Notes on Carbon Dioxide and Calcium Carbonate Chemistry

Carbon Dioxide Reaction with Hydroxide Ions

• The balanced chemical equation for the reaction between carbon dioxide gas and hydroxide ions:
  $CO<em>2(g) + 2OH^-(aq) \rightarrow CO</em>3^{2-}(aq) + H_2O(l)$

Precipitation of Calcium Carbonate

• When $CO<em>2(g)$ is bubbled into a saturated solution of $Ca(OH)</em>2$, a cloudy precipitate of $CaCO_3(s)$ forms.

• At $25^\circ C$, the solubility-product constant, $K<em>{sp}$, of $CaCO</em>3(s)$ is $3.4 \times 10^{-9}$. (Note the temperature dependence of $K_{sp}$).

Net Ionic Equation

• Net ionic equation for the precipitation of $CaCO<em>3(s)$ from the solution: $Ca^{2+}(aq) + CO</em>3^{2-}(aq) \rightarrow CaCO_3(s)$

Carbonate Ion Concentration

• Calculation of $[CO<em>3^{2-}]$ in the saturated solution of $Ca(OH)</em>2$ just before $CaCO_3(s)$ begins to precipitate:

• The $K<em>{sp}$ expression for calcium carbonate ($CaCO</em>3$) is given by: $K<em>{sp} = [Ca^{2+}][CO</em>3^{2-}]$

• In a saturated solution of $Ca(OH)_2$, the concentration of $Ca^{2+}$ ions can be determined.

• Just before precipitation, the ion product is equal to the $K_{sp}$.

• We are asked to find the concentration of carbonate ions $[CO<em>3^{2-}]$ just before the calcium carbonate $CaCO</em>3(s)$ begins to precipitate.

• Given that $K_{sp} = 3.4 \times 10^{-9}$, and we need to consider the calcium ion concentration $[Ca^{2+}]$ in the saturated calcium hydroxide solution.