QUESTION 124 (4 marks)
Solid calcium carbonate and solid calcium oxide coexist with carbon dioxide at equilibrium: . The vessel volume and temperature are constant.
(a) Write the equilibrium expression and explain why the two solids do not appear. [2 marks]
(b) Additional solid CaCO₃ is placed in the vessel without changing its gas volume. Predict the equilibrium CO₂ concentration. [1 marks]
(c) Some CO₂ is removed. Both solids remain present. State the net reaction direction while equilibrium is re-established. [1 marks]
Practice marking scheme
Answer
; pure solids have constant activity. Adding more CaCO₃ leaves the equilibrium CO₂ concentration unchanged. Removing CO₂ produces a net forward reaction.
Working
The activities of pure solids are constant and are incorporated into Kc. Because both solid phases are already present and gas volume is unchanged, adding more of one solid does not change the equilibrium gas concentration. Removing CO₂ makes Qc < Kc; CaCO₃ decomposes to restore CO₂.
Marking criteria
- Writes Kc = [CO₂]. [1 mark]
- Explains omission of pure solids using their constant activity/contribution. [1 mark]
- Predicts no change in equilibrium CO₂ concentration on adding solid. [1 mark]
- Predicts the forward reaction after CO₂ removal. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.