1 CHAPTER 16 Acid – Base Equilibria. 2 Properties of Aqueous Solutions of Acids & Bases n Acidic properties taste sour change the colors of indicators

1

CHAPTER 16

Acid – Base EquilibriaAcid – Base Equilibria

2

Properties of Aqueous Solutions of Acids & Bases

Acidic propertiesAcidic properties taste sourtaste sour change the colors of indicatorschange the colors of indicators

turn litmus redturn litmus red

react with metals to generate Hreact with metals to generate H2(g)2(g)

react with metal oxides and hydroxides to form react with metal oxides and hydroxides to form salts and watersalts and water

aqueous solutions conduct electricityaqueous solutions conduct electricity

3

Properties of Aqueous Solutions of Acids & Bases

Basic propertiesBasic properties taste bittertaste bitter feel slipperyfeel slippery change colors of indicatorschange colors of indicators

turn litmus blueturn litmus blue react with acids to form salts and waterreact with acids to form salts and water aqueous solutions conduct electricityaqueous solutions conduct electricity

4

Strong Electrolytes Strong electrolytes Strong electrolytes ionizeionize or or dissociatedissociate

completelycompletely Three classes of strong electrolytesThree classes of strong electrolytes1 Strong AcidsStrong Acids

HNO H O H O NO

or

HNO H NO

3 2100%

3+

3

3 3

5

Strong Electrolytes2 Strong Soluble BasesStrong Soluble Bases

K OH K OHH O 100%2

6

Strong Electrolytes3 Most Soluble SaltsMost Soluble Salts

Ca(NO ) Ca 2 NO3 2 sH O 100% 2

32

7

Strong Electrolytes

Calculation of concentrations of ions in Calculation of concentrations of ions in solution of strong electrolytes is easysolution of strong electrolytes is easy

Example: Calculate the concentrations of ions Example: Calculate the concentrations of ions in 0.050 in 0.050 MM nitric acid, HNO nitric acid, HNO33..

8

Strong Electrolytes

Calculation of concentrations of ions in Calculation of concentrations of ions in solution of strong electrolytes is easysolution of strong electrolytes is easy

Example: Calculate the concentrations of ions Example: Calculate the concentrations of ions in 0.050 in 0.050 MM nitric acid, HNO nitric acid, HNO33..

HNO H O H O NO

0.050 0.050 3 2

100%3 3

0 050. M M M

9

Arrhenius Theory Svante Augustus Arrhenius - 1884Svante Augustus Arrhenius - 1884 acids generate Hacids generate H++ in aqueous solutions in aqueous solutions

-2322

- 32

HCO O H O HH HCO

Cl O H O H HCl

10

Arrhenius Theory bases generate OHbases generate OH-- in aqueous solutions in aqueous solutions

aqaq423(g)

-aqaq

OH NH O H NH

OH Na NaOH

11

Arrhenius Theory

neutralization - combination of Hneutralization - combination of H++ (or H (or H33OO++) )

with OHwith OH-- strong acids - ionize 100% in waterstrong acids - ionize 100% in water

HCl, HBr, HI, H2SO4, HNO3, HClO4, HClO3

strong bases - ionize 100% in waterstrong bases - ionize 100% in waterLiOH, NaOH, KOH, RbOH, CsOH,

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

12

Arrhenius Theory

total ionic equation for strong acid with total ionic equation for strong acid with strong basestrong base

net ionic equation for strong acid with net ionic equation for strong acid with strong basestrong base

(l)O HCl Na OH Na Cl H 2-aqaq

-aqaqaqaq

(l)O H OH H 2-aqaq

13

Bronsted-Lowry Acid-Base Theory

acids - proton (Hacids - proton (H++) donor) donor

1221

-32

base acid base acid

Br O H O HHBr

14


bases - proton (Hbases - proton (H++) acceptor) acceptor

212 1

-423

base acid acid base

OH NH O H NH

15


acid-base reactions are proton transfer acid-base reactions are proton transfer reactionsreactions

– note that we are often making coordinate note that we are often making coordinate covalent bondscovalent bonds

1221

-43

base acid base acid

Cl NH NH HCl

16


conjugate acid-base pairsconjugate acid-base pairs– species that differ by a protonspecies that differ by a proton

