Inorganic reactions

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Q.1 Could you please explain why aqueous BaCl₂ and NaHCO₃ react to give white precipitate of BaCO₃, whereas aqueous MgCl₂ reacts with NaHCO₃ to give colourless solution of Mg(HCO₃)₂.
 
Q.2 Could you please explain why aqueous BaCl₂ and NaHSO₄ react to give white ppt of BaSO₄, whereas MgCl₂ reacts with NaHSO₄ to give colourless solution of Mg(HSO₄)₂.
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Corrie writes ...
 
Q.1 It's all to do with the relative solubilities of the Group 2 carbonates or sulphates compared to the solubilities of the corresponding hydrogencarbonates or hydrogensulphates, and the relative amounts of the various negative ions present in solution.
 
Barium carbonate is much more insoluble than barium hydrogencarbonate. So, although hydrogencarbonate ions are only weakly dissociated into carbonate ions and hydrogen ions in the equilibrium:
 
HCO₃^-(aq) <=> CO₃^2-(aq) + H⁺(aq)
 
the concentration of carbonate ions is sufficient to cause insoluble barium carbonate to form as a precipitate.
 
However, in the case of magnesium, magnesium carbonate is more soluble than barium carbonate, and the concentration of carbonate ions present is not sufficient to cause a precipitate to form.
 
The hydrogencarbonates are all much more soluble than the corresponding carbonates, so these do not form precipitates.
 
Q. 2 You can now apply the above reasoning to the case of the sulphates/hydrogensulphates.
 
Bear in mind that the hydrogensulphate ion is much more dissociated in solution:
 
HSO₄^-(aq) <=> SO₄^2-(aq) + H⁺(aq)
 
so the concentration of sulphate ions will be greater compared to that for carbonate ions. Secondly, barium sulphate is very insoluble, whereas magnesium sulphate is very soluble in water.
 
Hope this helps? It is possible to go into more, quantitative, detail for the equilibria involved, using solubility products and the dissociation constants for the hydrogencarboante and hydrogensulphate ions.

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updated: 14 August 2008