kc values for the above equilibrium at various temperatures for the respective hydrogen halides are shown in Table 1.2. Table 1.2 temperature / oC Kc values for dissociation of HX HCl HBr HI 800 10−13 10−9 10−5 1000 10−10 10−7 10−4 1200 10−9 10−5 10−3 1400 10−7 10−4 10−2 Using the information in Table 1.2 and relevant data from the Data Booklet, describe and explain the relative thermal stability of the hydrogen halides. ……………………………………………………………………………………………………… ……………………………………………………………………………………………………... ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ………………………………………………………………………………………………….. [3] [Total: 15]

6 © ACJC2023 9729/02/Prelim/2023 [Turn over 2 (a) Write the electronic configuration of Mg 2+ and explain why the ionic radius of Mg 2+ is smaller than the atomic radius of Mg. electronic configuration of Mg2+ …………………………………………………………… [1] explanation ……………………………………………………………………………………….. ……………………………………………………………………………………………………… ………………………………………………………………………………………………….. [1] (b) Table 2.1 gives the melting points, in °C, of the fluorides and chlorides of two elements in Period 3. Table 2.1 magnesium silicon fluoride 1261 –90 chloride 714 –70 Explain, in terms of structure and bonding, the differences in melting point between (i) MgCl2 and SiCl4 .…………………………………………………………………………………………….. ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... …………………………………………………………………………………………. [2] (ii) MgCl2 and MgF2 ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... …………………………………………………………………………………………. [1]

7 © ACJC2023 9729/02/Prelim/2023 [Turn over (c) The reaction scheme in Fig. 2.2 shows how a sample of anhydrous magnesium chloride is converted into a solution containing compound F. AgNO3(aq) filter white precipitate solution E F colourless solution MgCl2(s) Fig. 2.2 Upon analysis, F is found to contain the following percentage composition by mass: Ag, 52.6%; Cl, 17.3%; N, 13.6%; C, 11.7%; H, 4.8%. (i) Determine the empirical formula of F and hence suggest a possible id