# Rate law for thermolysis at temperatures well above the decomposition temperature

If the temperature is higher than the decomposition temperature, are thermal decomposition reactions reversible? For example, which of the following schemes would be more appropriate for temperatures higher than the decomposition temperature of calcium carbonate $$\ce{CaCO3}$$?

\begin{align} \ce{CaCO3(s) &-> CaO(s) + CO2(g)} \tag{R1}\\ \ce{CaCO3(s) &<=> CaO(s) + CO2(g)} \tag{R2} \end{align}

Also, which of the following equations would be correct for the reaction rate $$v,$$ where $$k_\mathrm{fwd}$$ and $$k_\mathrm{rev}$$ are the rates of the forward and reverse reactions, respectively?

\begin{align} v &= k_\mathrm{fwd} \tag{1} \\ v &= k_\mathrm{fwd} - k_\mathrm{rev} \cdot p(\ce{CO2}),\tag{2} \end{align}

• Jul 19 at 17:32

## 1 Answer

In principle, the pyrolysis of $$\ce{CaCO3}$$ is an equilibrium, which should be described with a double arrow. But usually this equation is considered as not reversible, because $$\ce{CO2}$$ does not have the opportunity to stay in contact with the solid $$\ce{CaO}$$. In practice, $$\ce{CaCO3}$$ is heated in an open system. $$\ce{CO2}$$ is eliminated as an unwanted gas, because $$\ce{CaO}$$ is the wanted goal of the operation. If this reaction would have been done in a closed flask, without any possibility for $$\ce{CO2}$$ to escape, the reaction would have been reversed by cooling the flask. And $$\ce{CaCO3}$$ would have been "back-synthesized" at room temperature. But, I repeat, this back-reaction is not wanted, as the whole reaction is carried out in order to produce the biggest amount of $$\ce{CaO}$$ as possible. Anyway it is not easy to do this reaction in a closed flask, as the pressure will quickly increase to a rather huge value, which is not simple to maintain at a high temperature.

• Hi, thank you for the answer. So if I understand correctly, in a closed flask at temperatures higher than the decomposition temperature, the backward reaction will take place simultaneously with the forward reaction, right? Given the forward reaction is much faster at temperatures higher than the decomposition temperature? Jul 20 at 13:48
• In theory, in closed flask, the backward reaction will take place simultaneously with the forward reaction. But this is impossible to make in the real world, because the pressure in the flask will soon increase to more than $100$ bars, and even maybe more than $1000$ bars. And no material (glass, ceramics or metal) will resist to such a huge pressure at the temperature of decomposition (about $800°$C). Jul 20 at 20:27