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Acids and Bases Topics to be covered: Acids and Bases Topics to be covered:

Acids and Bases Topics to be covered: - PowerPoint Presentation

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Acids and Bases Topics to be covered: - PPT Presentation

Definitions of acids and bases Bronsteds conjugate acidbase pairs concept Determination of H 3 O OH and pH for strong acids and strong bases Determination of H 3 O OH ID: 684297

base acid weak acids acid base acids weak solution bases strong conjugate ionization strength salt basic water salts reaction

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Slide1

Acids and Bases

Topics to be covered:

Definitions of acids and bases;

Bronsted’s conjugate acid-base pairs concept;

Determination of [H

3

O

+

], [OH

-

], and pH for strong acids and strong bases;

Determination of [H

3

O

+

], [OH

-

], pH,

K

a

, or

K

b

, and percent ionization for weak acids and weak bases;

Predicting acid/base properties of salts (ionic compounds) and oxides of metals and nonmetals;

The effect of molecular structures and bond energy on the strength of acids and bases.Slide2

Acids and Bases

Arrhenius definition:

Acid

: substance that increases [H

3

O

+

] in solution;

Base

: substance that increases [OH

-

] in solution;

Br

nsted-Lowry definition:

Acid

: reactant that donates proton in a reaction;

Base

: reactant that accepts proton in a reaction;

Lewis definition: -

in the formation of covalent bonds,

Acid

: one that accept electron-pairs

Base

: one that donate electron-pairsSlide3

Strong and Weak Acids

Strong acids ionize completely in aqueous solution

:

HCl

(

aq

)

+ H

2

O

H

3

O

+

(

aq

)

+ Cl

-

(

aq

)

;

H

2

SO

4

(

aq

)

+ H

2

O

H

3

O

+

(

aq

)

+ HSO

4

-

(

aq

)

;

Weak acids ionize only partially in aqueous solution:

HF

(

aq

)

+ H

2

O

H

3

O

+

(

aq

)

+ F

-

(

aq

)

;

HOCl

(

aq

)

+ H

2

O

H

3

O

+

(

aq

)

+ ClO

-

(

aq

)

.Slide4

Complete Ionization of Hydrochloric AcidSlide5

Dissociations of Strong and Weak Acids

Slide6

Strong and Weak Bases

Strong bases ionize completely in aqueous solution:

NaOH

(

aq

)

Na

+

(

aq

)

+ OH

-

(

aq

)

;

Ba(OH)

2

(

aq

)

Ba

2+

(

aq

)

+ 2 OH

-

(

aq

);

Weak bases ionize only partially in aqueous solution:

NH

3

(

aq

)

+ H

2

O

NH

4

+

(

aq

)

+ OH

-

(

aq

);

PO

4

3-

(

aq

) + H

2

O

HPO

4

2-

(

aq

)

+ OH

-

(

aq

)Slide7

Strong Acids and Bases

Some of the common strong acids and

bases.Slide8

Brønsted-Lowry: Conjugate Acids & Bases

Consider the following equilibrium:

HA + B

BH

+

+ A

-

;

Acid

1

Base

2

Conjugate

Conjugate

acid

2

base

1

A

-

is the conjugate base to acid HA;

HA & A

-

are conjugate acid-base pair;

BH

+

is the conjugate acid to base B;

BH

+

& B are also conjugate acid-base pair.Slide9

Brønsted

-Lowry Acid

An acid

HX

donates

proton to

water producing hydronium ion, H

3

O

+

, and the conjugate base X

-

.Slide10

Brønsted

-Lowry Base

A base

NH

3

takes a proton from water to produce hydroxide ion, OH

-

, and the conjugate acid

NH

3

+

.

