QUESTION 26 (3 marks)
One step of the contact process involves reacting sulfur dioxide (SO2) and oxygen (O2) to produce sulfur trioxide (SO3), as shown.
2SO2(g) + O2(g) ⇌ 2SO3(g)
The equilibrium constant (Kc) for the forward reaction is 5.6 × 104 at 350 °C and 9.2 × 101 at 800 °C.
a) Determine whether the forward reaction is exothermic or endothermic.
[1 mark]
b) Apply Le Châtelier’s principle to explain how changing pressure could be used to increase the production of SO3 gas.
[2 marks]
QCAA guide · typeset solution
QCAA sample response and mark allocation
26a) | The forward reaction is exothermic. | • determines that forward reaction is exothermic [1 mark] |
26b) | Increasing pressure would shift equilibrium to the products to produce more SO3(g). The reaction would shift towards the products to reduce pressure because there are fewer gas molecules as products (2 molecules) than reactants (3 molecules). | • determines that increasing pressure increases production of SO3(g) [1 mark] • explains that equilibrium shifts to the side with fewer molecules to reduce pressure [1 mark] |
QCAA sample response and marking criteria reproduced from the official guide.
Compare your working with the guide above.