Why do CaCO3 + CO2 + H2O form Ca(HCO3)2?

Is it because CO2 + H2O create the carbonic acid H2CO3 and it somehow reacts with CaCO3?

1 Answer
Feb 18, 2016

Answer:

Considering poor solubility of #CaCO_3(s)# a probable explanation may be given as follows

Explanation:

Calcium carbonate is poorly soluble in pure water (47 mg/L)
Ksp = 3.7×10−9 to 8.7×10−9 at #25^oC#
there exist an ionic equilibrium with the ions formed due to ionization of soluble calcium carbonate.
#CaCO_3(s) ⇌CO_3(soln) ⇌ Ca^(2+)(aq) + CO_3^(2−)(aq)#
Now in presence of #CO_2# in water #H_2CO_3# is formed and as a result medium becomes acidic.
In this acidic medium there exists
#HCO_3^-# ion and #H^+#.The #CO_3^(2−)(aq)#ions get protonated by this#H^+# ions and #HCO_3^-# ions are formed
.So decrease in concentration of #CO_3^(2−)(aq)#ions in solution shifts the above equilibrium towards right and dissolution of more #CaCO_3(s)# in solution takes place.
And the reaction proceeds as stated in question.

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