Why is the equivalence point of the reaction between hydrochloric acid and sodium carbonate "somewhat acidic"?
The balanced chemical equation for the described reaction is as follows:
`HCl + Na_2CO_3 -> H_2CO_3 + NaCl ` .
The equivalence point is not at pH 7 but is slightly acidic.
The equivalence point is defined as the point during titration wherein the moles of added acid (or base) are equal to the moles of the base (or acid) being titrated. In titration of strong acids and bases, like the reaction of NaOH and HCl, the product is NaCl, a neutral salt, and water, a neutral substance. This is true for any strong acid and strong base -- the products will always be a neutral salt and water -- and hence, the pH of the resulting solution at the equivalence point is 7.
This is not the case, however, if one of the reacting species isn't a strong acid or a strong base. Sodium carbonate dissociates in water into its ions -- the sodium ion and the carbonate ion. The carbonate ion is a weak base. Since it is a weak base, upon reaction with HCl, it produces its conjugate acid. The conjugate acid, just like any acid in a solution, will react with water to produce more hydronium ions. The generation of this extra hydronium makes the equivalence point of this given titration reaction slightly acidic.
In general, if a reaction is between a strong acid and a strong base, the equivalence point will be at pH 7. If it's between a strong acid and a weak base, it will be acidic. If it's between a weak base and a strong base, it will be basic.