H2CO3, dissociation equilibrium

An expression for the ionization of H2CO3 under such conditions (that is, in the presence of dissolved CO2) can be obtained from Kh, the equilibrium constant for the hydration of CO2, and from the first acid dissociation constant for H2CO3 ... 

Substituting from Table 3.3 for the equilibrium constant for dissociation of H2CO3, which is fast,... 

The quantity K = Z X lO" is commonly called the dissociation constant of carbonic acid. In reality it is the apparent dissociation constant. The true dissociation constant is much greater, for the equilibrium between CO2 and H2CO3 favors the... 

Unpolluted rainwater, as we have already seen, has pH close to 5.6 as a result of equilibrium of raindrops with ca. 350-360 ppmv CO2 in the troposphere. This yields the weak acid H2CO3 for which dissociation constant, Ka = 4.2 x 10 mol/L at 25 °C. 

The equilibrium (4.207) only describes the physical dissolved gas species (Fig. 4.7) without considering the subsequent protolysis equilibrium as discussed for the example of CO2 absorption in Chapter 2.S.3.2. In Eq. (2.117), we introduced an apparent Henry coefficient, where the dissolved matter comprises the anhydride (for example CO2 or SO2) and the acid (H2CO3, H2SO3). The acid can dissociate according to Eq. (4.174) and thereby increases the total solubility of gas A, as described by the effective Henry coefficient Hgf/. 

Carbonic acid is thus a relatively strong acid, comparable with formic acid, and its apparent weakness is due to the small proportion of dissolved carbon dioxide which is in the form H2CO3. As long as we are dealing with equilibrium properties, the distinction between CO2 and H2CO3 is unimportant, and it is quite satisfactory to operate with the apparent dissociation constant iC(C02). However, the distinction does become important as soon as we are dealing with time-dependent phenomena, especially rapid ones, and we shall see later (Chapter 10) that the catalytic effect of solutions containing carbon dioxide or bicarbonate ions can be understood only if we know both the true and apparent dissociation constants. 

Salomaa has used an ingenious procedure for separating the transfer effect in his studies of the first and second dissociation constants of carbonic and sulphurous acids in H2O-D2O mixtures. We have already seen (p. 38) that only 0.3% of dissolved carbon dioxide is in the form of H2CO3, and there is some evidence that the proportion of H2SO3 is also small. The product of the first and second dissociation constants thus represents the equilibrium constant for the process... 

Care must be taken when quoting and using the first dissociation constant of carbonic acid. In aqueous solution carbonic acid only exists in equilibrium with carbon dioxide, and the concentration of H2CO3 is much lower than the dissolved CO2 concentration. Since it is not possible to distinguish between H2CO3 and dissolved CO2 (referred to as C02(aq)) by conventional methods, H2CO3 is used to represent the two species when writing the aqueous chemical equilibrium equation. The equation may be rewritten as follows cf. sulfurous acid) ... 

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