copyright mcgraw-hill 20091 chapter 16 acids and bases insert picture from first page of chapter

Copyright McGraw-Hill 2009 1

Chapter 16

Acids and BasesInsert picture fromFirst page of chapter


16.1 Brønsted Acids and Bases

• Acid – proton donor

• Base – proton acceptor

• Conjugate base – what remains of the acid after the donation of a proton

• Conjugate acid – newly formed protonated species (base)


Conjugate Acid-Base Pairs


16.2 The Acid-Base Properites of Water

• Autoionization of water – water is amphoteric, can act as an acid or base

or


For pure water at 25 oC

Relative amounts determine if the solution isacid, base, or neutral.

• When [H3O+] = [OH], the solution is neutral.• When [H3O+] > [OH], the solution is acidic.• When [H3O+] < [OH], the solution is basic.

]][OHO[H3wK

1477w 10 x 110 x (1 x)10 x 1 )(K


16.3 The pH Scale

]log[HpH

[ ] pH3H O 10

]Olog[HpH 3

or

or

pH Meter: Laboratory Measurement of pH


Relative amounts determine if the solution isacid, base, or neutral.

• When pH = 7, the solution is neutral.• When pH < 7.00, the solution is acidic.• When pH > 7.00, the solution is basic.


1.What is the pH of a solution that has a hydronium ion concentration of 6.5 x 105M?

2. What is the hydronium ion concentration of a solution with pH 3.65?


4.19pH

]Olog[HpH 3

]10 x log[6.54pH 5

43 2x102]O[H .

1.

2. [ ] pH3H O 10

.[ ] 3 653H O 10


Relations among pH, [OH], pOH, and Kw

]log[OHpOH

[ ] pOHOH 10

14.00pOHpHp w K

]][OHO[H3wK


1. What is the pOH of a solution that has a hydroxide ion concentration of 4.3 x 102M?

2. What is the hydroxide ion concentration of a solution with pOH 8.35?


371pOH .

]log[OHpOH

]10 x log[4.3pOH 2

[ ] pOHOH 10

.[ ] 8 35OH 10

95x104][OH .

1.

2.


16.4 Strong Acids and Bases

• Ionization or dissociation goes to completion – not considered an equilibrium process

• Concentration of ions in solution is determined by the stoichiometry of the reaction only

– [H3O+] is equal to starting concentration of the acid

– [OH] is equal to starting concentration of the base (stoichiometry must be considered)

• Only a few acids and bases are strong


What is the pH of a 0.057 M solution of HBr?


]Olog[HpH 3

HBr completely ionizes, and the [H3O+] is equal to the initial concentration of HBr.

log[0.057]pH

1.24pH

HBr + H2O H3O Br +(l) (aq) (aq)(aq)


What is the pOH of a solution of 0.034 M solution of Ca(OH)2?


Ca(OH)2 completely ionizes, and the [OH] is equal to twice the initial concentration of Ca(OH)2.

(aq) (aq)(aq) + OHCa2+Ca(OH)2 2

[ ] . ( )( )

2

2

2 mol OHOH 0 034 M Ca OH x

1 mol Ca OH

]log[OHpOH

log[0.068]pOH

1.17pOH

M0.068][OH


16.5 Weak Acids and Acid Ionization Constants

• Weak acids ionize only to a limited extent in water.

• The degree to which a weak acid ionizes depends on

– the concentration of the acid

– the equilibrium constant for the ionization called the acid ionization constant, Ka.


or

[HA]

]][AO[H3a

K

[HA]

]][A[Ha

K

For a generic weak monoprotic acid


Format for solving problems of weak acids using an equilibrium table

• Fill in initial concentrations• Determine concentration changes in terms of x• Determine equilibrium concentrations in terms of initial concentrations (Ci) and x• Substitute into the Ka expression and solve for x


which produces a quadratic expression of the form:

If the initial concentration and Ka allow, the expression can be simplified by assuming that x is insignificant when compared to Ci.

ii C

x

C

xxK

2

))((

a

ii CxC

xC

xxK

i

))((a

0iaa CKxKx 2


Testing the approximation – it is acceptable to use this shortcut if the calculated value of x is less than 5 percent of the initial acid concentration.

x1005%iC

x


Calculate the pH at 25°C of a 0.18 M solution of a weak acid that has Ka = 9.2 x 106.


0 00.18 M

+x+x−x

0.18 M − x x x

xM

xx

0.1810x9.2 6 ))((

Use the approximation since Ka is small compared to Ci.

M

x

0.1810x9.2 6

2


M

x

0.1810x9.2 6

2

xM 310x1.3

Check the approximation:

0.72%1000.18

10x1.3 3

xM

M

5%0.72%

Approximation is valid.

( . x ) . 3pH log 1 3 10 2 89M


16.6 Weak Bases and Base Ionization Constants

• Weak bases ionize only to a limited extent in water.

• The degree to which a weak base ionizes depends on

– the concentration of the base

– the equilibrium constant for the ionization called the base ionization constant, Kb.


