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