How does H₂CO₃ form when CO₂ dissolves in water?

Here is a sketchy outline of the process of CO₂ dissolution.

CO₂ must first cross the surface of the liquid:

CO₂(g) doublearrow CO₂(aq)

CO₂ is a bit more soluble than other gases in air, because the oxygen ends of the molecule have a partial negative charge are better able to hydrogen-bond to the water as a result.

Waters first hydrogen-bond to the oxygen atoms on the ends of the CO₂ and additional The CO₂ rather slowly acquires a shell of water molecules. A fraction of these hydrated carbon dioxide molecules react with the water to produce carbonic acid (H₂CO₃):

CO₂(aq) + H₂O H₂CO₃(aq)

The equilibrium constant for this reaction is about 1.6×10^-3 around room temperature, which means that most of the dissolved carbon dioxide is present as hydrated CO₂. Only about 0.16% of the dissolved CO₂ is present as carbonic acid.

The activation energy for the reaction is rather high, since it involves bending a stable, linear CO₂ molecule (with a water parked oxygen-down over the carbon) into a Y-shaped O=C(OH)₂ molecule. As a result, the reaction proceeds very slowly.

Author: Fred Senese senese@antoine.frostburg.edu

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