[B]

]][OH[HBb

K

For a generic weak base

Solving problems involving weak bases requires the same approach as for weak acids. Solving for x in a typical weak base problem gives the hydroxide ion concentration rather than the hydronium ion concentration.


Determine the Kb of a weak base if a 0.50 M solution of the base has a pH of 9.59 at 25°C.


14.00pOHpH

14.00pOH599 .

.pOH 4 41

[ ] pOHOH 10

.[ ] . x 4 41 5OH 10 3 89 10 M


0 M 0 M0.050 M

+3.89x10-5 M +3.89x10-5 M

3.89x10-5 M 3.89x10-5 M

− 3.89x10-5 M

0.050 M*

[B]

]][OH[HBb

K

( . ).

.

59

b3 89x10 2

3 0 x 100 50

MK

M

MMM 0.05010 x (3.89 - 0.050* -5 )


16.7 Conjugate Acid-Base Pairs

• The conjugate base of a strong acid, is an example of a weak conjugate base.

• The conjugate base of a weak acid, is an example of a strong conjugate base.

• Conversely, a strong base has a weak conjugate acid and a weak base has a strong conjugate acid.


Reciprocal Relationship Between the Strength of an Acid or Base and the Strength of its Conjugate

wab KKK

Quantitative Relationship Between Ka and Kb


Determine Kb of the benzoate ion (C6H5COO).


a

wb K

KK

Benzoate ion is the conjugate base of benzoic acid.

C6H5COOH (aq) (aq)(aq)(l) +H3OH2O + C6H5COO

105

14

b 10x1.510x6.5

10x1.0

K

5a 10 x 6.5 K

+ H2O OH +(l) (aq)C6H5COO (aq) (aq)C6H5COOH


16.8 Diprotic and Polyprotic Acids

• Diprotic and polyprotic acids undergo successive ionizations, losing one proton at a time

• The conjugate base in the first ionization is the acid in the second ionization.

• Each ionization has a Ka associated with it.• Ionization constants are designated by the

step in the ionization involved.

.,, etcKK21 aa


Calculate the concentrations of H2C2O4, HC2O4

, C2O42, and H+ ions in a 0.20 M oxalic

acid solution at 25°C.


-2a 10x6.51

KHC2O4H +(aq) (aq)(aq)H2C2O4

HC2O4H +(aq) (aq)(aq)H2C2O4

− x +x +x

0.20 M

0.20 M − x x x

22

422

42a 6.5x10

x 0.20

x

]OC[H

]][HO[HC1

M

K

x 0.086 and 0.151 [H+]=0.086 M

[HC2O4+]=0.086 M

[H2C2O4]=0.20 M − 0.086 M = 0.11 M

010 x 1.3x10 x 6.5x 222


[ ][ ] ( . )( ) ( . )( ).

. . [ ]

2

2-52 4

a2 4

C O H 0 086 y y 0 086 y61x10

0 086 y 0 086HC O

MK

M M

[H+] = 0.086 M

[HC2O4] = 0.086 M

[C2O42] = 6.1 x 105 M

5a 10x6.1 2

KHC2O4 (aq) (aq)(aq) +H C2O4

− y +y +y

0.086 M

0.086 M − y 0.086 M + y y

HC2O4 (aq) (aq)(aq) +H C2O4

0.086 M

My 56.1x10


16.9 Molecular Structure and Acid Strength

• The strength of an acid is measured by its tendency to ionize.

• Two factors influence the extent to which the acid undergoes ionization.

– the strength of the bond

– polarity of the bond

HX H X +

H X

H X


Types of Acids

• Hydrohalic Acids – binary acids formed between hydrogen and the halogens

– the predominant factor in determining the strength of the hydrohalic acids is bond strength

HF << HCl < HBr < HI


• Oxoacids - contain hydrogen, oxygen, and a central, nonmetal atom– contain one or more bonds– an electronegative or a high oxidation

state central atom, will attract electrons, causing the bond to be more polar

– high electronegativity and oxidation result in stronger acids

O H

O H


To compare oxyacid strengths• Oxoacids having different central atoms

that are from the same group of the periodic table and that have the same oxidation number.

– acid strength increases with increasing electronegativity of the central atom

Example:


• Oxoacids having the same central atom but different numbers of oxygen atoms

– strength increases with increasing oxidation number of the central atom

Example:


• Carboxylic acids – organic acids with the structure

– Ionize to produce a carboxylate anion

– Increasing stability of the carboxylate anion increases acidity• Stability increases with the number of

electronegative groups in the R group

R C O

O

H

R C O

O

R C O

O


Indicate which is the stronger acid: (a) HBrO3

or HBrO4; (b) H2SeO4 or H2SO4.


b) In a group with different central atoms, we must compare electronegativities. The electronegativities of the central atoms in this group decrease as follows: S>Se. H2SO4 > H2SeO4

a)These two acids have the same central atom but differ in the number of attached oxygen atoms. In a group such as this, the greater the number of attached oxygen atoms, the higher the oxidation number of the central atom and the stronger the acid.