HNOHNO3 3 + H+ H22O O H H33OO+ + + NO + NO33--

HNOHNO33 - acid - acid11 NO NO33- - - base- base11

HH22O - baseO - base2 2 H H33OO+ + - acid- acid22

HF + HHF + H22O O H H33OO+ + + F + F--

HF - acidHF - acid11 F F- - - base- base11

HH22O - baseO - base2 2 H H33OO+ + - acid- acid22

17


differences between Arrhenius & Bronsted-differences between Arrhenius & Bronsted-Lowry theoriesLowry theories

reaction does not have to occur in an reaction does not have to occur in an aqueous solutionaqueous solution

bases do not have to be hydroxidesbases do not have to be hydroxides for example- ammonia is not a hydroxidefor example- ammonia is not a hydroxide

base acid acid base

OH NH O H NH

212 1

-423

18


weak acids have strong conjugate weak acids have strong conjugate basesbases

weak bases have strong conjugate weak bases have strong conjugate acidsacids

primary reason they are weak acids or primary reason they are weak acids or basesbases

strong conjugates recombine to form the strong conjugates recombine to form the original speciesoriginal species

19


NHNH44++ must be a strong acid-it gives up H must be a strong acid-it gives up H++

to reform NHto reform NH33

NaOH NaOH Na Na++ (aq) + OH (aq) + OH--(aq)(aq)

NaNa++ must be a weak acid or it would must be a weak acid or it would recombine to form NaOHrecombine to form NaOH

remember NaOH ionizes 100%remember NaOH ionizes 100%

-423 OH NH O H NH

20


amines are weak bases that behave like amines are weak bases that behave like ammoniaammonia

amines have organic groups attached to amines have organic groups attached to -NH-NH22 group group

-33223

-423

OH NHCH O H NHCH

OH NH O H NH

21


water can be either an acid or base in water can be either an acid or base in Bronsted-Lowry theoryBronsted-Lowry theory

amphotericamphoteric - species that can be either - species that can be either an acid or basean acid or base

amphiprotic amphiprotic - proton transfer reactions - proton transfer reactions that species behave as either an acid that species behave as either an acid or baseor base

2121

-322

base acid acid base

OH O H O H OH

22

The Auto-Ionization of Water

Pure water ionizes Pure water ionizes very slightlyvery slightly– less than one-millionth molarless than one-millionth molar

H O + H O H O + OH2 2 3+ -

23

The Auto-Ionization of Water

Because the activity of pure water is 1, the Because the activity of pure water is 1, the equilibrium constant for this reaction isequilibrium constant for this reaction is

K H O OHc 3+

24

The Auto-Ionization of Water Experimental measurements have determined that Experimental measurements have determined that

the concentration of each ion is 1.0 x 10the concentration of each ion is 1.0 x 10-7-7 MM at 25 at 2500C C

K H O OH

1.0 x 10 1.0 x 10

1.0 x10

c 3+

-7 -7

14

25

The Auto-Ionization of Water This particular equilibrium constant is called theThis particular equilibrium constant is called the ion- ion-

product for water, Kproduct for water, Kww..

K H O OH

1.0 x10

w 3+

14

26

The pH and pOH scales

A convenient way to express acidity and A convenient way to express acidity and basicitybasicity

pH is defined as pH is defined as

pH = -log H O3+

27


In general, a lower case p before a symbol is In general, a lower case p before a symbol is read as ‘negative logarithm of” the symbolread as ‘negative logarithm of” the symbol

pOH = -log OH

pAg = -log Ag

and so forth

-

+

28


If we know either [HIf we know either [H33OO++] or [OH] or [OH--], then we can calculate pH and pOH.], then we can calculate pH and pOH.

Example: Calculate the pH of a solution in which the [HExample: Calculate the pH of a solution in which the [H33OO++] =0.030] =0.030MM..

29


If we know either [HIf we know either [H33OO++] or [OH] or [OH--], then we can calculate pH and pOH.], then we can calculate pH and pOH.

Example: Calculate the pH of a solution in which the [HExample: Calculate the pH of a solution in which the [H33OO++] =0.030] =0.030MM..

52.1pH

100.3logpH

OH-log=pH2

+3

30


A convenient relationship between pH and A convenient relationship between pH and pOH may be derived for pOH may be derived for allall dilute aqueous dilute aqueous solutions at 25solutions at 2500C.C.

143 100.1]][OHO[H

31


Remember these two expressions!!Remember these two expressions!!

