TJC H1 CHEM P2 (B) QN

12 2015 TJC H1 Chemistry Preliminary Exam [Turn Over Section B Answer two questions from this section on separate answer paper. 5 (a) Hydrogen sulfide is a colourless gas with a characteristic ‘rotten egg’ smell. It occurs naturally in crude petroleum and reacts with methane according to the following equilibrium. CH4(g) + 2H2S(g) CS2(g) + 4H2(g) H = +232 kJ mol-1 (i) State and explain the effect on the number of moles of methane when the volume of the vessel is reduced at a constant temperature. (ii) Write an expression for the equilibrium constant, Kc, stating its units. (iii) In an experiment, 1.00 mol of CH 4, 2.00 mol of H 2S, 1.00 mol of CS 2 and 2.00 mol of H 2 are mixed in a 250 cm3 vessel at 960 °C. The concentration of methane is found to be 5.56 mol dm-3 when the system reaches equilibrium. Calculate the value of Kc at 960 °C. (iv) The equilibrium constant varies with temperature according to the van’t Hoff equation: 𝑙𝑛 𝐾2 𝐾1 = − ∆𝐻𝑜 𝑅 ( 1 𝑇2 − 1 𝑇1 ) where K1 is the equilibrium constant at temperature T1 K2 is the equilibrium constant at temperature T2 Ho is the enthalpy change in J mol-1 R is the ideal gas constant and T is temperature in Kelvin. Using your answer in (iii), calculate the value of equilibrium constant at 480 °C. (v) Hydrogen sulfide can be removed by passing it through nitric acid. 0.15 mol of H 2S was found to react with 0.10 mol of HNO3 to give sulfur and an oxide of nitrogen. Determine the oxidation state of nitrogen in the oxide. Hence, write a balanced equation fo r the reaction of H2S and HNO3. [10] (b) Adipic acid is one of the most important dicarboxylic acids and has a wide range of industrial applications. It is used as a monomer in the synthesis of nylon, and in the production of plasticizer and coatings. adipic acid cyclohexanone

13 2015 TJC H1 Chemistry Preliminary Exam [Turn Over (i) Predict and explain the difference in boiling points of adipic acid and cyclohexanone. (ii) Show a three -step synthetic route to obtain adipic acid from cyclohexanone. Suggest reagents and conditions you would use and s how the structures of any intermediates formed. (iii) Suggest a simple chemical test to distinguish between adipic acid and cyclohexanone. State the observations and write a balanced equation for any reaction that occurs. In the 1880s, German chemist Rainer Ludwig Claisen discovered that esters were able to react in the presence of a strong base to give a keto ester. (iv) Name the type of reaction that has taken place between the ester molecules. (v) Two molecules of an ester undergo the above reaction to give methanol and the following organic product. Suggest the structure of the ester. [10] [Total: 20] 6 Aluminium chloride, AlCl3, exhibits properties which differ from chlorides of other Period 3 elements. It sublimes at a relatively low temperature of 180 ⁰C at atmospheric pressure due to the original lattice structure of A lCl3 being converted into A l2Cl6 molecules. In the presence of excess water, aluminium chloride forms an acidic solution of pH 3 to 4. (a) Describe and explain the reactions of aluminium chloride with excess water, writing equations where appropriate. [2] (b) Aluminium chloride reacts with sodium chloride to form sodium chloroaluminate, NaAlCl4. Sodium chloroaluminate is one of the simplest compounds containing chloroaluminate anion and has a melting point of 185 ⁰C. (i) Explain why sodium chloroaluminate would be formed from the above reaction, stating the type of bond that is formed during this reaction. O O R O O O O  carbon

14 2015 TJC H1 Chemistry Preliminary Exam [Turn Over (ii) Draw the dot -and-cross diagram of chloroaluminate anion. Using the valence shell electron pair repulsion theory, state its shape and bond angle. (iii) A class of compounds called room temperature ionic liquids (RTILs) can be formed from chloroaluminate anion and organic cations such as 1-ethyl-3-methylimidazolium and pyridinium ions. 1-ethyl-3-methylimidazolium pyridinium ion Explain the differences in physical states between RTILs and sodium chloroaluminate in terms of bonding. [6] (c) To determine the rate equation of the following chlorate -chloride reaction, an experiment was conducted using 0.000480 mol dm-3 of ClO3 –, 0.1 mol dm-3 of Cl– and 0.4 mol dm-3 of H+. 2ClO3 – + 2Cl– + 4H+ → Cl2 + 2ClO2• + 2H2O At regular 5-minutes intervals, small samples of the reaction mixture were withdrawn, quenched and placed into the UV -vis spectrometer to record its absorbance value. The absorbance value corresponds to the concentration of the product ClO2•. (i) The results of the above experiment were shown below. Time/min 0 5 10 15 20 25  Absorbance/A 0.000 0.211 0.348 0.436 0.494 0.531 0.600 Plot the graph of absorbance/A against time/min. (ii) Using your graph plotted in (c)(i), determine the half-life of the experiment and hence the order of reaction with respect to [ClO3 –].

15 2015 TJC H1 Chemistry Preliminary Exam [Turn Over A research student carried out a series of experiments to further investigate the order of reaction with respect to [Cl–] and [H+] in the chlorate-chloride reaction. The following graphs were plotted: Using the above data, deduce the order of the reaction with respect to [C l–] and [H+]. (iv) Hence, write a rate equation for the chlorate-chloride reaction. [9] (d) With the aid of a sketch of the Boltzman distribution, explain how an increase in temperature affects the rate of a chemical reaction. [3] [Total: 20] 7 During the cracking process, large hydrocarbon molecules are broken into smaller and more useful hydrocarbons such as alkanes and alkenes. When a saturated hydrocarbon Z, CxHy, undergoes cracking, hydrocarbons A and B are produced as shown by the equation below: CxHy C4H10 + C5H10 A B Hydrocarbon A reacted with chlorine gas in the presence of u.v. light to form two monochlorinated products, C and D, in the ratio of 9 : 1 respectively. Hydrocarbon B on strong heating with acidified potassium manganate(VII) produced E, C 3H6O, and F, C 2H4O2. E gave a bright orange precipitate with 2,4-dinitrophenylhydrazine and a yellow precipitate with aqueous alkal ine iodine. Effervescence was observed when aqueous sodium carbonate was added to F. (a) Write the molecular formula of Z. [1] (b) State the type of reaction for the formation of the monochlorinated products. [1] Experiment 1 [ClO3 –] = 0.12 mol dm−3 [H+] = 0.5 mol dm−3 0.012 0.008 0.004 [Cl–] 0.000 0 0.002 0.006 0.010 5 10 15 20 Time / min 0 Experiment 2 Assuming that [ClO3 –] and [Cl–] are constant, Initial Rate [H+]2 0 (iii)

16 2015 TJC H1 Chemistry Preliminary Exam [Turn Over (c) Give the structures of C and D. Hence, deduce the structure of A. [3] (d) Suggest the structural formula of B, E and F, explaining your reasoning. [7] (e) (i) Define the term standard enthalpy change of formation of A, C4H10(g). (ii) Write an equation for the standard enthalpy change of