HBrO4 > HBrO3


16.10 Acid-Base Properties of Salt Solutions

• Salt solutions are affected by salt hydrolysis, in which ions produced by the dissociation of a salt react with water to produce either hydroxide ions or hydronium ions—thus impacting pH.

• Basic salt solutions - an anion that is the conjugate base of a weak acid reacts with water to produce hydroxide ion.


• Acidic salt solutions

– When the cation of a salt is the conjugate acid of a weak base, a solution of the salt will be acidic.

For example:


– Small, highly charged metal ions are hydrated by water molecules and polarization of one of the O-H bonds can produce hydrogen ions.


Determine the pH of a 0.25 M solution of

pyridinium nitrate (C5H5NHNO3) at 25°C.

[Pyridinium nitrate dissociates in water to

give pyridinium ions (C5H5NH+), the conjugate

acid of pyridine (Kb = 1.7 x 109), and

nitrate ions (NO3 ).]


]NHH[C

]ON][HH[C

65

365a

K

C5H5NH (aq) + H2O(l) C5H5N H3O (aq)(aq) +

14. .

. 1

6wa 9

b

1 00 x 105 9 x 10

1 7 x 0

KK

K


0.25 M

−x +x +x

xx0.25 − x M


. 1( . ) ( . )

2 26

b 5 9 x 00 25 x 0 25

x xK

M M

. : .

.

31 2 x 10

Check x 100 0 49%0 25

MM

[ ] . 33H O 1 2 x 10 M

og( . ) . 3pH l 1 2 x 10 2 92M

( . )( . ) . 6 35 9 x 10 0 25 1 2 x 10x M M


0.25 M

− x +x +x

xx0.25 − x M


• Neutral salt solutions

– A salt composed of the cation of a strong base and the anion of a strong acid produces a neutral solution.

– These ions do not hydrolyze in water.

For example:

NaCl or KNO3


Summary


• Salts in Which Both the Cation and the Anion Hydrolyze

–pH depends on the relative strengths of the conjugate acid and base

• When Kb > Ka, the solution is basic.

• When Kb < Ka, the solution is acidic.

• When Kb = Ka, the solution is neutral

or nearly neutral.

NH4NO2For example:


Predict whether 0.10 M solutions of the

following are acidic, basic or nearly neutral.

a) C5H5CNHCl

b) KF


a) C5H5CNHCl

b) KF

acidic

basic


(aq) + OH (aq)HFH2O(l)(aq) +F


16.11 Acid-Base Properties of Oxides and Hydroxides


• Basic metallic oxides react with water to form metal hydroxides.

• Acidic nonmetallic oxides react with water to form acids.


• Basic and Amphoteric Hydroxides– All the alkali and alkaline earth metal

hydroxides, except Be(OH)2, are basic.– Be(OH)2, as well as Al(OH)3, Sn(OH)2,

Pb(OH)2, Cr(OH)3, Cu(OH)2, Zn(OH)2, and Cd(OH)2, are amphoteric and insoluble in water.

– React with acids

– React with bases


16.12 Lewis Acids and Bases• Lewis base as a substance that can

donate a pair of electrons.

• Lewis acid is a substance that can accept a pair of electrons.

• A Lewis acid-base reaction is one that involves the donation of a pair of electrons from one species to another.

• Most general definition of acids and bases


coordinate covalent bond

A Lewis Acid-Base Reaction


Key Points

• BrØnsted acids and bases– Conjugate acid– Conjugate base

• Acid-base properties of water– Autoionization of water

– Kw

– Relation between hydronium and hydroxide ion concentrations


• The pH scale– Defining equation– pOH scale

– Relation among pH, pOH and Kw

• Strong acids and bases

• Weak acids and acid ionization constants– Definition of weak acids

– Ka, the acid ionization (equilibrium) constant

– Method to solve weak acid equilibrium problems


• Weak bases and base ionization constants– Definition of weak bases

– Kb, the base ionization (equilibrium) constant

– Method to solve weak base equilibrium problems

• Conjugate acid-base pairs– Strength of acid-base pairs

– Relation between Ka and Kb for conjugate pairs

• Diprotic and triprotic Acids– Successive ionization steps

– Associated Ka values

– Solving equilibrium problems


• Molecular structure and acid strength– Hydrohalic acids– Oxoacids– Carboxylic acids

• Acid-base properties of salt solutions– Basic salts– Acidic salts– Neutral salts– Complex salts in which both ions hydrolyze


• Acid-base properties of oxides and hydroxides– Oxides

• Metal• Nonmetal

– Basic and amphoteric hydroxides

• Lewis acids and bases

copyright mcgraw-hill 20091 chapter 16 acids and bases insert picture from first page of chapter

Documents

copyright mcgrawhill

strong slide

w slide

acid oh

page of chapter slide

solution of hbr

solution of caoh

proton conjugate acid