I was presented with the following reaction when we were just learning about acids and bases:

$$\ce{2NaOH + H2CO3 -> H2O + Na2CO3}$$ however I had thought it might've been something like

$$\ce{NaOH + H2CO3 -> H2O + NaHCO3}$$

They both seem to fit the notion of a neutralization reaction creating a salt so how do I know that the first one is correct? The second reaction looks fine to me (is it that bicarbonate is too reactive?) and more intuitive as the carbonic acid just gives a proton away ending up as an ion to bond with $Na$, so how do I distinguish between these kind of cases?


You're right, both of these reactions take place.

From Wikipedia, the Free Encyclopedia

In aqueous solution, carbonate, bicarbonate, carbon dioxide, and carbonic acid exist together in a dynamic equilibrium.

Now, the answer to the question depends on context. If you're asked to do this reaction in a mildly acidic/weakly basic medium, then the answer would be the bicarbonate, while in a strongly basic medium, the answer would be the carbonate.

Interesting for further reading:

This $\ce{pH}$ dependent equilibrium between carbonate and bicarbonate is one of the ways in which the blood $\ce{pH}$, and the $\ce{pH}$ of the mouth is maintained. It also helps aquatic life to survive in ocean waters. (Also see urea cycle and carbon cycle)

  • $\begingroup$ @ AbhigyanC What about $\pu{pH}$ at the second equivalence point, more or less than 7? $\endgroup$ – Adnan AL-Amleh Jan 31 at 15:35
  • 1
    $\begingroup$ @AdnanAL-Amleh $\ce{pK_{a1}}$ and $\ce{pK_{a2}}$ of carbonic acid are $4.5\times10^{-7}$ and $4.7\times10^{-11}$, respectively. So it happens at $\ce{pH}=8.34$, using the formula for pH of a amphiprotic anion is the average of the two pK values. See chembuddy.com/… $\endgroup$ – AbhigyanC Jan 31 at 15:44

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