Weak Acid/ Strong Base Titrations Weak Base/ Strong Acid Titrations Acid-Base Titrations Weak Acid/ Strong Base Titrations Weak Base/ Strong Acid Titrations

Titration of a Weak Acid with a Strong Base Must assume that the neutralization reaction goes to completion! Titration curve for 50.0ml of 0.100M Acetic acid with 0.100M NaOH

Titration Curves of Weak Acids

Titration of a Weak Acid with a Strong Base To analyze titration curve, break them down into four sections: Initial pH: Use the Ka and an ICE chart to run a weak acid equilibrium calculation to find pH.

Titration of a Weak Acid with a Strong Base 2. B/w initial pH and equivalence point: Prior to the equivalence point, the acid is being neutralized and the conjugate base is being generated. A) First use stoich to determine the new acid and conjugate base concentrations b) then use either an ICE or HH. (Weak acid and conjugate base  a Buffer Calculation) CH3COOH + OH- CH3COO- + H2O

Titration of a Weak Acid with a Strong Base 3. Equivalence point: All of the acid has been reacted with stoichiometrically equal moles of base. The remaining species is the conjugate base (salt) which will then hydrolyze with water to create a pH>7. Use a ICE chart for the conjugate base.

Titration of a Weak Acid with a Strong Base 4. After the equivalence point: (excess base) the [OH-] from the conjugate base reaction is negligible compare to the excess strong base. Use stoich to determine the excess base [ ] and use –log[base]. (Strong base titrant)

Ways That a Weak Acid Titration Differs from a Strong Acid Titration A solution of weak acid has a higher initial pH than a strong acid. The pH change near the equivalence point is smaller for a weak acid. (This is at least partly due to the buffer region.) The pH at the equivalence point is greater than 7 for a weak acid.

Ways That a Weak Acid Titration Differs from a Strong Acid Titration Note: It is the strength of the acid that determines the pH value at the equivalence point.

The pH Curve for the Titration of 100. 0 mL of 0 The pH Curve for the Titration of 100.0 mL of 0.050 M NH3 (Weak Base) with 0.10 M HCl (Strong Acid)

Strong Acid-Weak Base At the beginning, the weak base is in equilibrium with itself. Thus, the beginning pH is lower than that of a strong base. More moles of the weak base are required to completely react with the amount of strong acid present, because the base is not entirely dissociated. For every mole of acid, there is less than one mole of base able to react: there is an excess of acid. Strong acid-weak base titrations reach their equivalence point at a pH less than 7.

Titrations of Polyprotic Acids- AP When a polyprotic acid is titrated with a base, there is an equivalence point for each dissociation. Using the Henderson–Hasselbalch equation, we can see that half way to each equivalence point gives us the pKa for that step.

Other Curves Weak acid-weak base titrations are difficult because each solution is in equilibrium. The pH of the equivalence point is around 7. This is the diagram of the titration of a diprotic acid with strong base. There are two equivalence points: one after each H has been detached from the acid molecule. Consequently, there are two jumps in pH on the graph.

Calculation Time! Consider: 25.0 mL of 0.100 M HCHO2 is titrated with 0.100 M NaOH. Ka= 1.8 x 10-4 (pKa = 3.74) Determine the following: a) Volume of NaOH needed to reach equivalence. b) Initial pH of weak acid c) pH after adding 5.00 mL of NaOH(or any amount before equivalence point) d) pH at equivalence e) pH after equivalence point