A-LevelChemistryEquilibria9701/23 · May/June 2024 · Q214 Marks

N2(g) reacts with H2(g) in the Haber process, as shown in reaction 1. reaction 1 N2(g) + 3H2(g) ⇌ 2NH3(g) ∆H = -x kJ mol-1 Table 2.1 shows the different conditions used to produce three equilibrium mixtures, A, B and C. Table 2.1 initial molar ratio of N2 : H2 added 1:3 1:3 1:3 temperature/°C 500 500 1000 pressure/atm 1000 1000 1000 iron present in mixture no yes no percentage yield of NH3(g) at equilibrium 58 X y

(a)Describe and explain the change, if any, to the percentage yield of NH3(g) produced in B compared to A.[1]
(b)(i)Describe and explain the change, if any, to the percentage yield of NH3(g) produced in C compared to A.[1]
(b)(ii)Describe and explain the change to the rate of the forward reaction that occurs to establish the equilibrium in C compared to A. You do not need to refer to the Boltzmann distribution in your answer.[2]
(c)(i)Write an expression for the equilibrium constant, Kp, for reaction 1. State the units. Kp = units[2]
(c)(ii)Equilibrium mixture D is made when 1.0mol of N₂(g) and 3.0mol of H2(g) are added to a sealed container at 750°C and 1000 atm and left to reach equilibrium. This mixture contains 1.16 mol of NH3(g). Calculate the mole fraction of NH3(g) in D. mole fraction of NH3(g) =[2]
(c)(iii)The mole fraction of N2(g) is 0.625 in a new equilibrium mixture, E. Calculate the partial pressure of N₂(g) in E when the total pressure is 1000 atm. partial pressure of N2(g) = atm[1]
(d)(i)When oxides of nitrogen escape into the atmosphere they may be involved in: • formation of acid rain from sulfur dioxide • formation of photochemical smog. Identify the role of NO and NO2 in the formation of H2SO4 from SO2 in the atmosphere to produce acid rain. Use relevant equations to support your answer.[3]
(d)(ii)Outline how NO and NO2 may contribute to the formation of photochemical smog.[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

[1] 58% / no change / stays the same AND catalysts don't affect yield / iron is a catalyst
[1] less yield (in C than A) AND (as) exothermic (forward reaction)
[2] (increase in temperature / energy from A to C (so)) M1 more collisions with particles that have E > Ea AND increase frequency of effective collisions M2 (initial) rate (of the forward reaction) increases
[2] M1 Kp = (pNH3)2 / (pN2)(pH2)3 M2 atm-2
[2] M1 correct amount N2 at equilibrium = 1 - 1.16 / 2 = 0.42 AND correct amount H2 at equilibrium = 3.0 – (1.16 × 3/2) = 1.26 M2 express amount of ammonia as a mol fraction in terms of total amount of N2+H2+NH3 at equilibrium = 1.16/ (N2 + H2 + 1.16) = 0.408 OR 0.41
[1] 0.625 × 1000 = 625 (atm)
[3] M1 catalyst M2 2 equations to show the catalytic activity of NO2 in formation of SO3 from SO2 • SO2 + NO2 → NO + SO3 • 2NO + O2 → 2NO2 M3 describe production of H2SO4 from SO3 and water (can be shown in an equation). SO3 then reacts with (rain)water / moisture / water vapour to make sulfuric acid / H2SO4 OR SO3 + H2O → H2SO4
[2] M1 unburned hydrocarbons combine with NO and / NO2 M2 to form PAN / peroxyac(et)yl nitrate

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