Is iron(III) carbonate stable in water?

Question

I searched for this onsite already, and also found the following answer on Quora:

In aqueous environment, iron(III) is bound by water molecules and exists as $\ce{[Fe(H2O)6]^3+}$. Due to strong interaction between Fe(III) and some of the aqua ligands, the complex can dissociate into $\ce{[Fe(OH)_n(H2O)6^{n-}]3^{n-}}$ and $\ce{H+}$. Carbonate is a base, so it will capture proton and further facilitate the formation of hydroxyl complexes so much that it will produce $\ce{H2CO3}$ and $\ce{Fe(OH)3}$, the former eventually decomposes into water and $\ce{CO2}$. Therefore, iron(III) carbonate cannot exist in water.

In solid state, most of iron(III) compounds are hygroscopic, which means that they have strong affinity for moisture. And of course, when water gets involved, the above processes start happening, which eventually decomposes iron(III) carbonate, if it could ever be formed as solid.

Therefore, the compound itself is not stable in solutions or in solid states.

Is this correct? What happens in the following equation then:

$$\ce{2 FeCl3(aq) + 3 Na2CO3(aq) -> Fe2(CO3)3 + 6 NaCl}$$

Will we get different end products? What will be solid? Hydroxides or oxides? Will there be carbon dioxide evolved? Are any of the products going to be hydrates?

Answer 1

Apparently, ferric carbonate might be produced in solution by mixing ferric ions and carbonate ions. This is not the case, because ferric solutions are rather acidic, due to the hydrolysis reaction $$\ce{Fe^{3+} + 2 H2O -> [Fe(OH)]^{2+} + H3O+}$$ If a $\ce{Na2CO3}$ solution is added to a solution containing ferric ions like $\ce{Fe^{3+}}$ or $\ce{[Fe(OH)]^{2+}}$, $\ce{Fe2(CO3)3}$ cannot be produced. Instead, an insoluble basic carbonate is produced with the formula $\ce{FeCO3(OH)}$. The equation is : $$\ce{Fe^{3+} + 2 CO3^{2-} + H2O -> FeCO3(OH) + HCO3^-}$$ or $$\ce{[Fe(OH)]^{2+} + CO3^{2-} -> FeCO3(OH)}$$The basic carbonate $\ce{FeCO3(OH)}$ is a brown precipitate, which is decomposed by heating to boiling temperature according to : $$\ce{FeCO3(OH) + H2O -> Fe(OH)3 + CO2}$$ Ref.: R. Delaby, Analyse qualitative minérale, Masson, Paris 1950, p.76.

Answer 2

The reaction

Mixing iron (III) chloride and sodium carbonate solutions results in forming of a brownish precipitate with the liberation of a colourless gas: $$\ce{2FeCl3(aq) + 3Na2CO3(aq) + 3H2O(l) -> 2Fe(OH)3(s) + 3CO2(g) + 6NaCl(aq)}$$ The resulting precipitate is sometimes written as $\ce{Fe2O3\cdot aq}$. Adding an excess of sodium carbonate solution may lead to the formation of $\ce{HCO3-}$ ions in the resulting solution.

Explanation

Iron (III) carbonate is a weak electrolyte since it is formed by a weak acid and a weak base so it gets readily hydrolysed especially in the presence of a mineral acid: $$\ce{Fe2(CO3)3(s) + 3H2O(l) -> 2Fe(OH)3(s) + 3CO2(g)\ \ \ \ \ \ \ (pH\approx7)}$$ $$\ce{Fe2(CO3)3(s) + 6H+(aq) -> 2Fe^3+(aq) + 3CO2(g) + 3H2O(l)\ \ \ \ \ \ \ (pH<7)}$$

Useful links and references

1. Ronald L. Rich. Inorganic Reactions in Water (1st ed.): "Carbon oxide species", p. 178.
2. Wikipedia article on hydrolysis: https://en.wikipedia.org/wiki/Hydrolysis.
3. Demonstration #1 of the given reaction: https://www.youtube.com/watch?v=XaIPZhOB7CI.
4. Demonstration #2 of the given reaction: https://www.youtube.com/watch?v=qQp8Odsl5TE.

Answer 3

Basically, you just add $\ce{NaCl}$ to the solution, since $\ce{NaCl}$ does not change the pH of the solution, the reaction for $\ce{Fe_2(CO3)_3}$ described in the quote does not change.