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Why does $\ce{CaCl2}$ gives precipitation of $\ce{CaCO3}$ when $\ce{Na2CO3}$ is added, but $\ce{FeCl3}$ does not give the precipitation of $\ce{Fe2(CO3)3}$ in $\ce{Na_2CO_3}$ ?

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  • $\begingroup$ If it gives a precipitate then it isn't/is slightly soluble. $\endgroup$ – user60221 Apr 23 '18 at 18:07
  • $\begingroup$ @KianStevens ... I wanted to know the reason ... If I'm correct or not ... $\endgroup$ – Nehal Samee Apr 23 '18 at 18:08
  • $\begingroup$ What about the down vote ?? Is it not related .? $\endgroup$ – Nehal Samee Apr 23 '18 at 18:10
  • $\begingroup$ Obviously, $\ce{FeCl3}$ does not give the precipitate of $\ce{CaCO3}$, because, er... why would it? There is no $\ce{Ca}$ in it, to begin with. $\endgroup$ – Ivan Neretin Apr 23 '18 at 18:25
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    $\begingroup$ Now we've come to the heart of the matter. See, $\ce{Fe2(CO3)3}$ (yes, you've got the formula right this time around) is unstable due to hydrolysis. For all practical purposes, it doesn't exist. $\endgroup$ – Ivan Neretin Apr 23 '18 at 18:34
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This must have been a very good question for a geochemist. They'd give you a complete answer with details of why $\ce{Fe2(CO3)3}$ does not but $\ce{Fe2O3}$ exist in our surrounding. Yet, I would like to give only a simple answer for the question: No, $\ce{Fe2(CO3)3}$ does not form (in sort) when you added $\ce{Na2(CO3)3}$ to an aqueous solution of $\ce{FeCl3}$. The reason?

An aqueous solution of $\ce{FeCl3}$ is acidic because $\ce{FeCl3}$ is a salt of a strong acid $(\ce{HCl})$ and a weak base $(\ce{Fe(OH)3})$. Thus an aqueous solution of $\ce{FeCl3}$ is in following equilibrium: $$\ce {Fe^{3+} + 2H2O <=> [FeOH]^{2+} + H3O+ }$$ Thus, $\ce{H3O+}$ ions in solution can react with added $\ce{CO3^{2-}}$ ions to form carbonic acid $(\ce{H2(CO3)3})$,which can decomposed to form $\ce{CO2}$ and $\ce{H2O}$. Remaining $\ce{Fe^{3+}}$ will combine with left behind $\ce{OH-}$ ions to make $\ce{Fe(OH)3}$, which would decomposed to $\ce{Fe2O3}$. Thus, when $\ce{Na2CO3}$ is added to an aqueous solution of $\ce{FeCl3}$, one can expect the following sequence of reactions to be taken place: $$\ce {FeCl3 (aq) + Na2CO3 (aq) -> [Fe2(CO3)]^{3+} (aq) -> Fe2O3 (s) + CO2 (g) + H2O (l)}$$

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