General organic and biological chemistry structures off life 5th CH11 acids and bases GOB structures 5th ed

Conjugate Acid–Base PairsIn this acid–base reaction, • the first conjugate acid–base pair is HF, which donates H+ to form its conjugate base, F−.. • the other conjugate acid–base pair is

Lecture Presentation

Chapter 11 Acids and Bases

Clinical laboratory

technicians prepare

specimens for the detection

of cancerous tumors and

type blood samples for

transfusions They must

also interpret and analyze

the test results, which are

then passed on to the

physician.

Chapter 11 Acids and Bases

Chapter 11 Readiness

Core Chemistry Skills

• Writing Ionic Formulas (6.2)

• Balancing a Chemical Equation (7.1)

• Using Concentrations as a Conversion Factor (9.4)

• Writing the Equilibrium Constant Expression (10.3)

• Calculating Equilibrium Concentrations (10.4)

• Using Le Châtelier’s Principle (10.5)

Chapter 11 Acids and Bases

A soft drink contains

phosphoric acid

(H3PO4) and carbonic

acid (H2CO3).

Arrhenius Acids

Arrhenius acids

• produce hydrogen ions (H+) when they dissolve in water

HCl(g) H+(aq) + Cl−(aq)

• are also electrolytes, because they produce H+ in water

• have a sour taste

• turn blue litmus red

• corrode some metals

H2O(l)

Naming Acids

• Acids with a hydrogen ion (H+) and a nonmetal (or CN−) ion are named with the prefix hydro and end with ic acid

HCl(aq) hydrochloric acid

• Acids with a hydrogen ion (H+) and a polyatomic ion are

named by changing the end of the name of the polyatomic ion from

ate to ic acid or ite to ous acid

ClO3− chlorate ClO2− chlorite

HClO3 chloric acid HClO2 chlorous acid

Names of Common Acids

The name of an acid with a hydrogen ion (H+) and a

nonmetal uses the prefix hydro and ends with ic acid

2 H2CO3CO32−, carbonate A carbonic acid

An acid with a hydrogen ion (H+) and a polyatomic ion

ending in ate is called an ic acid

3 HBrO2 BrO2−, bromite C bromous acid

An acid with a hydrogen ion (H+) and a polyatomic ion

ending in ite is called an ous acid

Arrhenius Bases

Arrhenius bases

• produce hydroxide ions (OH−)

in water

• taste bitter or chalky

• are also electrolytes, because

they produce hydroxide ions

(OH−) in water

• feel soapy and slippery

• turn litmus indicator paper

• blue and phenolphthalein

indicator pink

An Arrhenius base produces cations and OH− anions in an aqueous solution.

Naming Bases

Typical Arrhenius bases are named as hydroxides

Ba(OH)2 barium hydroxide

Calcium hydroxide, Ca(OH)2,

is used in the food industry

to produce beverages, and in dentistry as a filler for root canals.

Characteristics of Acids and Bases

Study Check

Match the formulas of acids and bases with their names

Match the formulas of acids and bases with their names

3 _B H2SO4 B sulfuric acid

11.2 Brønsted–Lowry Acids and Bases

According to the Brønsted–Lowry theory,

• an acid is a substance that donates H+

• a base is a substance that accepts H+

General, Organic, and Biological Chemistry: Structures of Life, 5/e

Karen C Timberlake

© 2016 Pearson Education, Inc.

NH3, a Brønsted–Lowry Base

In the reaction of ammonia and water,

• NH3 acts as the base that accepts H+

• H2O acts as the acid that donates H+

Because the nitrogen atom of NH has a stronger attraction

Study Check

In each of the following equations, identify the Brønsted–

Lowry acid and base in the reactants:

A HNO3(aq) + H2O(l) H3O+(aq) + NO3−(aq)

B HF(aq) + H2O(l) H3O+ (aq) + F−(aq)

In each of the following equations, identify the Brønsted–

Lowry acid and base in the reactants:

A HNO3(aq) + H2O(l) H3O+(aq) + NO3−(aq)

Acid Base

B HF(aq) + H2O(l) H3O+ (aq) + F−(aq)

