NYJC H2 CHEM P2 ANS Prelim

1 NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME Answers CLASS TUTOR’S NAME CHEMISTRY 9647/02 Paper 2 Structured Questions 19 September 2011 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet. READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 12 2 10 3 12 4 20 5 18 Total 72 This document consists of 19 printed pages and 1 blank page. [Turn over

2 1 Planning (P) There are four colourless solutions, labelled as A1, A2, B1 and B2. You are told that A1 and A2 could be 1.0 mol dm –3 H 2SO4 or 2.0 mol dm –3 H 2SO4. B1 and B2 are either 1.0 mol dm–3 NaOH or 1.0 mol dm–3 Ba(OH)2. You are required to plan a simple experiment to identify A1, A2, B1 and B2. You are provided with 200 cm 3 of each solution, a thermometer polystyrene cup common apparatus in the laboratory No pH indicators are provided. For Examiner’s Use (a) Suggest how we can first identify Ba(OH) 2. Write an equation (with state symbols) to explain your answer. Add a few drops of A1 (or A2) to 2 cm 3 of B1. Repeat this with B2. The solution which gives a white precipitate is Ba(OH)2. Ba(OH)2 (aq) + H2SO4 (aq)  BaSO4 (s) + 2H2O ( l ) [2] (b) Write an equation for the reaction betw een sodium hydroxide and sulfuric acid. 2NaOH (aq) + H2SO4 (aq)  Na2SO4 (aq) + 2H2O (l) [1] (c) Outline the steps you will take to determine the identities of A1 and A2, using the sodium hydroxide solution. Step 1: Using a measuring cylinder, add 20 cm 3 of A1 into the polystyrene cup provided and record its initial te mperature. Using another measuring cylinder, measure 80 cm3 of NaOH and record its initial temperature. Step 2: Add the 80 cm 3 of NaOH into the polystyrene cup containing A1, stir with the thermometer and record the highest temperature reached. Rinse the cup. Step 3: Repeat steps 1 and 2, this time replacing A1 with A2. [Accept any specific volume suggested by student, but volume of NaOH should be 4 times of A2/A1 and total volume of mixture should be between 50-100 cm3.] [3]

3 (d) Let T1 be the temperature change for the reaction between A1 and NaOH(aq) and T2 be the temperature change for the reaction between A2 and NaOH(aq). Illustrate how the identities of A1 and A2 can be deduced from the temperature changes obtained in the procedure above. 2NaOH (aq) + H2SO4 (aq)  Na2SO4 (aq) + 2H2O (l) NaOHn used in both experiments = 1.0 x 80/1000 = 0.080 mol For 1.0 mol dm–3 H2SO4, 24HS On used = 1.0 x 20/1000 =0.020 mol n(H2O) formed = 2 x 24HS On = 0.040 mol For 2.0 mol dm–3 H2SO4, 24HS On used = 2.0 x 20/1000 =0.040 mol n(H2O) formed = NaOHn = 0.080 mol Since number of moles of water formed in the two reactions are in the ratio of 1:2, temperature change for the two reactions is expected to be around 1:2. If T2 = 2T1, A2 is 2.0 mol dm–3 H2SO4 and A1 is 1.0 mol dm–3 H2SO4. [Other methods are acceptable.] [3] (e) Define the term enthalpy change of neutralisation. The enthalpy change when one mole of water is formed during the neutralisation of an acid and alkali under standard conditions. [1] (f) Do you expect the enthalpy change of reaction per mole of acid to be more exothermic, less exothermic or the same if (i) hydrochloric acid was used instead of sulfuric acid? Explain your answer. Enthalpy change of reaction per mole of acid will be less exothermic Reason: HCl is also a strong acid, and the amount of water formed per mole of acid would be less. (ii) ethanedioic acid was used instead of sulfuric acid? Explain your answer. Enthalpy change of reaction per mole of acid will be less exothermic Reason: ethanedioic acid is a weak str ong acid, and is only partially dissociated. Part of the energy is used dissociate ethanedioic acid completely before neutralisation occurs. [2] [Total: 12]

4 2 Nitrobenzene may be made by reacting together concentrated nitric acid, concentrated sulfuric acid and benzene. Data about these three compounds and nitrobenzene are given in the table. Compound Boiling point /oC Density /g cm-3 Molar mass / g mol-1 Solubility in water Nitrobenzene 210 1.20 123.0 insoluble Benzene 80.1 0.88 78.0 insoluble Concentrated nitric acid 120 1.36 63.0 soluble Concentrated sulfuric ac id 337 1.84 98.0 soluble Preparation of nitrobenzene 1 Place 35 cm 3 of concentrated nitric acid in a 500 cm3 round bottomed flask, and add slowly 40 cm3 of concentrated sulfuric acid, keeping the mixture cool during the addition by immersing the flask in cold water. 2 Place a thermometer in this nitrating mixture, and then add very slowly 29 cm3 of benzene. This should be added about 3 cm3 at a time, and the contents of the flask thoroughly mixed after each addition. 3 When all the benzene has been added, fit a reflux water condenser to the flask, and place the latter in a water bath, which is then maintained at 55-60oC for 45 minutes. (a) (i) What is the role of sulfuric ac id in this reaction? Write a balanced equation when the two acids are mixed. Sulfuric acid acts as a catalyst that generates the stronger electrophile NO2  2H2SO4 + HNO3  NO2  + 2HSO4  + H3O (ii) For safety reasons, it is important to keep the mixture cool in step 1. Explain why the mixing of the acids is exothermic. The hydration of the H+ released by the sulfuric acid gives out heat. [3]

5 (b) Suggest with reasoning, one more safety precaution you would take when carrying out the experiment. Any of the following:  Concentrated sulfuric acid / Concentrated nitric acid is corrosive. Wear gloves when handling it wear safety goggles or gloves when handling it  Nitrobenzene/benzene is carcinogenic or flammable, carry out the experiment in a fume cupboard [1] (c) Calculate the mass of nitrobenzene fo rmed from benzene, assuming a 70% yield.     3 Benzene HNO nitrobenzenehenc given densityof benzene 0.88 mass of benzene used 29 0.88 25.52 25.52n = = 0.3272 mol,78.0 35 x 1.36n = = 0.7556 mol limiting reagent is Benzene.63.0 assuming 70% yield, n m V g      e nitrobenzenehence 70 100 70 25.52= 123.0 28.17 28.2100 78.0 benzenen mg The preparation of organic compounds usua lly produces a mixture of the required compound and other impurities. The purification of nitrobenzene is described below. Purification of impure nitrobenzene 4 After completion of heating, pour the cont ents of the flask into a large excess of cold water. Stir the mixture as vigorously as possible. Transfer the liquid to a separating funnel and separate the aqueous layer from the nitrobenzene. 5 Return the nitrobenzene to the separating funnel, and wash with an equal volume of dilute sodium carbonate solution, releasing the pressure as necessary. 6 Transfer the nitrobenzene into a small conical flask, and add some anhydrous granular potassium hydroxide. Shake until the liquid is completely clear. 7 Filter the nitrobenzene into a clean and dry distilling flask fitted with an air