MI H2 CHEM P2 Prelim

Class Adm No Candidate Name: This question paper consists of 18 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 8 41 7 72

Page 2 of 18 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.

Page 3 of 18 [Turn over ii) Show how you will use your da ta to estimate a value of the enthalpy change of combustion of C20H42 (l). Use c to represent specific heat capacity of water and C to represent the heat capacity of the calorimeter. [6] 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] …………………………………………………………………………………..………… ……………………………………………………………………………………………..

Page 4 of 18 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] 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] [Total: 12]

Page 5 of 18 [Turn over 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] …………………………………………………………………………………..……… ………………………………………………………………………………………….. b) Suggest how D2O is different from H2O. [1] …………………………………………………………………………………..……… ………………………………………………………………………………………….. 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] d) Write a balance equation to show the reaction between chlorine gas with NaOD dissolved in D2O at 15oC. [1] …………………………………………………………………………………..……… ………………………………………………………………………………………….. [Total: 4]

Page 6 of 18 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) a) Calculate Go of the dissolution per unit of CH2 group in aliphatic alcohols. [1] b) Suggest why the entropy change per CH2 unit is negative. [1] …………………………………………………………………………………..………… …………………………………………………………………………………………….. c) Predict the effect of temperature change on the spontaneity of dissolution of each CH2 unit. [2] …………………………………………………………………………………..………… …………………………………………………………………………………………….. ……………………………………………………………………………………………..

Page 7 of 18 [Turn over d) By studying a series of alkanes, the Go of transfer of a hydrocarbon molecule from a dilute aqueous solution into a dilute solution of another hydrocarbon is measured. The Go per CH2 unit was found to be -3.8 kJ mol-1. -CH2- (aq)  -CH2- (hexane) Go = -3.8 kJ mol-1 What could be deduced from the sign of Go above? [1] …………………………………………………………………………………..………… …………………………………………………………………………………………….. e) A students wrote, “The Go of transfer of –SiH2- (aq) to –SiH2- (hexane) is predicted to be about -3.0 kJ mol-1 as the C-H bond is less polar than Si-H bond.” Using your knowledge of periodicity, explain the phrase in italics. [1] ……………………………………………………………………………………..……… …………………………………………………………………………………………….. f) Solubility of an organic compound can be altered by a chemical change. Octan-1-ol was refluxed in the presence of NaCl and concentrated H2SO4. Draw the displayed formula of the product of the reaction. [1] [Total: 8]

Page 8 of 18 4 Saponification is a process where esters in triglycerides, commonly known as oil or fats, are hydrolyzed in the presence of a strong base to form soap – a mixture of glycerol and fatty acids. A fatty acid contains both a hydrophobic group that allows it to dissolve oil-based substances, and a h ydrophilic group that allows it to dissolve in water. This results in the cleaning action of soaps. A saturated triglyceride molecule has a general structure: C C C O O O H H H H H C C C O O O (CH2)n (CH2)n (CH2)n CH3 CH3 CH3 where n is a whole number, usually between 16 to 20. The Saponification (SAP) value is the mass of potassium hydroxi de (in m illigrams) required to hydrolyse 1 g of triglycerides under stated conditions. SAP value is inversely related to mean molecular mass: SAP = r 168300 Average M of triglyceride The general procedures of saponification are as follow: (1) To 5 g sample of triglyceride in a conical flask, add 50 cm 3 of 0.5 mol dm -3 potassium hydroxide dissolved in ethanol. (2) Reflux the mixture in an oil bath. (3) Titrate the excess potassium hydroxide with 0.5 mol dm -3 hydrochloric acid using phenolphthalein as indicator. a) i) Assuming that the triglyceride used is saturated and has n = 16 on all 3 side chains, write a balanced equation of the reaction occurring in Step (1) above.

Page 9 of 18 [Turn over ii) Provide a reason why the hydrolysis reaction was found to be exothermic. ……………………………………………………………………………………………… …………………………………………………………………………………………..…