Acid Base

Study Check

Identify each as a characteristic of

A an acid or B a base

1 has a sour taste

2 produces OH− in aqueous solutions

3 has a chalky taste

4 is an electrolyte

5 produces H+ in aqueous solutions

Identify each as a characteristic of

A an acid or B a base

B 2 produces OH− in aqueous solutions

A, B 4 is an electrolyte

A 5 produces H+ in aqueous solutions

Conjugate Acid–Base Pairs

In any acid–base reaction, there are two conjugate acid–

base pairs

• Each pair is related by the loss and gain of H+

• One pair occurs in the forward direction

• One pair occurs in the reverse direction

Acid and conjugate base pair 1

HA + B A− + BH+

Base and conjugate acid pair 2

Conjugate Acid–Base Pairs

In this acid–base reaction,

• the first conjugate acid–base pair is HF, which donates

H+ to form its conjugate base, F−

• the other conjugate acid–base pair is H2O, which

accepts H+ to form its conjugate acid, H3O+

• each pair is related by a loss and gain of H+

Conjugate Acid–Base Pairs

In the reaction of NH3 and H2O,

• one conjugate acid–base pair is NH3/NH4+

• the other conjugate acid–base pair is H2O/H3O+

2 Write the conjugate acid of each of the following bases:

(Add an H+ to each base to get the conjugate acid.)

A NO2− + H+ HNO2

B NH3 + H+

NH4+

C OH− + H+ H2O

Identify the sets that contain acid–base conjugate pairs

1 HNO2, NO2− acid, conjugate base

2 H2CO3, CO32− not acid–base conjugate pair

Amphoteric Substances

Substances that can act as both acids and bases are

amphoteric or amphiprotic

For water, the most common amphoteric substance, the

acidic or basic behavior depends on the other reactant

• Water donates H+ when it reacts with a stronger base

• Water accepts H+ when it reacts with a stronger acid

Guide to Writing Conjugate Acid–Base Pairs

Identify the conjugate acid–base pairs in the following reaction:

HNO3(aq) + NH3(aq) NO3−(aq) + NH4+(aq)

STEP 1 Identify the reactant that loses H + as the acid.

In the reaction, HNO3 donates H+ to NH3

STEP 2 Identify the reactant that gains H + as the base.

In the reaction, NH3 gains H+ to form NH4+ Thus,

• NH3 is the base and NH4+ is its conjugate acid

• HBr is the acid and Br− is its conjugate base

Identify the conjugate acid–base pairs in the following

reaction:

HNO3(aq) + NH3(aq) NO3−(aq) + NH4+(aq)

STEP 3 Write the conjugate acid–base pairs.

HBr/Br− is the acid and conjugate base pair

NH3/NH4+ is the base and conjugate acid pair

11.3 Strengths of Acids and Bases

Weak acids only partially

dissociate in water

Hydrofluoric acid, HF, is

the only halogen that

forms a weak acid.

Learning Goal Write equations for the dissociation of

strong and weak acids; identify the direction of reaction

Strong and Weak Acids

A strong acid completely ionizes (100%) in aqueous

solutions

HCl(g) + H2O(l) H3O+(aq) + Cl−(aq)

A weak acid dissociates only slightly in water to form a few

ions in aqueous solutions

H2CO3(aq) + H2O(l) H3O+(aq) + HCO3− (aq)

Strong Acids

In water, the dissolved molecules

of HA, a strong acid,

• dissociate into ions 100%

• produce large concentrations of

H3O+ and the anion (A−)

The strong acid HCl dissociates

completely into ions:

HCl(g) + H2O(l)

H3O+(aq) + Cl−(aq)

Weak Acids

In weak acids, only a few

molecules dissociate

• Most of the weak acid

remains as the undissociated

(molecular) form of the acid

Relative Strength of Acids and Bases

Strong and Weak Acid Dissociation

Strong and Weak Acid Dissociation

Figure 11.2 After dissociation in water, ▶ (a) the strong acid HI has high

concentrations of H3O + and I –, and (b) the weak acid HF has a high

concentration of HF and low concentrations of H3O + and F.

