MI H2 CHEM P2 MS Prelim

Class Adm No Candidate Name: This question paper consists of 21 printed pages. [Turn over 2011 Preliminary Examination II Pre-university 3 CHEMISTRY 9647/02 Paper 2 Structured Questions 16 September 2011 Candidates answer on the Question Paper. 2 hours 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 or graphs. Do not use 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. You are reminded of the need for good English and clear presentation in your answers. For Examiners Q 1 2 3 4 5 Total Score 12 4 7 41 5 72

Page 2 of 21 1 Planning (P) Biodiesel, which could be obtained from natural products such as soybean, is a renewable and generally cleaner-burning fuel than petroleum diesel. However, it was claimed that biodiesel is less efficient as the heat content of biodiesel is less than that of petroleum diesel. The enthalpy change of combustion of straight chain alkanes can be estimated by finding the enthalpy change of combustion of a –CH 2– unit. This data could be obtained from the standard enthalpy change of combustion of straight chain alkanes. The enthalpy change of combustion of straight chain alkanes with 1 to 5 carbons are as shown: No. of carbon atoms Alkane H o combustion / kJ mol-1 1 CH 4 -890 2 C 2H6 -1560 3 C 3H8 -2219 4 C 4H10 -2877 5 C 5H12 -3509 In this question, it is assumed that petroleum diesel contained only hydrocarbon C20H42 (l), and that soybean oil used as a biodiesel. a) i) Plan a series of experiments to find out the enthalpy change of combustion of successive alkanes shown in the table above. Illustrate by means of a well labelled diagram how the experiment will be set up. 1 m for set-up

Page 3 of 21 [Turn over 1. Weigh 30 g of CH4 into a burner set up. Ensure that the wick is soaked in the fuel. Adjust the wick such that it is about 2 cm above the cap of the burner. 2. Using a measuring cylinder, add 200 cm3 of water into the empty tin can. 3. Using the stirrer, gently stir the water for 5 seconds. Measure the initial temperature of the water with the thermometer until the temperature becomes steady. Record the initial temperature, T i. 4. Light the lamp, quickly place it in position and stir the water in the beaker gently with the thermometer. 5. As soon as a temperature rise of about 5°C has been obtained, blow out the flame. Record the final temperature , Tf. 6. Reweigh the burner and the unburnt fuel. Find the mass of alkane burnt. 7. Empty the water from the tin can and clean the soot off the bottom. 8. Repeat the experiment for the other 4 alkanes. 1 mark for clear written procedures on how temperature rise is measured. 1 mark for clear written procedures on how mass of fuel burnt is measured. Bonus mark if calibration of combustion calorimeter with a substance with a known enthalpy change of combustion is stated. ; ; ii) Show how you will use your da ta to estimate a value of the enthalpy change of combustion of C 20H42 (l). Use c to represent specific heat capacity of water and C to represent the heat capacity of the calorimeter. [6] Heat evolved, Q = mass of water in calorimeter x c x change in temperature of water + CT Amount of C20H42 (l) = r mass of alkane burnt M of alkane H = - Q Amount of fuel burnt (allow e.c.f.) Plot a graph of H o combustion against no. of carbon atoms. Extrapolate to C20 to find the H o combustion of C20H42. ; ; ;

Page 4 of 21 b) A student calculated a value of enthalpy change of combustion of C 20H42 (l) using the bond energy data from the Data Booklet. Explain if the value would be a reliable estimate of the actual value. [1] It will not be a reliable estimate as it assumes that the reactants and products are gases which are not true in this case / oil is not in gaseous state. (Do not accept: Bond energies are average values.) ; c) Soybean oil used as a biodiesel contained primarily the methyl ester of linoleic acid (C19H34O2). The enthalpy change of formation are given as follows: Substance H o formation / kJ mol-1 CO2 (g) -394 H2O (l) -286 C19H34O2 (g) -605 The standard enthalpy change of vaporisation of C19H34O2 (l) is +62.1 kJ mol-1. Using the information above, draw an energy cycle and use it to calculate the standard enthalpy change of combustion of C19H34O2 (l). [3] C19H34O2 (l) + 53/2 O2 (g)  19CO2 (g) + 17H2O (l) C19H34O2 (g) + 53/2 O2 (g) 19C (s) + 17H2 (g) + 53/2 O2 (g) H o combustion of C19H34O2 (l) = H o vap (C19H34O2 (l)) + 19 X H o f (CO2 (g)) + 17 X H o f (H2O (l)) - H o f (C19H34O2 (g)) = 62.1 + 19(-394) + 17(-286) – (-605) = -11680 = -11700 kJ mol -1 (3 s.f.) d) Using the data of enthalpy change of combustion of straight chain alkanes, estimate a value for the enthalpy change of combustion of C20H42 (l). Hence, justify the claim made about the efficiency of biodiesel. [2] Each CH2 unit averages -655 kJ mol-1. H o combustion of C20H42 = -890 + 19(-655) = -13335 ; 19 X H o f (CO2 (g)) 17 X H o f (H2O (l)) H o f (C19H34O2 (g)) H o vap (C19H34O2 (l))

Page 5 of 21 [Turn over  -13300 kJ mol-1 As the combustion of C20H42 (l) is more exothermic than C19H34O2 (l), the claim is true that biodiesel is a less efficient fuel. ; [Total: 12]

Page 6 of 21 2 Deuterium, 2D, is an isotope of hydrogen. D 2O can be used in the elucidation of molecular structures of substances. a) Deduce the total number of neutrons present in D2O. [1] Total number of neutrons = 1 + 1 + 8 = 10 ; b) Suggest how D2O is different from H2O. [1] D2O has a slightly larger molecular mass than H2O. ; c) Given the average relative atomic mass of hydrogen is 1.00015, assuming that 2D is the only other isotope of 1H, find the percentage abundance of deuterium. [1] Let percentage abundance of the 2D isotope be x. 1.00015 = 100 x X 2 + 100 x-100 X 1 x = 0.015% ; d) Write a balance equation to show the reaction between chlorine gas with NaOD dissolved in D2O at 15oC. [1] Cl2 (g) + 2NaOD (aq) → NaCl (aq) + NaClO (aq) + D2O (l) ; [Total: 4] 3 ‘Hydrophobic’ is a term derived from hydro- (water) and phobos (fear). In water, hydrophobic molecules generally tend to aggregate to minimize their surface contact, and associated surface energy, with water molecules. Smaller hydrophobic molecules could, however, dissolve to a small extent in water as water molecules can arrange around them without breaking hydrogen bonds or losing much energy. Adding CH 2 groups to aliphatic alcohols increases the heat produced on solution ( ∆Ho per CH 2 unit = -5.4 kJ mol -1) but causes a greater decrease in the entropy (∆So per CH2 unit = -23.8 J mol-1) e) Calculate Go of the dissolution per unit of CH2 group in aliphatic alcohols. [1] Go = ∆Ho – T∆So = -5.4 – 298(-23.8 x 10-3) = +1.69 kJ mol-1 ; f) Suggest why the entropy change per CH2 unit is negative. [1]

Page 7 of 21 [Turn over …………………………………………………………………………………..………… …………………………………………………………………………………………….. As the number of carbon increases, the hydroph