When we mix NaOH and H2SO4 we get the equation
OH-(aq) + H+(aq) --> H2O(l)
and if we mix H2CO3 and NaOH we get the equation
2OH-(aq) + H2CO3 --> 2H2O(l) + CO3-2(aq)
Why aren't the two equations the same? ie only one mol of NaOH reacts in the first equation but two mol of NaOH in the second - Why?
The difference is that the second acid is diprotic, so it takes two OH ions to neutralize all the hydrogen ions in the molecule.
According to my textbook H2SO4 is diprotic too.
That's correct. H2SO4 is diprotic; H2CO3 is diprotic. BUT H2SO4 is a strong acid; H2CO3; is a weak acid. Your first net ionic equation involves only H^+ and OH^- because both NaOH and H2SO4 are 100% ionized (actually, H2SO4 has a relatively large k2 of 1.2E-2 but for most freshman work we consider H2SO4 100% ionized).In the second equation, H2CO3 appears because it is a weak acid
To explain why the two equations are different, we need to understand the concept of acid-base reactions and the strength of acids.
In the first equation, we mix NaOH (sodium hydroxide) and H2SO4 (sulfuric acid). Sodium hydroxide is a strong base, and sulfuric acid is a strong acid. When they react, the sodium hydroxide dissociates completely to produce OH- ions, and the sulfuric acid dissociates completely to produce H+ ions. Therefore, only one mol of NaOH is needed to neutralize one mol of H2SO4, resulting in the net ionic equation: OH-(aq) + H+(aq) -> H2O(l).
In the second equation, we mix H2CO3 (carbonic acid) and NaOH. Unlike sulfuric acid, carbonic acid is a weak acid. It does not dissociate completely in solution. When carbonic acid reacts with sodium hydroxide, it takes two hydroxide ions (OH-) to neutralize each carbonic acid molecule. This is because carbonic acid is diprotic, meaning it can donate two protons (H+) during ionization. So, the net ionic equation becomes: 2OH-(aq) + H2CO3 -> 2H2O(l) + CO3-2(aq). The two hydroxide ions from sodium hydroxide react with one carbonic acid molecule to form two water molecules and a carbonate ion.
Thus, the difference in the number of moles of NaOH reacting in the two equations is due to the diprotic nature of carbonic acid. In the first equation, one mole of NaOH is enough to neutralize one mole of H2SO4, while in the second equation, two moles of NaOH are needed to neutralize one mole of H2CO3.