MJC H2 CHEM P2 ANS

Name ________________________ Class: ___________ Reg Number: _____ MERIDIAN JUNIOR COLLEGE JC2 Preliminary Examination Higher 2 ___________________________________________________________________ CHEMISTRY 9647/02 Paper 2 Structured Questions 19 September 2011 2 hours Additional Materials: Data Booklet. ___________________________________________________________________ INSTRUCTIONS TO CANDIDATES Write your name, class and register number in the spaces provided at the top of this page. Write your calculator brand and model/number in the box provided above. Answer all questions in the spaces provided on the question paper. All working must be shown clearly. INFORMATION FOR CANDIDATES This document consists of 20 printed pages. FOR EXAMINER’S USE Q1 / 12 Q2 / 17 Q3 / 21 Q4 / 10 Q5 / 12 Total / 72 The number of marks is given in brackets [ ] at the end of each question or part question. You are reminded of the need for good English and clear presentation in your answers. Calculator Model / No.

2 Answer all questions in the space provided. 1 Planning (P) You are provided with solutions FB 3 , FB 4 and FB 5 . FB 3 and FB 4 are either 1.0 mol dm –3 nitric acid or 2.0 mol dm –3 ethanoic acid whereas FB 5 is 1.0 mol dm –3 sodium hydroxide. You are to plan experiments that will enable you to identify the solution that matches each of FB 3 and FB 4 , hence, calculate /uni2206Hneutralisation for the reaction between ethanoic acid and sodium hydroxide. You are provided with the following apparatus: –10 ° C to 110 ° C thermometer, 100 cm 3 beaker, 50 cm 3 measuring cylinder The measuring cylinder should be rinsed thoroughly before each use. (a) (i) When determining the identity of the two acids lis ted above, the independent variable is the nature or strength of acids the dependent variable is temperature change (ii) Plan an experiment to find which of the solutions i s 1.0 mol dm –3 nitric acid and which is 2.0 mol dm –3 ethanoic acid. Your plan must identify the acids solely based on t he change in temperature. Mathematical processing of the tempera ture change is thus not required. Your experiment(s) must use only the solutions and apparatus above.

3 Your plan should give a step by step description of the method including: • the apparatus used for measurement • appropriate volumes of reagents • how you would measure the various variables [4] Method 1. Measure 25 cm 3 of FB 5 using the 50 cm 3 measuring cylinder, and transfer it into the 100 cm 3 beaker. 2. Measure and record the initial temperature in th e 100cm 3 beaker using a thermometer. 3. Measure 25 cm 3 of FB 3 using the 50 cm 3 measuring cylinder. 4. Carefully transfer the FB 3 into the 100 cm 3 beaker, stir, and record the highest temperature reached. 5. Repeat steps 1-4 by mixing FB 4 with FB 5. (b) Explain clearly how you could determine the identit y of the acids using the difference in temperature rise in the experiment proposed in (a) . [2] Ethanoic acid is a weak acid and nitric acid is a strong acid. The temperature rise will be lower when 2.0 mol dm -3 ethanoic acid is used compared to 1.0 mol dm -3 nitric acid for the same number of moles of water formed. This is because some of the energy evolved from the neutralisation process is used to further dissociate the ethanoic acid completely. (c) Show the mathematical expression for the enthalpy change of neutralization of sodium hydroxide, using the volume proposed in (a) with 25 cm 3 of ethanoic acid. The temperature change measured in (a) should be represented by /uni2206T. [4.2 J of heat energy raised the temperature of 1 cm 3 of any solution by 1 oC.] [2] Energy evolved by neutralization reaction = mc /uni2206T = 210 /uni2206T J /uni2206H neutralisation = - 210 /uni2206T / 0.025 = - 8400 /uni2206T J mol -1 (d) A student accidentally diluted both the ethanoic ac id and nitric acid prior to conducting the experiment above. (i) With an aid of a pH meter, a titrimetric method co uld still be used to differentiate the diluted ethanoic acid from the di luted nitric acid, even though their exact concentrations are not known.

4 Briefly describe the measurements to be taken, and how the measurements may lead to the identification of the two acids. Precise details of how a titration is performed, al ong with the apparatus used are not required, though the identity of any indicator (if used) should be clearly stated. Titrate the diluted acids separately with NaOH(aq) using phenolphthalein. Record the pH of the end-point (or pH when solution changes colour) using a pH meter. The pH of the end point of ethanoic acid and NaOH s hould be greater than 7 or ~ 9, and the pH of the end-point of HNO 3 and NaOH is ~ 7 (ii) Other than the fact that a titrimetric method is m ore effective in the determination of the identity of a weak or strong a cid when the concentration is unknown, compare the general relia bility of using a titrimetric and thermochemical method in the determ ination of the identity of a weak or strong acid. [4] Heat could be lost from the beaker easily resulting in inaccurate results in the thermochemical method. However, pH resulting from the presence of CH 3COO - in the solution is always higher than 7 at end point. Hence titration method is a better method. [Total: 12]

5 2(a) Alkenes are very useful compounds, which are used as fuel and in the manufacture of a wide variety of polymers. (i) Name and outline the mechanism in Step I to form A. Free radical substitution

6 (ii) Draw the structure of B. Cl Cl (iii) When C 10 H18 Br 2 reacts with hot NaOH in ethanol in Step II , the rate of reaction is found to be faster. Explain why. C-Br bond is weaker as compared to C-C l hence it is more susceptible to nucleophilic substitution. Therefore the rate of reaction is faster. (iv) When B reacts with hot NaOH in ethanol in Step II , another side- product, C (M r = 228.0), is formed. Suggest the structure of C. [7] OCH2CH3 OCH2CH3

7 (b) The flow scheme below involves the reaction of D. CH2=CHCH2CH2CH2OH Br2 in CCl4Step 1 CH2(COOH)2 D E F Step 2 excess CH 3NH2 heat G (i) Draw the structures for E, F and G in the boxes provided below. CH 2=CHCH 2CH=CH 2 E CH2CHCH2CH2CH2OH Br Br F CH2CHCH2CH2CH2OH NHCH3 NHCH3 G (ii) State the reagents and conditions for Step 1 and 2 in the spaces provided. Reagents Conditions Step 1 concentrated H 2SO 4 170 oC Step 2 KMnO 4, dilute H2SO 4 Heat

8 (iii) G contains a chiral carbon but the eventual reaction mixture was found to be optically inactive. Suggest a reason for this observation. F undergoes S N1 mechanism to form G. Hence, there is an equal probability of the nucleophile, CH 3NH 2, attacking from either plane of the carbocation intermediate, which is trigonal planar in shape. This will lead to the formation of a racemic mixture. Hence, G is optically inactive. (iv) State the type of hybridisation with respect to th e carbon atom indicated below by the asterix * in compound D. CH 2=*CHCH 2CH 2CH 2OH Sketch the shape of the hybrid orbitals of this ca rbon and indicate its bond angle. Type of Hybridization = sp 2 (v) It was found that when D is reacted with bromine, H, (C 5H9BrO) was formed . Suggest the structural formula of H. [10] O CH2Br [Total: 17]

9 3 Old mine workings are often the cause of the pollu tion of mountain streams. One of the main chemical culprits of such pollution is iron pyrites, FeS 2, in which iron has an oxidation state of +2.