I want to understand why the p$K_{\mathrm a}$ of $\ce{HCO3-}$ is approximately 10.2 and not less.

My hypothesis is this :

Bicarbonate could react in two ways in an aqueous solution:

$[1]\qquad\ce{HCO3- + H2O -> CO3^2- + H3O+}$

$[2]\qquad\ce{HCO3- + H2O -> H2CO3 + OH-}$

Since $\ce{H2CO3}$ is more stable than $\ce{CO3^2-}$ (because it has a lower charge and all the octets are completed), the p$K_{\mathrm a}$ must be high (more alkaline than acid).

In [1], the equilibrium will be shifted to the left and in [2] the equilibrium will be shifted to the right.

Is my line of thought correct?

  • 3
    $\begingroup$ No, it isn't. You can't meaningfully compare neutral molecules to ions in terms of stability. Also, bringing up completed octets doesn't help much, because all molecules and ions mentioned in your post share this feature. $\endgroup$ – Ivan Neretin Jun 20 '17 at 6:10
  • $\begingroup$ Your second equation is unbalanced. You have added an extra hydroxide ion to the right-hand side. $\endgroup$ – Tan Yong Boon Jun 20 '17 at 10:15

Carbonic acid is not necessarily more stable than the carbonate ion.

Why carbonic acid ( p$\ce{Ka = 6.3}$ ) is not stable?

The reason for this is same as the one given for the instability of aluminium carbonate, iron (III) carbonate etc. That is, the carbonate ion has a highly polarisable electron cloud, which can be distorted by densely positively-charged species, such as the hydrogen ion, in this case. Thus, carbonic acid is not a very stable species and compared to the bicarbonate ion, which only has one densely-charged hydrogen ion distorting its electron cloud, it is definitely more unstable. Thus, the position of the 2nd equilibrium mentioned in the question should be to the left, favouring the more stable species.

Why is the bicarbonate ion ( p$\ce{Ka = 10.3}$ ) more stable than the carbonate ion ?

This is due to there being a smaller negative charge on the bicarbonate ion and there being still 2 equivalent resonance structures to spread out the negative charge. Compared to the carbonate ion, which has twice the charge density but yet only 1 more resonance structure, the bicarbonate ion is more stable. Therefore, the position of equilibrium in the first reaction mentioned in the question should be to the left, not to the right.

Therefore, in solution, the dominant species among the three, should be the bicarbonate ion. This should be the correct line of thought.

  • $\begingroup$ "That is, the carbonate ion has a highly polarisable electron cloud, which can be distorted by densely positively-charged species, such as the hydrogen ion, in this case." I understood that , but i didnt get the realation between what you said and why H2CO3 is more unstable than HCO3-. For me you explained why CO3-2 is more unstable than HCO3-.Could you try explain to me again ? I aprreciate your anwser though. @Tan Yong Boon $\endgroup$ – user158657 Jun 20 '17 at 22:00
  • $\begingroup$ @user158657 Your first query can be answered very logically. Since in bicarbonate, there is only one H+ distorting the electron cloud while in carbonic acid, there are two H+ producing a much greater amount of distortion, carbonic acid is definitely much less stable due to the electron cloud of its carbonate ion being extremely polarised. $\endgroup$ – Tan Yong Boon Jun 21 '17 at 14:15
  • $\begingroup$ @user158657 Regarding your second query, we would have to assess stability based on both charge density (i.e. charge of the ion) and charge distribution (i.e. number of resonance structures). Although the carbonate ion has 3 equivalent structures, compared to the the bicarbonate ion, which only has 2 equivalent ones, the charge density of the ion is twice that of the bicarbonate ion. Thus, even though it can delocalise the charge over one more oxygen atom, it does not compensate for the extra unit of negative charge. Thus, the carbonate ion is more unstable compared to bicarbonate. $\endgroup$ – Tan Yong Boon Jun 21 '17 at 14:34

It makes no sense to ask why a compound has a specific value of pka. Values are meant to compare things together especially in terms of pka.

When you claim that one product is more stable, you need to mention in which condition otherwise we cannot make any prediction.

Furthermore, when talking about carbonates, you need to consider not only pKa of solution but pressure and proportion of carbon dioxide in gas phase. With no pressure applied, the amount of carbonate in solution if very low. Sodium bicarbonate is not as soluble as one might think it is.

The reason for it comes to what I assume is your question, which I will rephrase as such: I expect that second acidic proton to behave like a carboxylic acid because of it's structure but it appears to be much more basic than anticipated.

The answer to that question is to reverse it, why does the carboxylic acid in the carbonate at a pka of 3.6 (which is ballpark carboxylic acid). The answer to that question is obviously resonance structure of the carboxylic acid that stabilises the negative charge. Problem is that this system of resonance cannot stabilise two charges. Then what remains of the other OH in the carbonic acid, what is it? Well it's an OH linked to an SP2 carbon. Another OH on an sp2 carbon is phenol, and funny enough phenol pka is 9.95.

Evan pKa table is the perfect tool to answer most structural pKa questions, work by comparing species together.

  • $\begingroup$ In fact, the $\mathrm{pK_a}$ values themselves are telling you that the most stable species changes with pH, and these values are boundary values. $\endgroup$ – user41033 Jun 20 '17 at 17:22

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