19.4 Neutralization Reactions > 1 Copyright © Pearson Education, Inc., or its affiliates. All...

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19.4 Neutralization Reactions >19.4 Neutralization Reactions >

Chemists use acid-base reactions to determine the concentration of an

acid or a base in solution.

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Acid-Base Reactions

Acid-Base ReactionsAcid-Base Reactions

What products form when an acid and a base react?

• 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

Acid-Base Acid-Base

Reactions:Reactions: NEUTRALIZATIONNEUTRALIZATION

Neutralization Neutralization ReactionsReactions

NaOH + HCl NaOH + HCl → NaCl + H→ NaCl + H2200

REACT a base with an acid.REACT a base with an acid.

forms salt and waterforms salt and water

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

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

For hydrochloric acid and sodium hydroxide, the mole ratio is 1:1.

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)1 mol 1 mol 1 mol 1 mol

For sulfuric acid and sodium hydroxide, the ratio is 1:2.

1 mol 2 mol 1 mol 2 mol

• Two moles of the base are required to neutralize one mole of the acid.

H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l)

Similarly, hydrochloric acid and calcium hydroxide react in a 2:1 ratio.

2HCl(aq) + Ca(OH)2(aq) → CaCl2(aq) + 2H2O(l)2 mol 1 mol 1 mol 2 mol

Sample Problem 19.7Sample Problem 19.7

Finding the Moles Needed for Neutralization

How many moles of sulfuric acid are required to neutralize 0.50 mol of sodium hydroxide? The equation for the reaction is

H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O.

KNOWNS

mol NaOH = 0.50 mol

1 mol H2SO4/2 mol NaOH (from balanced equation)

Analyze List the knowns and the unknown.1

UNKNOWN

mol H2SO4 = ? mol

To determine the number of moles of acid, you need to know the number of moles of base and the mole ratio of acid to base.

Calculate Solve for the unknown.2

Use the mole ratio of acid to base to determine the number of moles of acid.

0.50 mol NaOH × 1 mol H2SO4

2 mol NaOH= 0.25 mol H2SO4

Evaluate Does the result make sense?3

Because the mole ratio of H2SO4 to NaOH is 1:2, the number of moles of H2SO4 should be half the number of the moles of NaOH.

Why are acid-base reactions called neutralization reactions?

The complete reaction of an acid with a base creates a solution of a salt in water. This solution has a neutral pH. It is neither acidic nor basic.

TitrationsTitrationsTitrationsTitrations

pHpHpHpH

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

At what point in a reaction does neutralization occur?

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

The steps in an acid-base titration are as follows:

1. A measured volume of an acid solution of unknownunknown concentration is added to a flask.

2. Several drops of an indicator are added to the solution while the flask is gently swirled.

3. Measured volumes of a base of known known concentrations are mixed into the acid until the indicator just barely changes color.

TitrationTitration

A flask with a known volume of acids (and an indicator) is placed beneath a buret that is filled with a base of known concentration.

The base is slowly added from the buret to the acid.

A change in the color of the solution is the signal that neutralization has occurred.

The solution of known concentration is the standard solution.

TitrationTitration

• You can use a similar procedure to find the concentration of a base using a standard acid.

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

TitrationTitration

• The point at which neutralization occurs is called

the equivalence point.

The indicator that is chosen for a titration must change color at or near the pH of the equivalence point.

TitrationTitration

• The point at which the indicator changes color is the end point of the titration.

This graph shows how the pH of a solution changes during the titration of a strong acid (HCl) with a strong base (NaOH).

• The initial acid solution has a low pH (about 1).

• As NaOH is added, the pH increases because some of the acid reacts with the base.

The pH change of a solution during the titration of a strong acid (HCl) with a strong base (NaOH).

• The equivalence point for this reaction occurs at a pH of 7.

• As the titration nears the equivalence point, the pH rises dramatically because hydrogen ions are being used up.

Interpret GraphsInterpret Graphs

• Extending the titration beyond the point of neutralization produces a further increase of pH.

• If the titration of HCl and NaOH could be stopped right at the equivalence point, the solution in the beaker would consist of only H2O and NaCl, plus a small amount of indicator.

Determining Concentration by Titration

A 25-mL solution of H2SO4 is neutralized by 18 mL of 1.0M

NaOH. What is the concentration of the H2SO4 solution?

H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l).

KNOWNS

[NaOH] = 1.0M

VNaOH = 18 mL = 0.018 L

VH2SO4 = 18 mL = 0.018 L

Analyze List the knowns and the unknown.1

UNKNOWN

[H2SO4] = ?M

The conversion steps are as follows:

L NaOH → mol NaOH → mol H2SO4 → M H2SO4.

Convert volume to liters because molarity is in moles per liter.

Use the molarity to convert the volume of base to moles of base.

0.018 L NaOH × 1.0 mol NaOH

1 L NaOH= 0.018 mol NaOH

Use the mole ratio to find the moles of acid.

0.018 mol NaOH × 1.0 mol H2SO4

2 mol NaOH= 0.0090 mol H2SO4

Calculate the molarity by dividing moles of acid by liters of solution.

molarity = mol of solute

L of solution

0.0090 mol

0.025 L=

= 0.36M H2SO4

Evaluate Does the result make sense?3

• If the acid had the same molarity as the base (1.0M), 50 mL of base would neutralize 25 mL of acid.

• Because the volume of the base is much less than 50 mL, the molarity of the acid must be much less than 1.0M.

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

19.2 Hydrogen Ions and Acidity >19.2 Hydrogen Ions and Acidity >

Measuring pH

Measuring pHMeasuring pH

What are two methods that are used to measure pH?

Either acid-base indicators or

pH meters can be used

Indicator

a compound that can reversibly a compound that can reversibly change color depending on the change color depending on the concentration of Hconcentration of H33OO++ ions ions..

Used to measure pHUsed to measure pH

Indicator

An indicator (HIn) is a weak acid or base that dissociates in a known pH range.

• Indicators work because their acid form and base form have different colors in solution.

Acid-Base Indicators

• The acid form of the indicator (HIn) is dominant at low pH and high [H+].

• The base form (In−) is dominant at high pH and high [OH−].

Acid-Base Indicators

Many indicators are needed to span the entire pH spectrum.

A pH meter can be easier to use than liquid indicators or indicator strips.

pH Meters

• The pH reading is visible in a display window on the meter.

• The color and cloudiness of the solution do not affect the accuracy of the pH value obtained.

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.

42Acetic 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 >

• 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 >

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

45Acid-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.

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

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 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?

48Acid-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?

49STOICHIOMETRY 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

50STOICHIOMETRY 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

51Equivalence 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

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

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

Kb 1.6 x 10-10 =

0.020 - x Kb 1.6 x 10-10 =

0.020 - x

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?

Equivalence Equivalence point point pH = 8.25pH = 8.25

54Acid-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 ] =

[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!

Figure 18.7Figure 18.7

Weak base (NH3) titrated with a

strong acid (HCl)

Key ConceptsKey Concepts

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

• neutralization reaction: a reaction in which an acid and a base react in an aqueous solution to produce a salt and water

• titration: process used to determine the concentration of a solution (often an acid or base) in which a solution of known concentration (the standard) is added to a measured amount of the solution of unknown concentration until an indicator signals the end point

Glossary TermsGlossary Terms

• standard solution: a solution of known concentration used in carrying out a titration

• equivalence point: the point in a titration where the number of moles of hydrogen ions equals the number of moles of hydroxide ions

• end point: the point in a titration at which the indicator changes color

Glossary TermsGlossary Terms

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

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