TJC H1 CHEM P2 (B) ANS

1 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 accordi ng 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.  When the volume of vessel is reduced, the pressure of the system increases. By Le Chatelier’s Principle, the system counteracts the change by reducing the total pressure by favouring the backward reaction which generates less number of moles of gases.  Hence, the number of moles of methane increases (answer must come with explanation to gain the credit). (ii) Write an expression for the equilibrium constant, Kc, stating its units.  Kc = [CS2][H2]4 [CH4][H2S]2  mol2dm-6 (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 cm 3 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.  working CH4(g) + 2H2S(g) CS2(g)+ 4H2(g) Initial conc/mol dm-3 4 8 4 8 Change/ mol dm-3 +1.56 +3.12 -1.56 -6.24 Equilibrium/ mol dm-3 5.56 11.12 2.44 1.76  Kc = 2.44 x 1.764 / (5.56 x 11.122) = 0.0341 mol2dm-6 (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 HƟ is the enthalpy change in J mol-1 R is the ideal gas constant and T is temperature in Kelvin.

2 2015 TJC H1 Chemistry Preliminary Exam [Turn Over Using your answer in (iii), calculate the value of equilibrium constant at 480 °C.  Working 𝑙𝑛 𝐾𝑐 0.0341 = − 232 × 103 8.31 ( 1 753 − 1 1233)  Kc = 1.84 x 10-8 mol2dm-6 allow ECF from (iii) (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 HNO 3 to give sulfur and an oxide of nitrogen. Determine the oxidation state of nitrogen in the oxide. Hence, write a balanced equation for the reaction of H2S and HNO3. [10] 3 H2S ≡ 3S ≡ 2 HNO3 ≡ 6e- HNO3 ≡ 3e-  Oxidation state of nitrogen decreases from +5 in HNO3 to +2 in the product  3H2S + 2HNO3 → 3S + 2NO + 4H2O (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 (i) Predict and explain the difference in boiling points of adipic acid and cyclohexanone.  Adipic acid has a higher boiling point than cyclohexanone.  Boiling adipic acid involves breaking of hydrogen bonds between its molecules, which are stronger and require more energy to overcome compared to the van der Waal’s forces of attraction between cyclohexanone molecules.

3 2015 TJC H1 Chemistry Preliminary Exam [Turn Over (ii) Show a three -step synthetic route to obtain adipic acid from cyclohexanone. Suggest reagents and conditions you would use and show the structures of any intermediates formed.  LiAlH4 in dry ether, room temperature and pressure Accept other reducing agents e.g. H2/Ni, high temp or NaBH4, alcohol, rtp  excess concentrated H2SO4, 180 °C  acidic KMnO4, reflux [reagent & condition + intermediate = 1 mark] (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.  2,4-dinitrophenylhydrazine, rtp  Orange ppt with cyclohexanone, no observable reaction with adipic acid  OR  Na2CO3 (s), rtp  Effervescence of CO 2 with adipic acid, no observable change with cyclohexanone  HO2C-(CH2)4-CO2H + Na2CO3 → Na+ –O2C-(CH2)4-CO2 –Na+ + CO2 + H2O

4 2015 TJC H1 Chemistry Preliminary Exam [Turn Over 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.  condensation/ nucleophilic (acyl) substitution/ nucleophilic addition followed by elimination (v) Two molecules of an ester undergoes the above reaction to give methanol and the following organic product. Suggest the structure of the ester.  CH3CH2CO2CH3 [10] [Total: 20] 6 Aluminium chloride, A lCl3, 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 Al2Cl6 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] AlCl3 undergoes hydration in water and dissolves to give an aqueous solution. AlCl3 + 6H2O [Al(H2O)6]3+ + 3Cl– Hydrolysis also occurs due to the high charge density of A l3+ ions. It polarizes the water molecule and weakens the O -H bond of water making i t easier for H+ ion to leave the water molecule. An acidic solution of pH = 3 – 4 is obtained. [Al(H2O)6]3+ [Al(H2O)5OH]2+ + H+ [Accept both full and equilibrium arrows for hydrolysis reaction] O O R O O O O

5 2015 TJC H1 Chemistry Preliminary Exam [Turn Over (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.  Aluminium in A lCl3 is electron deficient (or has vacant orbitals) as it only has 6 electrons around it and Cl– anion has lone pairs of electrons and thus,  dative bond is formed in the chloroaluminate anion. (ii) Draw the dot -and-cross diagram of chloroaluminate anion. Using the valence shell electron pair repulsion theory, state its shape and bond angle. 1m – dot-and-cross Shape: Tetrahedral Bond angle: 109.5⁰ (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] RTIL have a lower melting point and exists as a liquid and sodium chloroaluminate exists as a solid because the organic cation in RTIL is larger and thus, weaker ionic bonds exist between the oppositely -charged ions in RTIL as

6 2015 TJC H1 Chemistry Preliminary Exam [Turn Over (c) To determine the rate equation of the following chlorate -chloride reaction, an experiment was conducted using 0.000480 mol dm-3 of C lO3 –, 0.1 mol dm-3 of C l– 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,