An analytical chemist investigates the reactions of aqueous iron ions with sodium carbonate solution, Na2CO3(aq)\text{Na}_2\text{CO}_3(\text{aq})Na2CO3(aq). When Na2CO3(aq)\text{Na}_2\text{CO}_3(\text{aq})Na2CO3(aq) is added dropwise to a solution of hexaaquairon(III) ions, [Fe(H2O)6]3+(aq)[\text{Fe}(\text{H}_2\text{O})_6]^{3+}(\text{aq})[Fe(H2O)6]3+(aq), immediate effervescence is observed along with the formation of a brown precipitate.
Which option correctly identifies the formula of the precipitate, the gas evolved, and the chemical explanation for this behaviour?
Precipitate: Fe2(CO3)3(s)\text{Fe}_2(\text{CO}_3)_3(\text{s})Fe2(CO3)3(s); Gas: CO2(g)\text{CO}_2(\text{g})CO2(g); Explanation: The high charge-to-size ratio of Fe3+\text{Fe}^{3+}Fe3+ facilitates direct precipitation of iron(III) carbonate, with excess carbonate ions decomposing to carbon dioxide due to the low pH of the aqueous medium.
Precipitate: [Fe(H2O)3(OH)3](s)[\text{Fe}(\text{H}_2\text{O})_3(\text{OH})_3](\text{s})[Fe(H2O)3(OH)3](s); Gas: H2(g)\text{H}_2(\text{g})H2(g); Explanation: The strong oxidizing nature of the Fe3+\text{Fe}^{3+}Fe3+ central metal ion causes the reduction of coordinated water ligands to hydrogen gas, converting the complex to a neutral iron(III) hydroxide.
Precipitate: [Fe(H2O)3(OH)3](s)[\text{Fe}(\text{H}_2\text{O})_3(\text{OH})_3](\text{s})[Fe(H2O)3(OH)3](s); Gas: CO2(g)\text{CO}_2(\text{g})CO2(g); Explanation: The high charge density of Fe3+\text{Fe}^{3+}Fe3+ strongly polarizes the O−H\text{O}-\text{H}O−H bonds of the coordinated water ligands, making the complex acidic enough to donate protons to carbonate ions and evolve carbon dioxide.
Precipitate: FeCO3(s)\text{FeCO}_3(\text{s})FeCO3(s); Gas: CO2(g)\text{CO}_2(\text{g})CO2(g); Explanation: The Fe3+\text{Fe}^{3+}Fe3+ ion is easily reduced to Fe2+\text{Fe}^{2+}Fe2+ by carbonate ions, resulting in the precipitation of iron(II) carbonate and the simultaneous evolution of carbon dioxide.
