SRJC_2008 JC2 Prelim H2 P3 Solutions

SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 2 CHEMISTRY 9746/03 Preliminary Examination Paper 3 Free Response 19th August 2008 2 hours Candidates answer on the separate paper. Additional Materials: Answer Paper 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, graphs or rough work. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers The number of marks is given in the brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. This document consists of 11 printed pages and 1 blank page SRJC 2008 9746/03/Prelim/2008 [Turn over

2 SRJC 2008 9746/03/Prelim/2008 [Turn over Answer any four questions. 1. The Haber Process is the nitrogen fixation reac tion of nitrogen and hydrogen over magnetite, iron oxide, to produce ammonia. This process is important because ammonia is difficult to be produced on an industrial scale, and ammonia is the precursor for many nitrogen-containing products such as nitrate fertilizers and munitions. N2(g) + 3H2(g) 2 NH3(g) ΔHo = – 92.4 kJ mol-1 (a) Write an expression for Kp for the above reaction, stating its units. [2] ( ) () () 3 2 22 3 H N NH p P P PK = atm–2 (or Pa-2 or torr-2 or kg m-1 s-2 or N-2m4) (b) When a 1:3 mixture by volume of N 2 and H2 at an initial pressure of 200 atm is allowed to reach equilibrium at 500 oC, it was found that the percentage yield of NH3 is 30 %. ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ×= % 100% amount l theoretica amountactualyield (i) Calculate the equilibrium partial pressures of N2 and H2. (ii) Hence calculate a value of Kp at 500 oC. (iii) Suggest, with a reason, whether or not a higher temperature would favour the formation of ammonia. (iv) When some H2 is added to the existing equilibrium mixture, the partial pressure of ammonia is increased to 50 atm. Calculate the new equilibrium partial pressure of H2. (v) The value of Δ G is 0 kJ mol -1 at 196 °C. Use this information, and any other information to calculate the value of ΔS. (vi) Calculate ΔG for the forward reaction at 500 oC. (vii) Based on the answer in (vi), comment on the feasibility and relate it with the reaction conditions of 200 atm and 500 oC. [8] i. Using mole fraction, atm PN 50 2004 1 2 = × = atm PH 150 2004 3 2 = × =

3 SRJC 2008 9746/03/Prelim/2008 [Turn over ii. N2(g) + 3H2 (g) 2 NH3 (g) Initial amount/atm 50 150 0 Change /atm -15 -45 +30 Final amount/atm 35 105 30 100100 30 = × ( ) () (