2023 ACJC H2 Chem Prelim Paper 3 Suggested Solutions-10

10 © ACJC2023 9729/03/Prelim/2023 3 (a) (i) State and explain the conditions needed for a gas to behave ideally. [2] High temperature so that that molecules have sufficient energy to break free from each other and experience negligible forces of interaction. Low pressure so that the volume occupied by particles is negligible compared to the volume of the gas. (ii) Sketch and label a graph showing how the volume for a given mass of an ideal gas varies with temperature in Kelvin at constant pressure. [1] (b) Globally, ammonia is the second most produced chemical. The equation for the Haber process for manufacturing ammonia is shown. 3H 2 (g) + N 2 (g) 2NH 3 (g) ∆ H = – 92 kJ mol – 1 A mixture of H 2 and N 2 in the molar ratio 3:1 is added to a sealed vessel and heated to 450 o C with an iron catalyst. At equilibrium, 40% of the nitrogen had reacted. The total pressure in the vessel is 2.80 × 10 4 kPa at equilibrium. (i) Write the expression for the equilibrium constant, K p , for this reaction. Use your expression to calculate the value of K p for this reaction. Include its units. [4] 3H 2 (g) + N 2 (g) ⇌ 2NH 3 (g) Initial/kPa 3x x 0 Change/kPa – 1.2x – 0.4x +0.8x Eqm/kPa 1.8x 0.6x 0.8x 3.2x = 2.80 x 10 4 kPa ⟹ x = 8.75 x 10 3 kPa P NH3 = 7.00 x 10 3 kPa P H2 = 1.575 x 10 4 kPa P N2 = 5.25 x 10 3 kPa K p = (𝑃 𝑁𝐻 3 ) 2 (𝑃 𝐻 2 ) 3 (𝑃 𝑁 2 ) = (7.00×10 3 ) 2 (1.575×10 4 ) 3 (5.25×10 3 ) =2.39 x 10 – 9 kPa – 2 V T/K