1. Acid-Base Reactions: TITRATION
To play the movies and simulations included, view the presentation in Slide Show Mode.

2. Neutralization Reactions
You can neutralize a base with an acid.
The result will form water molecules
NaOH + HCl → NaCl + H20

3. Acid-Base Reactions Strong acid + strong base Strong acid + weak base
HCl + NaOH ----> SALT WATER
Strong acid + weak base
HCl + NH3 ---> ACID
Weak acid + strong base
HOAc + NaOH ---> BASIC
Weak acid + weak base
HOAc + NH3 ---> Ka / Kb

4. NH3(aq) + H2O n NH4+(aq) + OH-(aq)
pH of Aqueous NH3
QUESTION: What is the effect on the pH of adding NH4Cl to 0.25 M NH3(aq)?
NH3(aq) + H2O n NH4+(aq) OH-(aq)
Let us first calculate the pH of a 0.25 M NH3 solution.
[NH3] [NH4+] [OH-]
initial
change -x +x +x
equilib x x

5. Assuming x is << 0.25, we have
pH of Aqueous NH3
Assuming x is << 0.25, we have
[OH-] = x = [Kb(0.25)]1/2 = M
This gives pOH = 2.67
and so pH =
= for 0.25 M NH3

6. pH of NH3/NH4+ Mixture I 0.25 0.1 0 C -x +x +x E 0.25 - x 0.10 + x x
We expect that the pH will decline on adding NH4Cl. Let’s test that!
[NH3] [NH4+] [OH-] I
C x x +x
E x x x

7. pH drops from 11.33 to 9.65 on adding a common ion.
pH of NH3/NH4+ Mixture
[OH-] = x = (0.25 / 0.10)Kb
= 4.5 x 10-5 M
This gives pOH = 4.35 and pH = 9.65
pH drops from to 9.65 on adding a common ion.

8. Titrations pH
Titrant volume, mL

9. Strong acid titrated with a strong base
Figure 18.4

10. Acetic acid titrated with NaOH
Weak acid titrated with a strong base
Figure 18.5

11. Acid-Base Titrations
Additional NaOH is added. pH increases and then levels off as NaOH is added beyond the equivalence point.
Adding NaOH from the buret to acetic acid in the flask, a weak acid. In the beginning the pH increases very slowly.

12. Acid-Base Titration Section 18.3
You titrate 100. mL of a M solution of benzoic acid with M NaOH to the equivalence point. What is the pH of the final solution?
HBz + NaOH ---> Na+ + Bz- + H2O
Kb = 1.6 x 10-10
Benzoate ion = Bz-
C6H5CO2H = HBz

13. pH at equivalence point?
QUESTION: You titrate 100. mL of a M solution of benzoic acid with M NaOH to the equivalence point.
pH at half-way point?
pH at equivalence point?
pH of solution of benzoic acid, a weak acid
Benzoic acid + NaOH

14. Acid-Base Reactions
QUESTION: You titrate 100. mL of a M solution of benzoic acid with M NaOH to the equivalence point. What is the pH of the final solution?
Strategy — find the conc. of the conjugate base Bz- in the solution AFTER the titration, then calculate pH.
This is a two-step problem
1. stoichiometry of acid-base reaction
2. equilibrium calculation

15. STOICHIOMETRY PORTION 1. Calc. moles of NaOH req’d
Acid-Base Reactions
STOICHIOMETRY PORTION
1. Calc. moles of NaOH req’d
(0.100 L)(0.025 M) = mol HBz
This requires mol NaOH
2. Calc. volume of NaOH req’d
mol (1 L / mol) = L
25 mL of NaOH req’d

16. 125 mL Acid-Base Reactions [Bz-] = 0.0025 mol / 0.125 L = 0.020 M
QUESTION: You titrate 100. mL of a M solution of benzoic acid with M NaOH to the equivalence point. What is the pH of the final solution?
STOICHIOMETRY PORTION
25 mL of NaOH req’d
3. Moles of Bz- produced = moles HBz = mol
4. Calc. conc. of Bz-
There are mol of Bz- in a TOTAL SOLUTION VOLUME of
125 mL
[Bz-] = mol / L = M

17. Acid-Base Reactions
Equivalence Point
Most important species in solution is benzoate ion, Bz-, the weak conjugate base of benzoic acid, HBz.
Bz- + H2O D HBz + OH Kb = 1.6 x 10-10
[Bz-] [HBz] [OH-] I
C x x +x
E x x x

18. x = [OH-] = 1.8 x 10-6 pOH = 5.75 -----> pH = 8.25
Acid-Base Reactions
x = [OH-] = 1.8 x 10-6
pOH = > pH = 8.25

19. pH at half-way point? Equivalence point pH = 8.25
QUESTION: You titrate 100. mL of a M solution of benzoic acid with M NaOH to the equivalence point. What is the pH at half-way point?
pH at half-way point?
Equivalence point
pH = 8.25

20. pH = 4.20 = pKa of the acid Both HBz and Bz- are present.
Acid-Base Reactions
You titrate 100. mL of a M solution of benzoic acid with M NaOH.
What is the pH at the half-way point?
Both HBz and Bz- are present.
This is a BUFFER!
At the half-way point, [HBz] = [Bz-]
Therefore, [H3O+] = Ka = 6.3 x 10-5
pH = = pKa of the acid

21. Weak diprotic acid (H2C2O4) titrated with a strong base (NaOH)
Figure 18.6
Weak diprotic acid (H2C2O4) titrated with a strong base (NaOH)

22. Weak base (NH3) titrated with a strong acid (HCl)
Figure 18.7

23. Indicator
a compound that can reversibly change color depending on the concentration of H3O+ ions.
Used to measure pH

24. Indicators for Acid-Base Titrations