Slide11

Exercise #1: Conjugate Acids & Bases

Write the formulas of the conjugate bases for the following acids:

(a) H

2

CO

3

(

b) HSO

4

-

(

c) Al(H

2

O)

6

3+

(

d) Cr(OH)

3

(H

2

O)

3

(

Answer: (a) HCO

3

-; (b) SO

4

2-

; (c) Al(H

2

O)

5

(OH)

2+

;

(

d) Cr(OH)

4

(H

2

O)

2

-

)Slide12

Exercise #2: Conjugate Acids and Bases

Write the formulas of the conjugate acids for the following bases:

(a)

CH

3

NH

2

(

b) CO

3

2-

(

c) C

5

H

5

N

(

d)

Al(OH)

3

(H

2

O)

3

(

Answer: (a) NH

4

+

; (b) HCO

3

-

; (c) C

5

H

5

NH

+

;

(

d) Al(OH)

2

(H

2

O)

4

+

)Slide13

Selected Conjugate

Acid-Base Pairs

Acids: strongest on the top, weakest at the bottom;

Conjugate bases; weakest (top), strongest (bottom)

HNO

3

– NO

3

-

H

2

SO

4

– HSO

4

-

H

3

O

+

– H

2

O

HF – F

-

H

3

PO

4

– H

2

PO

4

-

CH

3

COOH – CH

3

COO

-

H

2

PO

4

-

– HPO

4

2-

NH

4

+

– NH

3

H

2

O – OH

-

Slide14

Relative Strength of Acids

and

Their Conjugate Bases

Acids

Conjugate Bases

Very Strong

Very Weak

Strong

Weak

Weak

Strong

Very

Weak

Very

Strong

__________________________________________

Strong

acids lose protons very readily

 weak conjugate bases;

Weak acids do not lose protons very readily

 strong conjugate bases.Slide15

Relative Strength of Acids and Their Conjugate BasesSlide16

Relative Strength of Acids and their Conjugate Bases

This diagram shows the relative strengths of conjugate acid-base pairs, as indicated by their

ionization constants

in aqueous solution.Slide17

Acid Strength and Ionization Constants

For the ionization or dissociation equilibrium of an acid in aqueous solution, such as:

HA

(

aq

)

+ H

2

O

H

3

O

+

(

aq

)

+

A

-

(

aq

)

;

The

ionization or dissociation constant is expressed as follows:

K

a

=

The

values of

K

a

indicate the relative strength of the acids. Strong acids have very large

K

a

, while weak acids have small Ka’s (Ka << 1)

 Slide18
Slide19

Acid-Base Properties of Water

Auto-ionization of water:

2H

2

O

H

3

O

+

(

aq

)

+ OH

-

(

aq

)

K

w

= [

H

3

O

+

][OH

-

] = 1.0 x 10

-14

at 25

o

C

Water ionizes to produce both

H

3

O

+

and OH

-

, thus it has both acid and base properties. Kw is called water ionization constant. Pure water at 25oC: [H3O+] = [OH-] = 1.0 x 10-7 M Slide20

Expressing Acidity in pH Scale

pH = -log[H

+

] (

note

: [H

+

] = [H

3

O

+

])

pOH = -log[OH

-

]

p

K

w

= -log(

K

w

); p

K

a

= -log(

K

a

); p

K

b

= -log(

K

b

)

For water,

K

w

= [H

3

O+][OH-] = 1.0 x 10-14 -log(Kw) = -log [H3O+] + (-log[OH-]) pKw = pH + pOH = 14.00At 25oC, pOH = 14 – pHSlide21

Acidity and pH Range

Acidic solutions:

[H

3

O

+

] > 1.0 x 10

-7

M

;

pH < 7;

Basic solutions:

[OH

-

] > 1.0 x 10

-7

M

or [H

3

O

+

] < 1.0 x 10

-7

M

pH > 7;

Neutral solutions:

[H

3

O

+

] = [OH

-

] = 1.0 x 10

-7

M

; pH = 7.00Slide22

Relationship

Between [H

3

O

+

]

and pHSlide23

pH of Common Household ItemsSlide24

Household SubstancesSlide25

[H

3

O

+

] and pH of Strong Acids

Strong acids like

HCl

and

HNO

3

ionize completely in aqueous solution:

HCl

(

aq

)

+ H

2

O

H

3

O

+

(

aq

)

+ Cl

-

(

aq

)

;

HNO

3

(

aq) + H2O  H3O+(aq) + ClO4-(aq);In solutions of strong monoprotic acids HA, such as HCl and HNO3

, H3O

+

concentration = initial acid concentration,

[H

3

O

+

] = [HA]

