Question

Decomposition of calcium carbonate is an irreversible reaction that goes to completion. How this reaction is made reversible by changing the conditions?

Answer 1

Answer. When CaCO3 is heated in an open flask, it decomposes to form calcium oxide and carbon dioxide. CO2 escapes out and reaction goes to completion;
CaCO3(S)      CaO(S) + CO2(g)  (decomposition)
 
                         Showing establishment of reversible reaction.
In these two reactions, decomposition is reverse to combination or vice versa. When calcium carbonate is heated in a closed flask so that CO2 can't escape out as shown in figure, for sometime only decomposition goes on (forward reaction), but after a while CO2 starts combining with CaO to form CaCO3 (reverse reaction). In the beginning forward reaction is fast and reverse reaction is slow. But eventually, the reverse reaction speeds up and both reactions go on at the same rate. At this stage
decomposition and combination take place at the same rate but in opposite directions, as a result amounts of CaCO3, CaO and CO2 do not change. It is written as

                   CaCO3(S) <========>CaO(S) + CO2(g)

Comments

I totally agree with the reversibility of the reaction, but the decomposition of CaCO3 takes place at high temp (800-1000 deg. cent.) and how is it possible that backward rxn also take place under same condition! (Provided we have maintained the temp)