# Alkali and alkali earth metal carbonates and bicarbonates decomposition

Which of the following can not decompose on heating to give $$\ce{CO_2}$$?

1. $$\ce{Li_2CO_3}$$
2. $$\ce{Na_2CO_3}$$
3. $$\ce{KHCO_3}$$
4. $$\ce{BaCO_3}$$

What I have read till now: Lithium carbonate is not so stable to heat and bicarbonates of alkali metals decompose at approximately $$\pu{300 ^\circ C}$$. The stability of alkaline earth metals increases down the group and they decompose on red heating.

This only leaves us with one option as correct (2nd), but what is the reaction of decomposition of $$\ce{Na_2CO_3}$$?

From your question, the heating temperature is unclear. See these links for sodium and lithium carbonates, the hydrocarbonate or barium.

They are all also mentioned on Wikipedia.

In other words, they all seem to decompose at some point, so what is meant by heating?

The reaction for sodium carbonate decompositon at melting point should be:

$$\ce{Na2CO3 -> Na2O + CO2}$$

• Is there any temperature where $Na_2CO_3$ doesn't give $CO_2$ on thermal decomposition? – Kartik Watwani Jul 5 '16 at 13:48
• Take a look at this paper: oasis.postech.ac.kr/bitstream/2014.oak/11644/1/OAIR002402.pdf They discuss the decomposition of sodium and lithium carbonate in detail. – Zubo Jul 5 '16 at 21:23
• @KartikWatwani sorry for the late follow-up, but I wanted to add this: so you're asking as to whether if you heat up sodium carbonate, at what temperatures does is decompose WITHOUT producing carbon dioxide? Aside from some arcane extreme conditions (extreme pressures and temperatures combined, for example), I think that if it decomposes, it will probably produce carbon dioxide. That's just my opinion though, I am happy to be corrected. – Zubo Nov 2 '16 at 15:39

Here the comparison is highly relative(or subjective). The stability of group 1 and 2 carbonates increases down the group therefore you can expect that the lower the metal in the periodic table the more stable is its corresponding carbonate.

Also, for the same period the group 1 carbonate is relatively more stable. Not getting into the energetics one can safely state that the reason for these trend is the polarising power of the metal cation.

We know that in the carbonate ion, the charge is delocalized and is thus highly polarizable.

Group one and two cations have a very high charge to radius ratio(e/r) which implies that their polarizing powers are fairly high. This makes the C-O bond weaker allowing the -CO2 group to depart easily.

This e/r decreases down the group making the compound relatively stable. Thus the most stable species here shall be BaCO3

Na2CO3 is more thermally stable. So there is no effect of heating on it. Hence there is no reaction and carbon dioxide is not released.

• Please cite references for your claims. Your "answer" lacks credibility otherwise. – paracetamol Nov 15 '17 at 5:35