0

For example, in 0.10 M

HCl

, [H

3

O

+

] = 0.10 M, and pH = -log(0.10) = 1.00Slide26

[OH

-

] and pH of Strong Bases

S

trong

bases also ionize

completely:

Examples:

NaOH

(

aq

)

Na

+

(

aq

)

+ OH

-

(

aq

)

;

Ba(OH)

2

(

aq

)

Ba

2+

(

aq

)

+ 2 OH

-

(aq);In base solution such as NaOH, [OH-] = [NaOH]0 For example, in 0.10 M NaOH solution, [OH

-] = 0.10 M

;

In

base

solution such as

Ba(OH)

2

,

[OH

-

] = 2 x [Ba(OH)

2

]

0

For example, in 0.10

M

Ba(OH)

2

solution, [OH

-

] = 0.20

M

;Slide27

[H

3

O

+

] and pH of Weak Acids

In weak acid solutions, [H

3

O

+

] < [HA

]

0

;

[H

3

O

+

] and pH can be calculated from the initial concentration of the acid and its

K

a

value.

For example, in

0.10

M

acetic acid,

CH

3

CO

2

H (

K

a

= 1.8 x

10

-5

)

[H

3

O+] and pH can be calculated using the following “ICE” table. (next slide)Slide28

ICE Table for Acetic Acid

Ionization: CH

3

CO

2

H + H

2

O

H

3

O

+

+ CH

3

CO

2

-



[

I

nitial] 0.10

M

0.00 0.00

[

C

hange] -

x

+

x

+

x

[

E

quil’m

]

(0.10 – x) x x Ka =

=

= 1.8 x 10

-5

By approximation, x

=

= 0.0013

M

;

x

= [H

3

O

+

] = 0.0013

M

; pH = -log(0.0013) =

2.89

 Slide29

Calculating [H

3

O

+

] from initial concentration and

K

a

using

approximation method

.

If

K

a

<< [HX]

0

, [H

3

O

+

] can be calculated using approximation method, by assuming that, since

K

a

<<

[HX]

0

, then

x

<< [HX]

0

and ([HX]

0

x

)

[HX]

0

.

For example,

=

= 1.8 x 10-5

Since

K

a

(1.8 x 10

-5

)

<< 0.10,

we approximate that

(0.10 –

x

)

0.10;

Then,

= 1.8 x 10

-5

Which leads to:

x

= 1.8 x 10-5)} = 0.0013 M = [H3O+] Slide30

Calculation Percent Ionization

We calculated that, in 0.10

M

CH

3

COOH, the concentration of acid that ionizes is 1.34 x 10

-3

M

.

The percent ionization of 0.10

M

acetic acid:Slide31

Increase in Percent Ionization with Dilution

Percent ionization of weak acid increases as the dilution increases.

For example, if [

CH

3

COOH]

0

= 0.0100

M

[CH

3

COOH]

ionized

4.24 x 10

-4

M

(

calculated using approximation method);

 Slide32

Percent Ionization Increases with Dilution

At [CH

3

COOH]

initial

=

0.0010

M

,

[CH

3

COOH]

ionized

=

1.3

x 10

-4

M

(by approximation)

Percent ionization =

x 100 = 13%

The more we dilute the acid solution, the higher the fraction of the acid that will ionize, which increases the degree of ionization.

Is this related to the Le

Chatelier’s

principle? Explain.

 Slide33

Exercise #3:

pH and Percent Ionization

Nitrous acid, HNO

2

, has

K

a

= 4.0 x 10

-4

at 25

o

C.

Calculate the pH and percent ionization of HNO

2

in 0.10

M

solution of the acid

.

Chlorous

acid,

HOCl

, has

K

a

= 3.5 x 10

-8

at 25

o

C.

Calculate the pH and percent ionization of

HOCl

in 0.10

M

solution of the acid.

(

Answer:

(1)

pH = 2.20; % ionization = 6.3

%; (2) pH = 4.23; % ionization = 0.059%)Slide34

[OH

-

] and pH of a Weak Base

In a weak base, [OH

-

] < [Base]

0

;

[OH

-

] and pH can be calculated from the initial concentration of the base and its

K

b

value.

