1

TitrationsTitrationsTitrationsTitrations

pHpHpHpH

Titrant volume, mLTitrant volume, mLTitrant volume, mLTitrant volume, mL

At what point in a reaction does neutralization occur?

19.4 Neutralization Reactions >19.4 Neutralization Reactions >

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You can use a neutralization reaction to determine the concentration of an acid or base.

• The process of adding a measured amount of a solution of known concentration to a solution of unknown concentration is called a

titration.

TitrationTitration

3

Acid-Base ReactionsAcid-Base Reactions

• Strong acid + strong 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

19.4 Neutralization Reactions >19.4 Neutralization Reactions >

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A reaction between an acid and a base will go to completion when the solutions contain equal moles of hydrogen ions and hydroxide ions.

Acid-Base ReactionsAcid-Base Reactions

• The balanced equation provides the correct ratio of acid to base.

5

In strong acids, are all ionizable hydrogens completely ionized? In

weak acids?

In strong acids, all ionizable hydrogens are completely ionized. In weak acids, all ionizable hydrogens are partially ionized. Some hydrogens in these acids (those with larger Ka values) have a greater degree of ionization.

6Acetic acid titrated with NaOHAcetic acid titrated with NaOH

Figure 18.5Figure 18.5

Weak acid titrated with a strong base

19.5 Salts in Solution >19.5 Salts in Solution >

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Interpret GraphsInterpret Graphs

• One curve is for the addition of sodium hydroxide, a strong base, to ethanoic acid, a weak acid.

• An aqueous solution of sodium ethanoate exists at the equivalence point.

CH3COOH(aq) + NaOH(aq) →

CH3COONa(aq) + H2O(l)

19.5 Salts in Solution >19.5 Salts in Solution >

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Interpret GraphsInterpret Graphs

• This difference in pH exists because hydrolysis occurs with some salts in solution.

9Acid-Base TitrationsAcid-Base Titrations

Adding NaOH from the buret to acetic acid in

the flask, a weak acid. In the beginning

the pH increases very slowly.

Additional NaOH is added. pH increases and then levels off as NaOH is added beyond

the equivalence point.

10

Acid-Base TitrationSection 18.3

You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the final solution?

HBz + NaOH ---> Na+ + Bz- + H2O

You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the final solution?

HBz + NaOH ---> Na+ + Bz- + H2O

CC66HH55COCO22H = HBzH = HBz Benzoate ion = Bz-

Kb = 1.6 x 10-10

11

QUESTION:QUESTION: You titrate 100. mL of a You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point.0.100 M NaOH to the equivalence point.

QUESTION:QUESTION: You titrate 100. mL of a You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point.0.100 M NaOH to the equivalence point.

pH at pH at equivalence equivalence point?point?

pH at pH at equivalence equivalence point?point?

pH of solution of pH of solution of benzoic acid, a benzoic acid, a weak acidweak acid

pH of solution of pH of solution of benzoic acid, a benzoic acid, a weak acidweak acid

Benzoic acid Benzoic acid + NaOH+ NaOHBenzoic acid Benzoic acid + NaOH+ NaOH

pH at pH at half-way half-way point?point?

pH at pH at half-way half-way point?point?

12Acid-Base ReactionsAcid-Base Reactions

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

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

QUESTION: You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the

final solution?

13STOICHIOMETRY PORTION

M * V = mol1. Calc. moles of NaOH req’d

(0.100 L)(0.025 M) = 0.0025 mol HBz

This requires 0.0025 mol NaOH

2. Calc. volume of NaOH req’d

0.0025 mol (1 L / 0.100 mol) = 0.025 L

25 mL of NaOH req’d

M * V = mol1. Calc. moles of NaOH req’d

(0.100 L)(0.025 M) = 0.0025 mol HBz

This requires 0.0025 mol NaOH

2. Calc. volume of NaOH req’d

0.0025 mol (1 L / 0.100 mol) = 0.025 L

25 mL of NaOH req’d

14STOICHIOMETRY PORTION

3. Moles Bz- produced = moles HBz = 0.0025 mol

4. Calc. conc. of Bz-

There are 0.0025 mol of Bz- in a TOTAL SOLUTION VOLUME of

3. Moles Bz- produced = moles HBz = 0.0025 mol

4. Calc. conc. of Bz-

There are 0.0025 mol of Bz- in a TOTAL SOLUTION VOLUME of

125 mL125 mL

[Bz[Bz--] ] = 0.0025 mol / 0.125 L= 0.0025 mol / 0.125 L = = 0.020 M0.020 M

15Equivalence Point

Most important species in solution is benzoate ion, Bz-. It will react to form the weak conjugate base,

benzoic acid, HBz.

Bz- + H2O HBz + OH- Kb = 1.6 x 10-10

[Bz-] [HBz] [OH-]

I 0.020 0 0

C - x +x +x

E 0.020 - x x x

Most important species in solution is benzoate ion, Bz-. It will react to form the weak conjugate base,

benzoic acid, HBz.

Bz- + H2O HBz + OH- Kb = 1.6 x 10-10

[Bz-] [HBz] [OH-]

I 0.020 0 0

C - x +x +x

E 0.020 - x x x

16

Acid-Base ReactionsAcid-Base Reactions

x = [OH-] = 1.8 x 10-6

pOH = 5.75 -----> pH = 8.25

Kb 1.6 x 10-10 =

x2

0.020 - x Kb 1.6 x 10-10 =

x2

0.020 - x

17

QUESTION:QUESTION: You titrate 100. mL of a You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. 0.100 M NaOH to the equivalence point. What is the pH at half-way point?What is the pH at half-way point?

QUESTION:QUESTION: You titrate 100. mL of a You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. 0.100 M NaOH to the equivalence point. What is the pH at half-way point?What is the pH at half-way point?

pH at pH at half-way half-way point?point?

pH at pH at half-way half-way point?point?

Equivalence Equivalence point point pH = 8.25pH = 8.25

Equivalence Equivalence point point pH = 8.25pH = 8.25

18Acid-Base ReactionsAcid-Base Reactions

You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH.

What is the pH at the half-way point?

[H3O ] = [HBz]

[Bz- ]• Ka[H3O ] =

[HBz]

[Bz- ]• Ka

At the half-way point, [HBz] = [Bz-]Therefore, [H3O+] = Ka = 6.3 x 10-5

pH = 4.20 = pKa of the acid

Both HBz and Both HBz and BzBz-- are present. are present.

This is a This is a BUFFER!BUFFER!

Both HBz and Both HBz and BzBz-- are present. are present.

This is a This is a BUFFER!BUFFER!

19

Figure 18.7Figure 18.7

Weak base (NH3) titrated with a

strong acid (HCl)

19.4 Neutralization Reactions >19.4 Neutralization Reactions >

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Key ConceptsKey Concepts

In general, acids and bases react to produce a salt and water.

Neutralization occurs when the number of moles of hydrogen ions is equal to the number of moles of hydroxide ions.

19.4 Neutralization Reactions >19.4 Neutralization Reactions >

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Chemists use acid-base reactions to determine the concentration of an acid or a base in solution.

BIG IDEABIG IDEA

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