SAJC H2 CHEM P2 Prelim

1 Name: Class: ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examinations Chemistry Higher 2 9647/02 Paper 2 19 September 2011 2 hours Candidates answer in the spaces provided on the question paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write in dark blue or black pen. 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. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use: Question Marks 1 2 3 4 5 Total This document consists of 13 printed pages including this page.

2 1. Planning (P) FA 1 is a solution of sulphuric acid, H2SO4, of concentration approximately 1 mol dm-3. FA 2 is a solution of 1.50 mol dm-3 sodium hydroxide, NaOH When sulphuric acid neutralises sodium hydroxide, water is produced. H2SO4 + 2NaOH  Na2SO4 + 2H2O The neutralisation process is exothermic, that is, gives out heat, and the amount of heat produced changes with the amount of water produced. From the temperature rise in various experiments, the actual concentration of H2SO4 in FA 1 can be determined. (a) Calculate the approximate temperature rise, T, when 10 cm3 of FA 1 and 40 cm3 of FA 2 were mixed. In your calculations, you are to assume specific heat capacity of the mixture = 4.18 J cm-3 K-1 standard enthalpy change of neutralisation (per mole of water) = – 57.4 kJ mol-1 T: ………………………. [3] (b) Fill in the table below, the approximate temperature rise when various volumes of FA 1 and FA 2 were mixed. In each case, the total volume of mixture is 50 cm3. Volume of FA 1 / cm3 10 20 30 40 Volume of FA 2 / cm3 40 30 20 10 Temperature rise, T / oC [2]

3 1. (c) With the help of (b), describe the steps you would carry out to determine the exact concentration of H2SO4 in FA 1. [3] (d) Identify one source of error in your procedure and explain how you would minimise the error. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. [2] (e) From the graph of temperature rise against volume of FA 1, show how you would treat the data in order to determine the actual concentration of H2SO4 in FA 1. [2] [Total: 12] [Turn over

4 2. The preparation of iodo -compounds can be prepared from alcohols and phosphorus tri-iodide, PI3. However, PI3 are not readily available. Hence, it is generated in situ from red phosphorus and iodine. The following equations and steps show how iodoethane can be prepared. 2P + 3I2 2PI3 ΔH = -92 kJ mol-1 3CH3CH2OH + PI3 3CH3CH2I + H3PO3 (a) 2.5 g of red phosphorus was placed in a 250 cm 3 round-bottomed flask, followed by 25 cm 3 of ethanol. 25 g of pow dered iodine was added to the contents of the flask in small quantities of about 3 - 4 g at a time, allowing about two minutes between consecutive additions. When all the iodine has been adde d, the product was allowed to stand for 10 minutes. (i) Explain why powdered iodine was used instead of large crystals ………………………………………………………………………………………… ………………………………………………………………………………………… (ii) Suggest why iodine was added in small quantities. ………………………………………………………………………………………… (iii) Give a possible reason why it is necessary for the mixture to stand for 10 minutes. ……………………………………………………………………………………....... [4] (b) After standing for 10 minutes, the flask was then heated in a water bath for 1 hour. After heating, the flask was fitted with a reflux condenser and distillation was carried out. T he distillate obtained consisted of iodoethane, ethanol, phosphorus acid and a small amount of ether, CH3CH2OCH2CH3. (i) With the aid of an equation, explain the formation of the ether in the distillate. ………………………………………………………………………………………… …………………………………………………………………………………………

5 2 (b) (ii) When the distillate was allowed to stand, a small amount of iodine was formed. Using data from the Data Booklet, explain the formation of iodine in the distillate. ………………………………………………………………………………………… ………………………………………………………………………………………… ……………………...………………………………………………………………… [5] (c) The distillate was then transferred to a separating funnel . An equal volume of sodium carbonate solution was added and the mixture was well shaken. The lower layer of iodoethane was collected and the upper aqueous layer was discarded. The iodoethane was transferred back to the separating funnel. An equal volume of water was added and the solution was mixed well. The lower iodoethane layer was collected into a small conical flask. A few granules of anhydrous calcium chloride were added and the conical flask was swirled until the liquid is clear. Iodoethane was filtered into a 50 cm3 round-bottomed flask, and was distilled. The distillate was collected between 68°C – 73°C. This process was repeated until the temperature was fixed to 71°C. (i) State the purpose of adding sodium carbonate solution in the separating funnel. ………………………………………………………………………………………… (ii) State the purpose of adding water in the separating funnel. ………………………………………………………………………………………… (iii) Suggest another compound that can be used in place of anhydrous calcium chloride. ………………………………………………………………………………………… (iv) Explain why distillation was repeated until the temperature was fixed at 71°C. ………………………………………………………………………………………… [4] [Turn over

6 2. (d) In a separate experiment, water was added using a thistle funnel to a conical flask containing excess PI3. The mixture was heated and violet fumes were seen. A similar experiment was done for PCl3 and white fumes were seen instead. (i) Using VSEPR theory, explain why the bond angle of PCl3 is smaller than PI3. ………………………………………………………………………………………… ………………………………………………………………………………………… (ii) Given that PI3 reacts similarly as PCl3 with water, write the equation for the reaction between PI3 and water. ………………………………………………………………………………………… (iii) PI3 and PCl3 produce different colour fumes for their separate reactions with water when heated. Explain. ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… [7] [Total: 20]

7 3. In a reaction where compound A is converted to compound C, the following mechanism was proposed. Step 1: A (g) B (g) Step 2: B (g) C (g) Overall: A (g) C (g) ΔH < 0 To investigate the rate of the reaction, various concentrations were monitored over time and the following graph was obtained. (a) (i) Using the graph of formation of B and C above, comment on the relative rate of the 2 steps proposed in the reaction mechanism. ………………………………………………………………………………………… ………………………………………………………………………………………… (ii) Hence, determine which step is the rate determining step of the reaction. ………………………………………………………………………………………… [Turn over Time / min Concentration / mol dm-3 1.2 - - 1.0 - - 0.8 - - 0.6 - - 0.4 - - 0.2 - - 0.0 | | | | | | | | | | | | | | | | | | | | | | | | 3 1 2 4 5 6 A B C

8 3. (a) (iii) Using the graph above, show that the reaction is first order with respect to compound A. ………………………………………………………………………………………… ………………………………………………………………………………………... (iv) Draw an energy profile diagram of the reaction. [8] (b) At t = 4 min, B and C are at equilibr