Carbonic Acid - Chemical Equilibrium

Chemical Equilibrium

When carbon dioxide dissolves in water it exists in chemical equilibrium producing carbonic acid:

CO₂ + H₂O H₂CO₃

The hydration equilibrium constant at 25 °C is called K_h, which in the case of carbonic acid is / = 1.70×10−3: hence, the majority of the carbon dioxide is not converted into carbonic acid, remaining as CO₂ molecules. In the absence of a catalyst, the equilibrium is reached quite slowly. The rate constants are 0.039 s−1 for the forward reaction (CO₂ + H₂O → H₂CO₃) and 23 s−1 for the reverse reaction (H₂CO₃ → CO₂ + H₂O). Carbonic acid is used in the making of soft drinks, inexpensive and artificially carbonated sparkling wines, and other bubbly drinks. The addition of two equivalents of water to CO₂ would give orthocarbonic acid, C(OH)₄, which exists only in minute amounts in aqueous solution.

Addition of base to an excess of carbonic acid gives bicarbonate. With excess base, carbonic acid reacts to give carbonate salts.