NJC 2008 H2 CM Prelim P2

Registration Number ST group Name Do not open this booklet until you are told to do so. Write your name, class and registration number in the spaces provided at the top of this page. For Examiner’s Use 1 2 3 4 5 6 Total READ THESE INSTRUCTIONS FIRST Do not open this booklet until you are told to do so. Write your name, class and registration number in the spaces provided at the top of this page. A Y nswer ALL questions. ou are advised to show all the working in calculations. You may use a calculator. The number of marks is given in brackets [ ] at the end of each question or part of the question. This question paper consists of 14 printed pages (including this page) NATIONAL JUNIOR COLLEGE Preliminary Examinations 2008 CHEMISTRY Higher 2 9746/02 P A P E R 2 10 Sept 2008 1 hour 30 min Candidates are to answer on the Question paper. Additional Materials: Data Booklet 1

1 A section of the periodic table showing elements in periods 2 and 3 is shown below. T W X Y Element X has the following successive ionization energies. IE/ kJmol−1 1011 1907 2914 4963 6273 21267 25431 29872 (a) (i) Which group does element X belong to? [1] (ii) Do you expect the first ionization energy of element Y to be greater or smaller than that of X? Explain. [2] (b) (i) When dissolved in water, the chlorides of elements W and X produce solutions that turn blue litmus red. Explain. [2] 2

(ii) Deduce whether the same observation is seen for the chloride of element T. [2] (c) Element Z is in the same group as X but with a much greater proton number. The oxide of Z is seen to react with hydrochloric acid. Explain. [2] [Total: 9] 3

2 (a) (i) State two assumptions of the kinetic theory of gases. [2] (ii) Under what conditions is the behaviour of ammonia most nearly ideal? Explain your answer. [2] (b) The graph below shows experimental values of the product PV for a mass of ammonia against P at constant temperature. Which graph (A , B or C) shows the relationship between the product PV and P for carbon dioxide? Explain your choice. [2] 4

(c) Ethylamine can be synthesized by reacting ammonia with chloroethane. (i) State the reagents and conditions required for the reaction. [1] (ii) Chloroethane may be converted to dipropylamine by the following route: CH 3CH2Cl E F (CH 3CH2CH2)2NH Stage I Stage II Stage III (I) Suggest reagents and conditions for Stages I and II. Stage I : Stage II: [2] (II) Draw the structural formulae of E and F. Compound E Compound F [2] 5

(III) Describe the mechanism in Stage I. [3] (iii) Comment on the base strength of ammonia, ethylamine and dipropylamine. [2] [Total: 16] 6

3 Very often water contains various amounts of impurities which render it unsuitable for human consumption and other specific purposes. Thus, it is necessary to purify water before using it. The method of purification depends on how the water is to be used, or has been used. (a) One of the methods involves the removal of excess iron from drinking water. This involves oxidation of the Fe 2+ ions by chlorate(I) ion, OCl −, followed by addition of a base. The precipitate formed will then be removed. (i) Given OCl − will be converted to Cl−, construct a balanced equation to show its reaction with Fe2+. [2] (ii) The recommended percentage of iron permitted in drinking water in Singapore is 0.3 mg dm −3. Calculate the concentration of chlorate(I) ion in mg dm−3 needed to lower Fe2+(aq) from 15 mg dm−3 to the required safety level. [2] 7

(b) Steel boilers that convert water to steam can be easily corroded by the dissolved oxygen in the water. Fortunately, liquid hydrazine, N 2H4, is an efficient oxygen scavenger and can be used to overcome the problem as it can remove the dissolved oxygen. (i) Draw a ‘dot-and-cross’ diagram of N 2H4. [1] (ii) Suggest the size of the bond angle in N 2H4. [1] (iii) Liquid N 2H4 is a scavenger of dissolved oxygen as shown by N2H4 (l) + O2 (aq) → N2 (g) + 2 H2O (l) Explain the feasibility of such method of removing dissolved oxygen from boiler water in terms of enthalpy and entropy changes. (No calculations are required.) [2] (c) Hydrazine, N 2H4, is not only useful for purifying water but also finds many applications in organic chemistry. One of such applications is the reaction with the carbonyl group. (i) Draw the structure of the organic product formed when equimolar quantities of benzaldehyde (C 6H5CHO) and hydrazine react. [1] 8

(ii) Explain why the product from the reac tion of benzaldehyde with hydrazine may exists as two stereoisomers but the product from C 6H5COC6H5 with hydrazine does not have stereoisomers. [2] (d) Engineers in Japan are developing a fuel cell that uses hydrazine instead of hydrogen to power automobiles and other motorized vehicles. N2H4 O2 Aqueous NaOH Given that one of the products formed is nitrogen gas, write the half-cell equations occurring at the anode and the cathode, and hence the overall reaction of the fuel cell. Anode: Cathode: Overall: [2] [Total: 13] 9

4 The solubilities of PbCl2 in various concentrations of HCl at 25oC are as shown: HCl /mol dm−3 0 0.5 1.0 1.20 2.90 4.02 5.78 Solubility of PbCl2 (× 10−2 mol dm−3) 6.60 5.10 4.91 4.93 5.90 7.48 14.0 (a) Explain the data as fully as you can. (b) Calculate a value for the solubility product of PbCl 2 at 25oC. [5] [Total: 5] 10