why is the equivalence point for the reaction between hydrochloric acid and sodium carbonate acidic?

Expert Answers
mvcdc eNotes educator| Certified Educator

The case of titrating sodium carbonate and hydrochloric acid is a weak base - strong acid titration.

In such cases, the solution is really expected to be acidic at the equivalence point -- the first equivalence point, in particular. In this particular case, we are expecting 2 equivalence points because we can add two successive protons to the base. There are two equivalence points because sodium carbonate is polyprotic (diprotic):

`CO_3^(2-) + H^+ harr HCO_3^-`

`HCO_3^(-) + H^+ harr H_2CO_3`

The first equivalence point is for the first reaction, while the second equivalence point is for the second reaction.

The pH at the equivalence point (second) is acidic because at the equivalence point, the solution contains a salt of the weak base. Note that there is an equilibrium also, the dissociation of `H_2CO_3` . The result is an acidic solution. [The pH at the first equivalence point is around 8.31, while it is 3.69 at the second equivalence point -- where the solution contains more acidic species as seen in the second reaction.)

Multiple equivalence points are always seen for polyprotic acids/bases, and the pH depends on the nature (strong/weak) of the acids/bases.