IJC H2 Chem 2012 Prelim P2 Soln

2 PRELIM 2  INNOVA 9647/02/2012 [Turn over For Examiner’s Use (c) Write a plan which will allow an identification of the three solutions to be made. You may use the reagents and apparatus normally found in a school or college laboratory. However, you are not provided with an indicator. In your plan you should give essential details, including quantities, of the identification procedure. To insert lines………………………………………………………………………………[6] Procedure 1.By using a 50cm 3/100cm3 measuring cylinder, place 50cm 3 of NaOH, into the dry polystyrene cup provided supported with a glass beaker. 2. Measure and note the initial temperature of NaOH solution using a 1.0 oC interval thermometer. 3.By using a second 50cm 3/100cm3 measuring cylinder (otherwise must mention washing & drying of the previous m.cyliner), measure 25 cm3 of acidic solution X. 4.Note the initial temperature of solution X. (both initial temp taken to find the average initial temp) 5.Add solution X to the cup in step 1. 6.Stir gently with a thermometer and note the highest temperature reached 7. Repeat the experiment (steps 1 to 6) with solutions Y and Z, using different measuring cylinders or mentioned in procedure measuring cylinder must be dry. Record all your readings (temperature) in an appropriate table. or a sample table is drawn. (d) Explain how you can make use of your experimental data to identify the three solutions. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ………………………………………………………………………………………[2] Feasibility of the identification depends on volume used in the experiment. Must use results obtained (either ∆H neu calculated or ∆T) . When V NaOH should be greater than VHCl:

3 PRELIM 2  INNOVA 9647/02/2012 [Turn over For Examiner’s Use -using enthalpy change of neutralisation calculated: Due to partial dissociation of CH 3COOH, some heat is absorbed to dissociate the weak acid. Thus, the ∆Hneu is less exothermic/smaller ∆ T than HCl of same concentration. In this experiment, more no. of moles of water produced with 2.0 mol dm -3 HCl used is than when 1.0 mol dm -3 HCl used. Thus, the one with a larger/twice the ∆ T is the one using solution 2.0 mol dm-3 HCl. (Although different concentration of HCl used in the experiment, they will have same ∆H neu calculated. ) (e) Discuss one possible source of error in conducting your experiment and suggest improvement to produce a more reliable result. ........................................................................................................................... ........................................................................................................................... ………………………………………………………………………………………[1] - Heat loss to the surrounding is not taken into account, the enthalpy change of neutralisation is less exothermic than expected. Use a lid to cover the cup to reduce heat loss. - (dependent on thermometer used) There is a percentage error in the enthalpy change calculated as the 1 oC interval thermometer is used. Use a thermometer with higher precision (0.2oC interval) to reduce the percentage error. - Slow response of thermometer. Use a data logger with temperature sensor instead. [Total: 12] 2 (a) Iodine is not very soluble in water but in the presence of iodide ions, it can dissolve to form a tri-iodide complex as follows: I 2 + I-  I3 - (i) Draw a dot and cross diagram for the tri-iodide ion and hence state its shape and bond angle. Bond angle of 180. Thus the complex is linear in shape.

4 PRELIM 2  INNOVA 9647/02/2012 [Turn over For Examiner’s Use (ii) Explain why fluorine does not form the corresponding F3 - ion. Fluorine, being in Period 2, is not able to expand it outermost shell to accommodate 10 electrons. [3] (b) IF7 is a colourless gas prepared by the direct combination of iodine and fluorine. The I−F bond energy can be obtained from thermochemical data. Standard enthalpy change of formation of IF7(g) = −944 kJ mol−1 Standard enthalpy change of atomisation of iodine = +107 kJ mol−1 (i) Define what is meant by the standard enthalpy change of formation of IF7. It is the enthalpy change when 1 mole of IF7 is formed from its constituent elements, iodine and fluorine in their standard states under standard conditions of 1 atm and 298 K. (ii) Draw an energy cycle diagram to determine the I-F bond energy. Incorporate values from the data above and any other relevant data from the Data Booklet into the diagram. 7BE(I-F) -944.0 = +107+ 7 2 (+158) BE(I-F) = +229 kJ mol-1       (iii) Explain why the I−F bond energy is not the mean of the I−I and F−F bond values. Fluorine is more electronegative than iodine. The covalent bond between the atoms is polarised, giving rise to a stronger bond. [6] (c) The typical daily food requirement of a person can be considered to be 1.2 kg of carbohydrate. The person obtains energy by oxidation of the carbohydrate, which can be represented by the formula (CH 2O)n.

5 PRELIM 2  INNOVA 9647/02/2012 [Turn over For Examiner’s Use (i) Construct an equation for the complete oxidation(combustion) of the carbohydrate (CH2O)n. (CH2O)n + nO2  nCO2 + nH2O (ii) The empirical relative formula mass of the carbohydrate is 30. Use your equation in c(i) to calculate the number of moles of oxygen required by the person each day. Moles of carbohydrate = 1200/30n = 40/n mol Moles of oxygen = n x 40/n = 40 mol (iii) Calculate the volume of oxygen at room temperature and pressure for a human with a lifetime of 70 years. Moles of oxygen in 70 years = 40 x 70 x 365 = 1.02 x 106 mol Volume at rtp = 1.02 x106 x 24 = 2.45 x 107 dm3 [3] [Total:12] 3 (a) Panadol is the trade name for paracetamol or acetaminophen which is an over the counter analgesic (pain reliever) and antipyretic (fever reducer). It can be synthesised in the lab from phenol via a series of steps. OH OH N C O CH3 H B dil HNO3 I II A paracetamol (i) State the reagents and conditions for step I and step II. Step I: Reagent(s): ………...………………………………………………………………….. Condition(s): ….……………………………………………………………………….. Step II:

6 PRELIM 2  INNOVA 9647/02/2012 [Turn over For Examiner’s Use Reagent(s): ………...………………………………………………………………….. Condition(s): ….……………………………………………………………………….. Step 1 : Sn, concentrated HCl; heat with reflux Then NaOH (aq); room temperature Step 2: CH3COCl, r.t. (ii) Draw the structures of compound A and B in the boxe