Titration Curves w/ Acids & Bases

[laurence14b2f]

Hi all! I had a question regarding the equivalence point in titration curves. How does the equivalence point in a titration curve of a weak acid with a strong base differ from the equivalence point in the titration curve of a strong acid with a strong base? Thanks!

[Chem_Mod]

In both cases, the "equivalence point" is the point at which all of your original molecules of analyte have been neutralized with your titrant. That means the moles of analyte you started with are equal to the moles of titrant you've added, regardless of which is the base and which is the acid.

When you neutralize a strong acid with an equivalent amount of strong base, you're reacting H+ with OH- and making H2O, which is exactly neutral. The other ions in solution (or the "conjugates" of your strong acid and base) will be neutral. So your overall pH at the equivalence point will be 7, or neutral.
HCl --> H+ + Cl-: Cl- must be a netural ion because by nature of HCl being a strong acid, the reaction will not run backwards, and Cl- does not behave as a base.

However, when you neturalize a weak acid with a strong base, each time you neturalize a weak acid molecule you're creating one of it's conjugate base molecules. When you've neutralized all of your weak acid, you'll have made a whole bunch of your conjugate base. This base behaves as a base, and causes the equivalence point pH to be slightly basic.
CH3COOH <--> H+ + CH3COO-: This reaction is an equilibrium, and runs both ways, so CH3COO- will behave as a base.

The difference is what kind of conjugate you're leaving in solution as you neutralize. The "conjugate" of a strong acid is a neutral ion, where the conjugate of a weak acid is a weak base.