18.2 The Chemistry of CO₂

Chemical Concept Demonstrated: Chemistry of CO₂

Demonstration:

The three beakers contain limewater (Ca(OH)2). A student blows into the first beaker with a straw. Dry-ice is added to the second beaker. Club soda is added to the third Compare.

Observations:

1.  A cloudy white precipitate forms.
2.& 3.  At first, a precipitate forms in the beaker.  When additional dry ice and club soda are added, the precipitate redissolves.

Explanations (including important chemical equations):

    Carbon dioxide dissolves in water and slowly reacts with water to produce carbonic acid.

    CO₂ (g) + H₂O (l) <=> H₂CO₃ (aq)

    The cloudy white solution observed when CO₂ is bubbled into limewater results from a reaction between Ca(OH)₂ and either CO₂ or H₂CO₃ to form an insoluble calcium carbonate precipitate.

    Ca(OH)₂ (aq) + H₂CO₃ (aq) <=> CaCO₃ (s) + 2 H₂O (l)    K_sp = 2.8 x 10^-9

    Excess CO₂ or H₂CO₃ , however, reacts with the CO₃ ^2- ion in this solution to form HCO₃ ^-.

    H₂CO₃ (aq)  + CO₃ ^2- (aq) <=> 2 HCO₃ ^- (aq)

    Since calcium bicarbonate is soluble in water, the CaCO₃ precipitate dissolves in the presence of excess carbonic acid.

    CaCO₃ (s) + H₂CO₃ (aq) <=> Ca ²⁺ (aq) + 2 HCO₃ ^- (aq)