O-LevelChemistryRates of Reaction5070/21 · May/June 2016 · Q910 Marks

Carbon reacts with steam in a reversible reaction. C(s) + H2O(g) ⇌ H2(g) + CO(g) ∆H = +131kJ/mol The reaction reaches an equilibrium if carried out in a closed container.

(a)Explain, in terms of bond breaking and bond forming, why this reaction is endothermic.[2]
(b)When one mole of hydrogen, H2, is formed, 131 kJ of energy is absorbed. Calculate the amount of energy absorbed when 240dm³ of hydrogen, measured at room temperature and pressure, is formed.[2]
(c)(i)Predict, with a reason, how the position of equilibrium of this reaction changes as the pressure is increased at constant temperature,[2]
(c)(ii)temperature is increased at constant pressure.[2]
(d)(i)The reaction between carbon and steam is a possible source of hydrogen. Suggest one disadvantage of using this reaction as a source of hydrogen.[1]
(d)(ii)Another source of hydrogen is the cracking of hydrocarbons from crude oil. Give one advantage of manufacturing hydrogen from the reaction of carbon with steam rather than from crude oil.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2016). The mark or mark type is shown in brackets.

[2] Bond breaking absorbs energy and bond making releases energy/bond breaking is endothermic and bond making is exothermic (1) More energy absorbed than released/less energy released than absorbed/ endothermic energy change is more than exothermic energy change/ exothermic energy change is less than endothermic energy change (1)
[2] Moles of H2 = 10 (1) Energy absorbed = 1310 (kJ) (1)
[2] Position of equilibrium moves to the left/equilibrium shifts to the reactant side (1) More moles of gas on product side/fewer moles of gas on reactant side/less volume (of gas) on left (or reverse argument)/fewer molecules of gas on the left(or reverse argument) (1)
[2] Position of equilibrium moves to the right/equilibrium shifts to the product side (1) Reaction is endothermic/backward reaction is exothermic (1)
[1] Produces carbon monoxide which is toxic or poisonous gas/reaction involves a very high temperature (1)
[1] Saves crude oil/crude oil can be used to make other chemicals/coal or coke is less of a finite resource than crude oil (1)

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