Diprotic Acids: Carbonic Acid

• Some weak acids, such as carbonic acid, are diprotic

acids that have two H +, which dissociate one at a time

H2CO3(aq) + H2O(l) H3O+(aq) + HCO3−(aq)

• Because HCO3− is also a weak acid, a second dissociation can take place to produce another hydronium ion and the

carbonate ion, CO32−

HCO3−(aq) + H2O(l) H3O+(aq) + CO32−(aq)

Diprotic Acids: Sulfuric Acid

• Some strong acids, such as sulfuric acid, are

diprotic acids that have two H+, which dissociate one at a time.

H2SO4(aq) + H2O(l) H3O+(aq) + HSO4−(aq)

• Because HSO4− is a weak acid, a second

dissociation can take place to produce another H+

and the sulfate ion, SO42− .

HSO4−(aq) + H2O(l) H3O+(aq) + SO42−(aq)

Strong Bases

Strong bases as strong electrolytes

• are formed from metals of

Groups 1A (1) and 2A (2)

Ba(OH)2, Sr(OH)2, and Ca(OH)2

• dissociate completely in water

KOH(s) K+(aq) + OH−(aq)

• are found in household products

used to remove grease and

unclog drains

Weak Bases

Weak bases are weak electrolytes

• that are poor acceptors of H+ ions.

• produce very few ions in solution.

• include ammonia.

NH3(g) + H2O(l) NH4+(aq) + OH−(aq)

Ammonia Ammonium hydroxide

Strong and Weak Bases

Strong Bases

Lithium hydroxide LiOH

Sodium hydroxide NaOH

Potassium hydroxide KOH

Rubidium hydroxide RbOH

Cesium hydroxide CsOH

Calcium hydroxide Ca(OH) 2*

Strontium hydroxide Sr(OH) 2*

Barium hydroxide Ba(OH)2*

*Low solubility, but they

Strong Bases

Drain cleaner, oven cleaner, NaOH

Direction of Reaction

Strong acids have weak conjugate bases that do not readily

accept H+

• As the strength of the acid decreases, the strength of its

conjugate base increases

In any acid–base reaction, there are two acids and two bases

• However, one acid is stronger than the other acid, and one base is stronger than the other base

• By comparing their relative strengths, we can determine the direction of the reaction

Direction of Reaction: H2SO4

Sulfuric acid, H2SO4, is a strong acid that readily gives up H+

to water

H2SO4(aq) + H2O(l) H3O+(aq) + HSO4−(aq)

Stronger Stronger Weaker Weaker

acid base acid base

• The hydronium ion H3O+ produced is a weaker acid than

H2SO4

• The conjugate base HSO4− is a weaker base than water

Direction of Reaction: CO3 2−

The carbonate ion from carbonic acid, H2CO3, reacts with water.

• Water donates one H + to carbonate, CO32− to form HCO3−

and OH −

• From Table 11.3, we see that HCO3− is a stronger acid than H2O.

• We also see that OH − is a stronger base than CO32−

To reach equilibrium, the strong acid and strong base react in the

direction of the weaker acid and weaker base.

CO32− (aq) + H2O(l) OH −(aq) + HCO3−(aq)

Weaker Weaker Stronger Stronger

acid base base acid

Identify each of the following as a strong or weak acid

or base:

D H2SO4 strong acid

Using Table 11.3, identify the stronger acid in each pair

A HNO2 or H2S HNO 2 is the stronger acid.

B HCO3− or HBr HBr is the stronger acid.

C H3PO4 or H3O+ H 3 O + is the stronger acid.

11.4 Dissociation Constants for

Acids and Bases

HCHO 2(aq) + H2O(l) H 3 O +(aq) + CHO2 −(aq)

Learning Goal Write the expression for the dissociation

constant of a weak acid or weak base

Dissociation of a Weak Acid

Because the dissociation of strong acids in water is

essentially complete, the reaction is not considered to be an

equilibrium process

• Weak acids partially dissociate in water as the ion

products reach equilibrium with the undissociated weak

acid molecules

• Formic acid is a weak acid that dissociates in water to

form hydronium ion, H3O+, and formate ion, CHO2−

HCHO2 (aq) + H2O(l) H3O+(aq) + CHO2−(aq)

Writing Dissociation Constants

As with other dissociation expressions,

• the molar concentration of the products is divided by the

molar concentration of the reactants

• water is a pure liquid with a constant concentration and

is omitted

• the expression is called acid dissociation constant, Ka

HCHO2 (aq)  +  H2O(l)   H3O+(aq)  +  CHO2−(aq) 

