Chapter 19PP.pdf · Chapter 19 Acids, Bases, and Salts 19.1 Acid-Base Theories 19.2 Hydrogen Ions and Acidity 19.3 Strengths of Acids and Bases 19.4 Neutralization Reactions

19.3 Strengths of Acids and Bases >

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Chapter 19 Acids, Bases, and Salts

19.1 Acid-Base Theories

19.2 Hydrogen Ions and Acidity

19.3 Strengths of Acids and

Bases

19.4 Neutralization Reactions

19.5 Salts in Solution



Acids and bases are classified as strong

or weak based on the degree to which

they ionize in water.

Strong and Weak

Acids and Bases



In general, a strong acid is completely

ionized in aqueous solution.

• Hydrochloric and sulfuric acid are examples

of strong acids.

HCl(g) + H2O(l) → H3O+(aq) + Cl–(aq)

100%

Strong and Weak

Acids and Bases



A weak acid ionizes only slightly in

aqueous solution.

• The ionization of ethanoic acid (CH3COOH), a

typical weak acid, is not complete.

Strong and Weak

Acids and Bases



Interpret Data



Interpret Graphs

Dissociation of an acid

(HA) in water yields

H3O+ and an anion, A–.

The bar graphs

compare the extent of

the dissociation of a

strong acid and a weak

acid.



A strong acid, such as hydrochloric acid,

completely dissociates in water.

• As a result, [H3O+] is high in an aqueous

solution of strong acid.

Acid Dissociation Constant

Strong and Weak

Acids and Bases



By contrast, weak acids remain largely

undissociated.

• In an aqueous solution of ethanoic acid, less

than 1 percent of the molecules are ionized.


Strong and Weak

Acids and Bases



You can use a balanced equation to write

the equilibrium-constant expression for a

reaction.

• The equilibrium-constant expression shown

below is for ethanoic acid.


Keq= [H3O

+] × [CH3COO–

] [CH3COOH] ×

[H2O]

Strong and Weak

Acids and Bases



The acid dissociation constant (Ka) is the

ratio of the concentration of the dissociated

form of an acid to the concentration of the

undissociated form.

• The dissociated form includes both the H3O+

and the anion.


Strong and Weak

Acids and Bases

Keq × [H2O] = Ka

=

[H3O+] × [CH3COO–]

[CH3COOH] × [H2O]



The acid dissociation constant (Ka) reflects

the fraction of an acid that is ionized.

• For this reason, dissociation constants are

sometimes called ionization constants.


Strong and Weak

Acids and Bases



If the degree of dissociation or ionization of

the acid is small, the value of the

dissociation constant will be small.

• Weak acids have small Ka values.

• If the degree of ionization of an acid is more

complete, the value of Ka will be larger.

• The stronger an acid is, the larger its Ka

value will be.


Strong and Weak

Acids and Bases



Nitrous acid (HNO2) has a Ka of 4.4 × 10−4,

and ethanoic acid (CH3COOH) has a Ka of

1.8 × 10−5.

• Which one is stronger?


Strong and Weak

Acids and Bases



Strong and Weak

Acids and Bases

•Nitrous acid is stronger

Answer



Interpret Data



Some acids have more than one

dissociation constant because they have

more than one ionizable hydrogen.


Strong and Weak

Acids and Bases

• Oxalic acid is a diprotic acid.

– It loses two hydrogens, one at a time.

– Therefore, it has two dissociation

constants.

– Oxalic acid is found naturally in certain

herbs and vegetables.



Observe what happens to the Ka with each

ionization.

– The Ka decreases from first ionization to second.

– It decreases again from second ionization to third.


Strong and Weak

Acids and Bases



To calculate the acid dissociation constant

(Ka) of a weak acid, you need to know the

initial molar concentration of the acid and the

[H+] (or alternatively, the pH) of the solution

at equilibrium.

• You can use these data to find the equilibrium

concentrations of the acid and the ions.

• These values are then substituted into the

expression for Ka.

Calculating Dissociation Constants

Strong and Weak

Acids and Bases



You can find the Ka of an acid in water by

substituting the equilibrium concentrations

of the acid, [HA], the anion from the

dissociation of the acid, [A−], and the

hydrogen ion, [H+], into the equation below.

Calculating Dissociation Constants

Ka =

[H+][A−]

[HA]

Strong and Weak

Acids and Bases



Sample Problem 19.6

Calculating a Dissociation Constant

In a 0.1000M solution of ethanoic

acid, [H+] = 1.34 × 10−3M.

Calculate the Ka of this acid. Refer

to the table for the ionization

equation for ethanoic acid.



KNOWNS

[ethanoic acid] = 0.1000M

[H+] = 1.34 × 10−3M

UNKNOWN

Ka = ?

Analyze List the knowns and the unknown. 1

Sample Problem 19.6



Calculate Solve for the unknown. 2

Start by determining the equilibrium

concentration of the ions.

[H+] = [CH3COO−] = 1.34 × 10−3M

Each molecule of CH3COOH

that ionizes gives an H+ ion

and a CH3COO– ion.

Sample Problem 19.6




Determine the equilibrium

concentrations of each component.

