The value quoted above is correct for the H 2CO 3 molecule, and shows that it is a stronger acid than acetic acid or formic acid: this might be expected from the influence of the electronegative oxygen substituent. HCO 3 − ⇌ CO 3 2− + H + K a2 = 5.61×10 −11 mol/L p K a2 = 10.25 at 25 ☌.Ĭare must be taken when quoting and using the first dissociation constant of carbonic acid. The rate constants are 0.039 s −1 for the forward reaction (CO 2 + H 2O → H 2CO 3) and 23 s −1 for the reverse reaction (H 2CO 3 → CO 2 + H 2O).Ĭarbonic acid has two acidic hydrogens and so two dissociation constants: In the absence of a catalyst, the equilibrium is reached quite slowly. The equilibrium constant at 25☌ is K h= 1.70×10 −3, which indicates that most of the carbon dioxide is not converted into carbonic acid and stays as CO 2 molecules. The presence of carbonic acid in the blood and other body fluids helps control the pH level (acidity) of those fluids.Ĭarbon dioxide dissolved in water is in equilibrium with carbonic acid:
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