14.00pOHpH

100.1OHOH 143

32


The usual range for the pH scale isThe usual range for the pH scale is

and for pOH the scale isand for pOH the scale is

14.00pH 0pH

100.1OH down to 0.1OH 1433

MM

0pOH 00.14pOH

0.1OH toup 100.1OH 14

MM

33


34

Strengths of Acids

Binary AcidsBinary Acids acid strength increases with decreasing acid strength increases with decreasing

bond strengthbond strength hydrogen halideshydrogen halides bond strengthbond strength

HF>>HCl>HBr>HIHF>>HCl>HBr>HI acid strengthacid strength

HF<<HCl<HBr<HIHF<<HCl<HBr<HI

35

Strengths of Acids

36

Strengths of Acids

VIA hydridesVIA hydrides bond strengthbond strength

HH22O>> HO>> H22S> HS> H22Se> HSe> H22TeTe

acid strengthacid strengthHH22O<< HO<< H22S< HS< H22Se< HSe< H22TeTe

37

Strengths of Acids

for HBr in waterfor HBr in water

HBr + HHBr + H22O O H H33OO+ + + Br + Br--

essentially 100%essentially 100% can only distinguish acid strength can only distinguish acid strength

differences of strong acids in differences of strong acids in nonaqueous solutions like acetic acidnonaqueous solutions like acetic acid

38

Strengths of Acids AcidAcid Conjugate BaseConjugate Base Strongest acid Weakest baseStrongest acid Weakest base HClOHClO4 4 ClOClO44

--

HH++(H(H33OO++) -H) -H++ H H22OO

CHCH33COCO22H H CHCH33COCO22--

HH22O +HO +H++ OH OH--

NHNH33 NH NH22--

Weakest acid Strongest baseWeakest acid Strongest base

39

Strengths of Acids strongest acid in water is Hstrongest acid in water is H33OO++

HCl + HHCl + H22O O H H33OO+ + + Cl + Cl--

HCl is so strong it forces water to accept HHCl is so strong it forces water to accept H++

strongest base in water is OHstrongest base in water is OH--

NHNH22- - + H+ H22O O NH NH33 + OH + OH--

NHNH22- - is strong enough to remove His strong enough to remove H+ + from waterfrom water

because water is amphiproticbecause water is amphiprotic

40

Strengths of Bases

Strong Bases are strong electrolytes Strong Bases are strong electrolytes Dissociate completely in solutionDissociate completely in solution Bases do not need to contain OHBases do not need to contain OH-- ion ion

OO2-2-(aq) + H(aq) + H22O (l) O (l) 2OH 2OH--(aq)(aq)

HH--(aq) + H(aq) + H22O(l) O(l) H H22(g) + OH(g) + OH--(aq)(aq)

NN33-- (aq) + H (aq) + H22O(l) O(l) NH NH33(aq) + 3OH(aq) + 3OH--(aq)(aq)

41

Ionization Constants for Weak Monoprotic Acids and Bases

Let’s look at the dissolution of acetic acid, a Let’s look at the dissolution of acetic acid, a weak acid, in water as an example.weak acid, in water as an example.

The equation for the ionization of acetic acid is:The equation for the ionization of acetic acid is:

COOCHOHOH COOHCH -3323

42


The equilibrium constant for this ionization is The equilibrium constant for this ionization is expressed as:expressed as:

OHCOOHCH

COOCHOHK

23

33c

43


The water concentration in dilute aqueous The water concentration in dilute aqueous solutions is very high.solutions is very high.

1 L of water contains 55.5 moles of water.1 L of water contains 55.5 moles of water. Thus in dilute aqueous solutions:Thus in dilute aqueous solutions:

M5.55OH2

44


The water concentration is many orders of The water concentration is many orders of magnitude greater than the ion concentrations.magnitude greater than the ion concentrations.

Thus the water concentration is essentially Thus the water concentration is essentially constant.constant.

