inorganic chemistry - Thermal decomposition of magnesium bicarbonate

I was trying to solve a problem from a regional contest where it was a mixture of bicarbonates of $\ce{Na}$ and $\ce{Mg}$ which was put at high temperature. What are the reactions of decomposition?

I've thought that the reactions are:

$$\ce{2NaHCO3 -> Na2CO3 + H2O + CO2}$$ and $$\ce{Mg(HCO3)2 -> MgCO3 + H2O + CO2}$$

But, I found out that the second reaction is wrong. Actually, the reaction is

$$\ce{Mg(HCO3)2 -> MgO + H2O + 2CO2}$$

I searched on the web and I found the decomposition of other bicarbonates, like $\ce{Ca(HCO3)2}$ is similar to the first reaction. Is there any rule? When is carbonate and when is oxide? Or is $\ce{Mg}$ an exception?

Thanks!

Answer

I can quite easily remember from my class XI studies that carbonates of alkali metals decompose on heating to give carbon dioxide and the corresponding metal oxide.

Moreover the thermal stability of the alkali metal carbonates increases with increasing cationic size because carbonate ion is big in size and increased cationic size leads to better bonding and hence greater stability.

In this regard the beryllium carbonate is the least stable alkali carbonate or in other words, an unstable alkali carbonate and readily decomposes into $\ce{BeO}$ and $\ce{CO2}$. $\ce{BeCO3}$ is so unstable that it can only be kept in an atmosphere of $\ce{CO2}$. $\ce{MgCO3}$ is also a bit unstable taking into consideration the above fact.

So the reaction you mentioned basically gives $\ce{MgCO3}$ as the product but $\ce{MgCO3}$ decomposes readily to give $\ce{MgO}$ and $\ce{CO2}$.

$$\ce{Mg(HCO3)2 -> MgCO3 + H2O + CO2}$$ and $$\ce{MgCO3 -> MgO + CO2}$$ giving $$\ce{Mg(HCO3)2 -> MgO + H2O + 2CO2}$$