Acid Dissociation Constant, Ka

When the value of the Ka

• is small, the equilibrium lies to the left, favoring the

reactants

• is large, the equilibrium lies to the right, favoring the

products

Weak acids have small Ka values, while strong acids have

very large Ka values

Base Dissociation Constant, Kb

When the value of the Kb,

• is small, the equilibrium lies to the left, favoring the reactants

• is large, the equilibrium lies to the right, favoring the

products

The stronger the base, the larger the Kb value

CH3—NH2(aq) + H2O(l) CH3—NH3+(aq) + OH−(aq)

The concentration of water is omitted from the base

dissociation constant expression

Weak Acids and Bases: Ka and Kb Values

Strength versus Equilibrium Position

Table 11.5 summarizes the characteristics of acids and

bases in terms of strength and equilibrium position

Study Check

Write the acid dissociation constant expression for nitrous acid, HNO2

HNO2 (aq) + H2O(l) H3O+(aq) + NO2−(aq)

The acid dissociation constant is the molar concentration of the products divided by the molar concentration of the reactants

11.5 Dissociation of Water

The equilibrium reached between the conjugate acid–base

pairs of water produces both H3O+ and OH−

H2O(l) + H2O(l) H3O+(aq) + OH−(aq)

Learning Goal Use the water dissociation constant to

calculate the [H3O+] and [OH−] in an aqueous solution

-Dissociation Constant of Water, Kw

Water is amphoteric—it can act as an acid or a base

In water,

• H + is transferred from one H2O molecule to another.

• one water molecule acts as an acid, while another acts as

a base.

• equilibrium is reached between the conjugate acid–base pairs.

Writing the Dissociation Constant, Kw

In the equation for the dissociation of water, there is both a

forward and a reverse reaction

H2O(l) + H2O(l) H3O+(aq) + OH−(aq)

• In pure water, the concentrations of H3O+ and OH− at 25 °C

Dissociation Constant, Kw

The ion product constant for water, Kw, is defined as

• the product of the concentrations of H3O+ and OH−

• equal to 1.0  10−14 at 25 °C (the concentration units are

omitted)

When

• [H3O+] and [OH−] are equal, the solution is neutral

• [H3O+] is greater than the [OH−], the solution is acidic

• [OH−] is greater than the [H3O+], the solution is basic

Using Kw to Calculate [H3O+] and [OH−]

• If we know the [H3O+] of a solution, we can use the

Kw to calculate the [OH−]

• If we know the [OH−] of a solution, we can use the

Kw to calculate the [H3O+]

Pure Water Is Neutral

[H3O+] = 1.0  107 M

[OH−] = 1.0  107 M

[H3O+] = [OH−]

Pure water is neutral

In pure water, the ionization of water molecules produces

small but equal quantities of H3O+ and OH− ions

Basic Solutions

Adding a base to pure water

• increases the [OH−]

• causes the [OH−] to exceed

Comparison of [H3O+] and [OH−]

Neutral, Basic, and Acidic Solutions

Guide to Calculating [H3O+] and [O–] in

Aqueous Solutions

Calculating [H3O+]

What is the [H3O+] of a solution if [OH−] is 5.0 × 10−8 M?

STEP 1 State the given and needed quantities.

STEP 2 Write the Kw for water and solve for the

unknown [H 3 O + ].

ANALYZE      Given         Need        Know THE       [OH − ] = 5.0 × 10 −8  M    [H3O +]      Kw = [H3O + ][OH − ]  PROBLEM       = 1.0 × 10 −14

ANALYZE      Given         Need        Know THE       [OH − ] = 5.0 × 10 −8  M    [H3O +]      Kw = [H3O + ][OH − ]  PROBLEM       = 1.0 × 10 −14

Calculating [H3O+]

What is the [H3O+] of a solution if [OH−] is 5.0 × 10−8 M?

STEP 3 Substitute in the known [H 3 O + ] or [OH − ] and

calculate.

Because the [H3O+] of 2.0 × 10–7 M is larger than the

[OH−] of 5.0 × 10–8 M, the solution is acidic