For example, in 0.100

M

ammonia, NH

3

, with

K

b

= 1.8 x 10

-5

, [OH

-

] and pH can be calculated using the following “ICE” table.Slide35

ICE Table for the Ionization of Ammonia

Ionization: NH

3

+ H

2

O

 NH

4

+

+ OH

-



I

nitial [M]

0.10

0.00 0.00

C

hange [M] -

x

+

x

+

x

E

quilm

. [M] (

0.10

x

)

x x



K

b

=

=

= 1.8 x 10

-5

By approximation,

x

=

= 0.0013

M

;

x

= [OH

-

] = 0.0013

M

; pOH = 2.89, and

pH = 11.11

 Slide36

Exercise #4: pH of Strong &

Weak Bases

Calculate [OH

-

] and [H

3

O

+

]

in 0.10

M

solution of:

Ethylamine

,

C

2

H

5

NH

2

(

K

b

= 5.6 x

10

-4

);

Pyridine,

C

5

H

5

N (

K

b

= 1.7 x 10-9)What is the pH of each solution?(Answer: [OH-] = 7.5 x 10-3 M; [H3O+] = 1.3 x 10-12 M; pH = 11.89)(Answer: [OH-] = 1.3 x 10-5 M; [H3O+] = 7.7 x 10-10 M; pH = 9.11)Slide37

Polyprotic Acids

Acids with more than one

ionizable

H

+

,

such as

H

2

SO

4

, H

3

PO

4

, H

2

C

2

O

4

,

H

3

C

6

H

5

O

7

, etc.

They ionize

in stages, for

example:

1) H

3

PO

4

(aq) + H2O ⇄ H3O+(aq) + H2PO4-(aq); Ka1 = 7.5 x 10-32) H2PO4-

(aq)

+

H

2

O

H

3

O

+

(

aq

)

+

HPO

4

2-

(

aq

)

;

Ka2 = 6.2 x 10-83) HPO42-(aq) + H2O ⇄ H3O+(aq) + PO43-(aq); Ka3 = 4.8 x 10

-13

Acid strength decreases in the order:

H

3

PO

4 >> H2PO4- >> HPO42-; pH of solution is determined mainly by ionization of H

3

PO

4

Slide38

Exercise #5:

Concentrations of Species

and pH

of

H

2

SO

4

solution

Calculate the concentrations of H

2

SO

4

, H

3

O

+

, HSO

4

-

, and SO

4

2-

, in 0.10

M

H

2

SO

4

solution. What is the pH of the solution? (H

2

SO

4

is a strong acid and HSO

4

-

has

K

a

= 1.2 x 10-2)(Answer: [H2SO4] = 0.0 M; [H3O+] = 0.11 M; [HSO4-] = 0.090 M; [SO42-] = 0.0098 M; pH = 0.96]Slide39

Salts and Their Corresponding Acids & Bases

What acid

and

base will form

each of the following salts

(ionic compounds)?

Acids

Bases

NaCl

HCl

NaOH

KNO

3

(NH

4

)

2

SO

4

NaC

2

H

3

O

2

BaCl

2

Na

3

PO

4Slide40

Acid-Base Properties of Salt Solutions

Soluble salts dissociate completely when dissolved in

water, producing cations and anions;

Ions produced

by salts may

react with water

to produce

H

3

O

+

, which would make

the solution

acidic

, or

produce

OH

-

, which would make

the solution

basic

.

pH of solution depends on which ion dominates in solution, and on relative values of

K

a

and

K

b

of ions.Slide41

Acid-Base Properties of Salt Solutions

The acid-base property of a salt solution depends on whether the compound is a product of:

Strong acid-strong base reaction

: produces

neutral

salt

Strong acid-weak base reaction

: produces

acidic

salt

Strong base-weak acid reaction

: produces

basic

salt

Weak acid-weak base reaction

: produces salt that is

either

acidic

or

basic

, depending on the relative strength of the acid and the base.Slide42

Types of Salts and Their Solutions

Salts of Strong

Acids-Strong Bases

:

NaCl

, NaNO

3

,

KBr

, etc.; solutions are neutral

Salts of Weak

Acids-Strong Bases

:

