Sulfur trioxide forms from sulfur dioxide and oxygen according to the exothermic…
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
Sulfur trioxide forms from sulfur dioxide and oxygen according to the exothermic reaction
2 SO2(g) + O2(g) ⇌ 2 SO3(g) ΔH° < 0
A 3.0 L rigid container is initially at 700 K and known to be at equilibrium with the following composition (particle counts shown):
• 12 SO2 molecules
• 6 O2 molecules
• 14 SO3 molecules
(a)–(d) refer to the same sample of gas.
At time t = 0 s the container is suddenly heated to 800 K. Immediately after the temperature increase, no reaction has yet occurred, so the composition above is still valid. The equilibrium constant values are
Kc(700 K) = 4.5 and Kc(800 K) = 1.3.
(a) Calculate the reaction quotient Qc immediately after the temperature change (t = 0 s, 800 K) using the particle counts provided.
(b) Compare the Qc value from part (a) with Kc(800 K) and determine the direction in which the reaction will proceed to reach the new equilibrium at 800 K.
(c) For each species (SO2, O2, SO3), indicate whether its concentration will increase, decrease, or remain unchanged as the system moves toward the new equilibrium. Justify each prediction based on your answer to part (b).
(d) Using Le Châtelier’s principle, explain why the numerical value of Kc decreases when the temperature is raised for this particular reaction.