Question 15.6: Each of these mixtures was placed in a closed container and ...
Each of these mixtures was placed in a closed container and allowed to stand:
(a) CaCO_{3} (s)
(b) CaO(s) and CO_{2} (g) at a pressure greater than the value of K_{p}
(c) CaCO_{3} (s) and CO_{2} (g) at a pressure greater than the value of K_{p}
(d) CaCO_{3} (s) and CaO(s)
Determine whether or not each mixture can attain the equilibrium
CaCO_{3} (s) ⇌ CaO (s) + CO_{2} (g)Analyze We are asked which of several combinations of species can establish an equilibrium between calcium carbonate and its decomposition products, calcium oxide and carbon dioxide.
Plan For equilibrium to be achieved, it must be possible for both the forward process and the reverse process to occur. For the forward process to occur, some calcium carbonate must be present. For the reverse process to occur, both calcium oxide and carbon dioxide must be present. In both cases, either the necessary compounds may be present initially or they may be formed by reaction of the other species.
Solve Equilibrium can be reached in all cases except (c) as long as sufficient quantities of solids are present. (a) CaCO_{3} simply decomposes, forming CaO(s) and CO_{2} (g) until the equilibrium pressure of CO_{2} is attained. There must be enough CaCO_{3}, however, to allow the CO_{2} pressure to reach equilibrium. (b) CO_{2} continues to combine with CaO until the partial pressure of the CO_{2} decreases to the equilibrium value. (c) Because there is no CaO present, equilibrium cannot be attained; there is no way the CO_{2} pressure can decrease to its equilibrium value (which would require some CO_{2} to react with CaO). (d) The situation is essentially the same as in (a): CaCO_{3} decomposes until equilibrium is attained. The presence of CaO initially makes no difference.