A-LevelChemistryReaction kinetics9701/22 · May/June 2017 · Q318 Marks

The hydrogen halides, HCl, HBr and HI, can undergo thermal decomposition. In a sealed container an equilibrium is established according to the equation shown. 2HX(g) ⇌ H₂(g) + X₂(g) (where X = Cl, Br or I)

(a)Some bond energies are shown in the table. bond energy/kJmol⁻¹ H-Br | 366 H-H | 436 Br-Br | 193 Use these data to calculate a value for the enthalpy change, ΔH, for the thermal decomposition of hydrogen bromide, HBr, according to the equation shown. ΔH = ...................................... kJ mol⁻¹ [1][1]
(b)(i)Using the same axes, sketch a second curve to indicate the Boltzmann distribution at a higher temperature.[2]
(b)(ii)With reference to the curves, state and explain the effect of increasing temperature on the rate of decomposition of HBr.[3]
(b)(iii)The decomposition of HBr is endothermic. State the effect of increasing temperature on the percentage of HBr that decomposes. Use Le Chatelier's principle to explain your answer.[3]
(b)(iv)At 700K HBr is approximately 10% decomposed but hydrogen iodide, HI, is approximately 20% decomposed. Explain this difference with reference to bond strengths and the factors that affect them.[3]
(c)(i)Calculate the amounts, in mol, of H₂ and HCl present in the equilibrium mixture. H₂ = ...................................... mol HCl = ...................................... mol [2][2]
(c)(ii)Calculate the mole fraction of each gas in the equilibrium mixture. mole fraction of HCl = ...................................... mole fraction of H₂ = ...................................... mole fraction of Cl₂ = ...................................... [1][1]
(d)(i)Write the expression for the equilibrium constant, Kp, for the decomposition of HCl. 2HCl(g) ⇌ H₂(g) + Cl₂(g) Kp = ...................................... [1][1]
(d)(ii)Explain why the total pressure of the system does not need to be known for Kp to be calculated for this experiment.[1]
(d)(iii)Calculate the value of Kp for this experiment.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[1] (+) 103
[1] general shape of the curve and peak are displaced to right of original and starts at origin
[1] the peak is lower and curve crosses once only finishing above original
[1] rate increases AND correct explanation in terms of 'more collisions'
[1] at higher T area above Ea is greater / more molecules with E > Ea
[1] higher frequency of successful collisions OR more successful collisions per unit time / higher chance of successful collisions per unit time / higher proportion of successful collisions per unit time
[1] increases (%) decomposition (of HBr)
[1] (increasing T) shifts equilibrium to the right / in the forward direction / endothermic direction / towards H2 + Br2
[1] to oppose the change or oppose the increase in temperature OR to absorb (additional) energy / heat OR to decrease the temperature
[1] H-I bond strength less than H-Br OR less energy needed to break H-I ora
[1] I (atom) is big(ger) (than Br) OR I (atom) has more shielding (than Br) ora
[1] Br (atom) has greater (%) orbital / outer shell overlap OR attraction (of nucleus in iodine) for shared (pair of) electrons is weak(er) OR attraction (of nucleus in iodine) for bonding pair (or electrons) is weak(er) ora
[1] H2 = 0.015 (mol)
[1] HCl = 0.27 (mol)
[1] HCl = 9/10 AND XH2 = 1/20 AND Cl2 = 1/20 OR HCl = 0.9(0) AND H2 = 0.05 AND Cl₂ = 0.05
[1] (Kp =) pH2 × pCl2 / PHC12
[1] equal number of moles (of gas) on either side (of equation) / (total) pressure cancels
[1] 4.649 x 10⁻³