(0.1000 – 0.00134)M = 0.0987M

Concentration [CH3COOH] [H+] [CH3COO−]

Initial 0.1000 0 0

Change −1.34 × 10−3 1.34 × 10−3 1.34 × 10−3

Equilibrium 0.0987 1.34 × 10−3 1.34 × 10−3

Sample Problem 19.6




Substitute the equilibrium values into the

expression for Ka.

Ka =

[H+] × [CH3COO–]

[CH3COOH] =

(1.34 × 10−3M) × (1.34 × 10−3M)

0.0987

= 1.82 × 10−5

Sample Problem 19.6



Evaluate Does the result make sense? 3

The calculated value of Ka is consistent

with that of a weak acid.

Sample Problem 19.6



Just as there are strong acids and weak

acids, there are strong bases and weak

bases.

Base Dissociation Constant

Strong and Weak

Acids and Bases





bases.

• A strong base dissociates completely into

metal ions and hydroxide ions in aqueous

solution.


Strong and Weak

Acids and Bases





bases.

• A strong base dissociates completely into

metal ions and hydroxide ions in aqueous

solution.

• A weak base reacts with water to form the

conjugate acid of the base and hydroxide ions.


Strong and Weak

Acids and Bases

– For a weak base, the amount of dissociation is

relatively small.



Ammonia is an example

of a weak base.

• Window cleaners often

use a solution of

ammonia in water to

clean glass.

Strong and Weak

Acids and Bases


NH3(aq) + H2O(l) NH4+(aq) + OH–(aq)

Ammonia Water Ammonium

ion

Hydroxide

ion



When equilibrium is established, only

about 1 percent of the ammonia is present

as NH4+.

• This ion is the conjugate acid of NH3.

• The concentrations of NH4+ and OH− are

low and equal.


Strong and Weak

Acids and Bases



ion

Hydroxide

ion



The equilibrium-constant expression for

the dissociation of ammonia in water is as

follows:


Keq = [NH4

+] × [OH−]

[NH3] × [H2O]

Strong and Weak

Acids and Bases



ion

Hydroxide

ion



Recall that the concentration of water is

constant in dilute solutions.


Keq × [H2O] = Kb

=

[NH4+] × [OH−]

[NH3]

Strong and Weak

Acids and Bases

• This constant can be combined with the Keq for

ammonia to give a base dissociation constant

(Kb) for ammonia.



The base dissociation constant (Kb) is

the ratio of the concentration of the

conjugate acid times the concentration of

the hydroxide ion to the concentration of

the base.


Kb = [conjugate acid] × [OH−]

[base]

Strong and Weak

Acids and Bases



The magnitude of Kb indicates the ability of

a weak base to compete with the very

strong base OH– for hydrogen ions.


Strong and Weak

Acids and Bases






• Because bases such as ammonia are weak

relative to the hydroxide ion, the Kb for such

a base is usually small.

– The Kb for ammonia is 1.8 × 10−5.


Strong and Weak

Acids and Bases






• Because bases such as ammonia are weak

relative to the hydroxide ion, the Kb for such

a base is usually small.

– The Kb for ammonia is 1.8 × 10−5.

– The smaller the value of Kb, the weaker the

base.


Strong and Weak

Acids and Bases



Sometimes people confuse the concepts

of concentration and strength.

• The words concentrated and dilute indicate

how much of an acid or base is dissolved in

solution.

– These terms refer to the number of moles of the

acid or base in a given volume.

• The words strong and weak refer to the

extent of ionization or dissociation of an acid

or base.

Concentration Versus Strength

Strong and Weak

Acids and Bases



The table below shows four possible combinations

of concentration and strength for acids.


Comparing Concentration and Strength of Acids

Acidic solution

Concentration

Strength Quantitative (or Molar) Relative

Hydrochloric acid 12M HCl Concentrated Strong

Gastric juice 0.8M HCl Dilute Strong

Ethanoic acid 17M CH3COOH Concentrated Weak

Vinegar 0.2M CH3COOH Dilute Weak

Interpret Data



The gastric juice in your stomach is a

dilute solution of HCl.


Strong and Weak

Acids and Bases

• The relatively small number of HCl molecules

in a given volume of gastric juice are all

dissociated into ions.

• Even when concentrated hydrochloric acid is

diluted with water, it is still a strong acid.



Conversely, ethanoic acid (acetic acid) is a

weak acid because it ionizes only slightly

in solution.

• Vinegar is a dilute solution of ethanoic acid.

• Even at a high concentration, ethanoic acid

is still a weak acid.


Strong and Weak

Acids and Bases



The same concepts apply to bases.

• A solution of ammonia can be either dilute

or concentrated.

• However, in any solution of ammonia, the

relative amount of ionization will be small.

• Thus, ammonia is a weak base at any

concentration.

• Likewise, sodium hydroxide is a strong base

at any concentration.


Strong and Weak

Acids and Bases



Key Concept &

Key Equation

Acids and bases are classified as

strong or weak based on the degree to

which they ionize in water.

Ka =

[H+][A−]

[HA]



Example problem 1



Example problem 2

Documents

Chapter 19PP.pdf · Chapter 19 Acids, Bases, and Salts 19.1 Acid-Base Theories 19.2 Hydrogen Ions and Acidity 19.3 Strengths of Acids and Bases 19.4 Neutralization Reactions