H2co3 - 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 Kh, 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.

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