NaF

, NaNO

2

, NaC

2

H

3

O

2

, etc.; solutions are basic

Salts of Strong

Acids-Weak Bases

:

NH

4

Cl, NH

4

NO

3

, (CH

3

)

2

NH

2Cl, C5H5NHCl; Solutions of these salts are acidicSalts of Weak Acids-Weak Bases: NH4C2H3O2, NH4CN, NH4NO2, etc.. These compounds can be acidic, basic, or neutral, which depends on the relative strength of the acid and the base.Slide43

Salt of

Strong Acid-Strong Base

Reaction

Dissociation and reaction of a neutral salt:

NaCl

(

aq

)

Na

+

(

aq

)

+ Cl

-

(

aq

)

;

Na

+

(

aq

)

+ H

2

O

“NR”;

(no reaction)

Cl

-

(

aq) + H2O  “NR”(Neither Na+ nor Cl- will react with water to produce H+ or OH-, respectively. Concentrations of H

3O

+

and OH

-

in

solution

are the same as in pure

water; therefore,

solution is neutral.)Slide44

Salt

of Weak Acid-Strong Base

Reaction

:

Dissociation and reaction of a basic salt:

NaNO

2

(

aq

)

Na

+

(

aq

)

+ NO

2

-

(

aq

)

;

Na

+

(

aq

)

+ H

2

O

“NR”

NO

2

-

(

aq) + H2O  HNO2(aq) + OH-(aq) (NO2- is a stronger conjugate base than H2O; it takes proton from water to produce HNO2 and OH-

, which causes solution to have [OH-

] > [H

3

O

+

]; solution

basic

)Slide45

Salt

of Strong Acid-Weak Base

Reaction

Dissociation and reaction of an acidic salt:

NH

4

NO

3

(

aq

)

NH

4

+

(

aq

)

+ NO

3

-

(

aq

)

;

NH

4

+

(

aq

)

+

H

2

O

NH

3(aq) + H3O+(aq); NO3-(aq) + H2O  “NR”;(NH4+

is a stronger conjugate acid than water; it donates proton to H2

O producing H

3

O

+

and NH

3

, which causes

[H

3

O

+

] > [OH

-

], and

solution

i

s

acidic

)Slide46

Salts of Weak Acid-Weak Base Reactions

A salt

produced by

reaction

of

a weak acid

and

a weak base

can be

neutral, acidic, or

basic

; it depends

on the relative

magnitude

of

the

K

a

of

cation

and the K

b

of

anion from the salt.

If

K

a

~

K

b

,

salt is

neutral

;

example

: NH

4

C

2

H3O2If Ka > Kb, salt is acidic; example: NH4NO2If Ka < Kb, salt is basic; example

: (NH4

)

2

CO

3

Slide47

Predicting Acid-Base Property of Salts

Consider a solution containing

NH

4

CH

3

CO

2

NH

4

CH

3

CO

2

(

aq

)

NH

4

+

(

aq

)

+

CH

3

C

O

2

-

(

aq

)

;

NH

4

+(aq) + H2O  H3O+(aq) + NH3(aq); Ka = 5.6 x 10-10

CH3

C

O

2

-

(

aq

)

+ H

2

O

CH

3

CO

2

H

(

aq

)

+ OH

-(aq); Kb = 5.6 x 10-10Ka (NH4+) = Kb (CH3CO2-) = 5.6 x

10-10

NH

4

C2H3O2 is neutralSlide48

Predicting Acid-Base Property of Salts

Consider a solution containing

NH

4

NO

2

NH

4

NO

2

(

aq

)

NH

4

+

(

aq

)

+

NO

2

-

(

aq

)

;

NH

4

+

(

aq

)

+ H

2

O

 H3O+(aq) + NH3(aq); Ka = 5.6 x 10-10NO2-(

aq)

+ H

2

O

HNO

2

(

aq

)

+ OH

-

(

aq

)

;

K

b

=

2.5

x

10-11Ka (NH4+) > Kb (NO2-)  NH4NO2 is acidic;Slide49

Predicting Acid-Base Characteristics of Salts

Now consider a solution containing NH

4

CN.