COOHCH

COOCHOHOHK

3

332c

45


Since a constant multiplied by a constant is a Since a constant multiplied by a constant is a new constant - let’s give this new constant its new constant - let’s give this new constant its own name and symbolown name and symbol

KKaa = ionization constant = ionization constant

acid aceticfor

108.1COOHCH

COOCHOHK 5

3

33a

46


In In simplified formsimplified form the equation and the equation and expression are written as:expression are written as:

5

3

3a

-33

108.1COOHCH

COOCHHK

COOCHHCOOHCH

47


Values for several ionization constantsValues for several ionization constants

48


From the above table we see that the order of From the above table we see that the order of increasing acid strength for these weak acids increasing acid strength for these weak acids is:is:

The larger KThe larger Kaa The stronger the acid The stronger the acid

If KIf Kaa >> 1 then the acid is completely ionized >> 1 then the acid is completely ionized

and the acid is a strong acid.and the acid is a strong acid.

HF > HNO > CH COOH > HClO > HCN3 3

49


The order of increasing base strength of the The order of increasing base strength of the anions (conjugate bases) of these acids is:anions (conjugate bases) of these acids is:

F < NO < CH COO < ClO < CN-3-

3- - -

50


Using Ka, the concentration of H+ (and hence the pH) can be calculated.– Write the balanced chemical equation clearly showing the

equilibrium.

– Write the equilibrium expression. Find the value for Ka.

– Write down the initial and equilibrium concentrations for everything except pure water. We usually assume that the change in concentration of H+ is x.

Substitute into the equilibrium constant expression and solve. Remember to turn x into pH if necessary.

A sample problem can be found at end of the slides.

51


Percent Ionization

HA(aq) + H2O(l) ↔ H3O+ (aq) + A-(aq)

% ionization = [H+]equ x 100

[HA]

Relates equilibrium H+ concentration to the initial HA concentration

52


Percent Ionization

The higher the percent ionization

The stronger the acid

For weak acids

Percent ionization decreases as the molarity of the solution increases

acetic acid 0.05M 2.0% ionized

0.15 M 1.0% ionized

53


54

Polyprotic Acids Many weak acids contain two or more acidic hydrogens.Many weak acids contain two or more acidic hydrogens.

– polyprotic acids ionize stepwisepolyprotic acids ionize stepwise– ionization constant for each stepionization constant for each step

Consider arsenic acid, HConsider arsenic acid, H33AsOAsO44, which has three ionization constants, which has three ionization constants

1 KK11=2.5 x 10=2.5 x 10-4-4

2 KK22=5.6 x 10=5.6 x 10-8-8

3 KK33=3.0 x 10=3.0 x 10-13-13

55

Polyprotic Acids

The first ionization step isThe first ionization step is

H AsO H H AsO

KH H AsO

H AsO

3 4 2 4

12 4

3 4

2 5 10 4.

56

Polyprotic Acids

The second ionization step isThe second ionization step is

81

43

242

2

24242

106.5AsOH

AsOHHK

AsOHHAsOH

57

Polyprotic Acids

The third ionization step isThe third ionization step is

HAsO H AsO

KH AsO

HAsO

42-

4

34

42-

3

3133 0 10.

58

Polyprotic Acids

Notice that the ionization constants vary in the following fashion:Notice that the ionization constants vary in the following fashion:

This is a general relationship. It is always easier to remove the first This is a general relationship. It is always easier to remove the first proton.proton.K K K1 2 3

59

Polyprotic Acids

60

Weak Bases

Remove protons from other substancesRemove protons from other substances Equilibrium establishedEquilibrium established

Weak base + HWeak base + H22O O ↔ Conjugate acid + OH↔ Conjugate acid + OH--

Calculate KCalculate Kbb base dissociation constant base dissociation constant

61

Weak Bases

62

Weak Bases

Generally have lone pair or negative Generally have lone pair or negative chargecharge

Neutral weak bases contain NNeutral weak bases contain N Anions of weak acids are weak basesAnions of weak acids are weak bases

63

Relationship between Ka and Kb

When two reactions are added to give a When two reactions are added to give a third, the equilibrium constant for the third, the equilibrium constant for the third reaction is the product of the third reaction is the product of the equilibrium constants for the first two.equilibrium constants for the first two.

Reaction 1 + Reaction 2 = Reaction 3Reaction 1 + Reaction 2 = Reaction 3

KK11 x K x K22 = K = K33

64

Relationship between Ka and Kb

For a conjugate acid base pair:For a conjugate acid base pair:

KKaa x K x Kbb = K = Kww

pKpKaa x pK x pKbb = pK = pKww

The larger the KThe larger the Kaa, the smaller the K, the smaller the Kbb

The stronger the acid, the weaker the The stronger the acid, the weaker the conjugate base.conjugate base.