NH

4

CN

(

aq

)

NH

4

+

(

aq

)

+ CN

-

(

aq

)

;

NH

4

+

(

aq

)

+

H

2

O

H

3

O

+(aq) + NH3(aq); Ka = 5.6 x 10-10CN-(aq) + H

2O 

HCN

(

aq

)

+ OH-

(

aq

)

;

K

b

= 1.6 x 10

-5

K

b

(CN

-

)

>

K

a

(NH4+)  NH4CN is basicSlide50

Effect of Structure on Acid-Base Properties

Relative bond strength:

H─F >

H─Cl

>

H─Br

> H─I

Relative acid strength:

HI >

HBr

>

HCl

> HF

(only HF

is

classified as a

weak acid in this group)

Relative acid strength:

H

2

Te > H

2

Se > H

2

S > H

2

O (all very weak acids)Slide51

Trends of

Acid Strength in the Periodic Table

As you move from left to right and down the periodic table, the acid strength increases. As you

move from

right to left and up, the base strength increases.Slide52

Effect of Structure on Acid-Base Properties

Electronegativity Effect on Relative Strength of Oxo-Acids:

Same central atom, but different numbers of oxygen

atoms (more O-atoms, stronger acidic):

HClO

4

>

HClO

3

>

HClO

2

>

HClO

(or

HOCl

)

Same number of

oxygen atoms,

but different central

atoms (more electronegative, stronger acid):

HClO

2

>

HBrO

2

>

HIO

2

; (

En

: Cl > Br > I)

CF

3CO2H > CCl3CO2H > CH3CO2H; (En: F > Cl > H)HClO4 > H2SO4 >

H3PO

4

; (

En

: Cl > S > P)Slide53

O

H Bond and Strength of Oxo-Acids

 

The relative strength of

oxo

-acids, such as HNO

2

and HClO

2

, depends on the strength of O

H bond (bond

b

), which is

influenced by the electronegativity of atom ”E” and other O-atoms attached to it.

M

ore electronegative atom ”E” (and more O-atoms attached to it) leads to weaker

O

H bond and a stronger acid. Stronger

O

H bond produces weaker

acid.

 Slide54

Acid-Base Properties of Oxides

Metal oxides are basic or amphoteric

Basic oxides:

Na

2

O

(s)

+ H

2

O

2NaOH

(

aq

)

2Na

+

(

aq

)

+ 2OH

-

(

aq

)

;

MgO

(

aq

)

+

HCl

(

aq

)

 MgCl2(aq) + H2OAmphoteric oxides: Al2O3(s) + HCl(aq)  2AlCl3(aq) + H2O Al

2O3

(s)

+ 2NaOH

(

aq

)

+ 3H

2

O

2NaAl(OH)

4

(

aq

)

Slide55

Acid-Base Properties of Oxides

Nonmetal oxides are

acidic:

N

2

O

5

+ H

2

O

2HNO

3

(

aq

)

;

SO

3

+ H

2

O

H

2

SO

4

(

aq

)

;

Cl

2

O

7

+ H2O  2HClO4(aq)Trends: Acidity increases left to right across a period SiO2 < P4O10 < SO3 < Cl2O7 Acidity decreases top-to-bottom down the group

N2O

5

> P

4

O

10

> As

2

O

5

> Sb

2

O

5Slide56

Acid-Base Properties of Oxides

Trend of acid-base properties of oxides in periodic table: from the most basic (on the left) to the most acidic (on the right):

Na

2

O, MgO, Al

2

O

3

, SiO

2

, P

4

O

10

, SO

3

, Cl

2

O

7

Trend of acid-base properties going down a group: most acidic on the left to the most basic on the right:

N

2

O

5

, P

4

O

10

, As

2

O

5

, Sb

2

O

5Slide57

Lewis Acids and Bases

Identify the Lewis acids and Lewis bases in the following reactions:

(a) H

2

O +

CO

2

(

g

)

H

2

CO

3

(

aq

)

;

(b) Cu

2

+

(

aq

)

+

4NH

3

(

aq

)

Cu(NH

3)42+(aq); (c) AlCl3(s) + Cl-(aq)  AlCl4-(aq)

(d) BF

3

+ NH

3

F

3

B─NH

3Slide58