65

Calculation of Ionization Constants

Example: In 0.12 Example: In 0.12 MM solution, a weak solution, a weak monoprotic acid, HY, is 5.0% ionized. monoprotic acid, HY, is 5.0% ionized. Calculate the ionization constant for the weak Calculate the ionization constant for the weak acid.acid.

66


Example: In 0.12 Example: In 0.12 MM solution, a weak solution, a weak monoprotic acid, HY, is 5.0% ionized. monoprotic acid, HY, is 5.0% ionized. Calculate the ionization constant for the weak Calculate the ionization constant for the weak acid.acid.

HY H + Y

KH Y

HY

+ -

a

+ -

67


Since the weak acid is 5.0% ionized, it is also Since the weak acid is 5.0% ionized, it is also 95% unionized.95% unionized.

Calculate the concentrations of all species in Calculate the concentrations of all species in solution.solution.

H Y

HY

+

0 05 012 0 0060

6 0 10

0 95 012 011

3

. ( . ) .

.

. ( . ) .

M M

M

M M

68


Substitute into the ionization constant Substitute into the ionization constant expression to get the value of Kexpression to get the value of Kaa

KH Y

HYa

6 0 10 6 0 10

011

3 3 10

3 3

4

. .

.

.

69

Calculations Based on Ionization Constants

Example: Calculate the concentrations of the various Example: Calculate the concentrations of the various species in 0.15 species in 0.15 MM acetic acid, CH acetic acid, CH33COOH, solution.COOH, solution.

It is always a good idea to write down the ionization It is always a good idea to write down the ionization reaction and the ionization constant expression.reaction and the ionization constant expression.

5

3

-33

a

-3323

108.1COOHCH

COOCHOHK

COOCHOH OHCOOHCH

70


Next we combine the basic chemical Next we combine the basic chemical concepts with some algebra to solve the concepts with some algebra to solve the problemproblem

M0.15 [] Initial

COOCH OH OHCOOHCH -3323

71



xMxMxM

M

- Change

0.15 [] Initial


72



xMxM-x)M.(

xMxMxM

M

150 [] mEquilibriu

- Change

0.15 [] Initial


73

Substitute these algebraic quantities into the Substitute these algebraic quantities into the ionization expression.ionization expression.

5

3

33a

108.115.0

COOHCH

COOCHOHK

x

xx


74

Solve the algebraic equation, using simplifying Solve the algebraic equation, using simplifying assumption.assumption.


52

52

108.115.0

108.115.0

xx

x

x

75

Solve the algebraic equation, using simplifying Solve the algebraic equation, using simplifying assumption.assumption.


52

3a

52

52

108.115.0

assumption thismake then 10K If

108.115.0

108.115.0

x

xx

x

x

76

Complete the algebra and solve for concentrations.Complete the algebra and solve for concentrations.


MM

Mx

x

15.0106.115.0COOHCH

COOCHOH106.1

107.2

33

333

62

77

Note that the properly applied simplifying assumption gives Note that the properly applied simplifying assumption gives the same result as solving the quadratic equation does. the same result as solving the quadratic equation does.


2a

4acbb

c b a

0107.2108.1

108.115.0

2

652

5

x

xx

X

xx

78


3-3

6255

101.6- and 106.1

12

107.214108.1108.1

x

x

79

Synthesis Question One method of increasing the solubility One method of increasing the solubility

and the absorption of a medication is to and the absorption of a medication is to convert weakly acidic drugs into sodium convert weakly acidic drugs into sodium salts before making the pills that will be salts before making the pills that will be ingested. How does this preparation ingested. How does this preparation method enhance the drug’s solubility in method enhance the drug’s solubility in the stomach? the stomach?

80

Synthesis Question

The sodium salt of a weakly acidic The sodium salt of a weakly acidic compound is a strong conjugate base. compound is a strong conjugate base. In the presence of stomach fluids, 1.0 M In the presence of stomach fluids, 1.0 M HCl, the conjugate base readily reacts HCl, the conjugate base readily reacts with the HCl generating the active and with the HCl generating the active and soluble form of the medication. soluble form of the medication.

81

Group Question

Medicines that are weakly basic are not Medicines that are weakly basic are not absorbed well into the bloodstream. absorbed well into the bloodstream. One method to increase their absorption One method to increase their absorption is to take an antacid at the same time is to take an antacid at the same time that the medicine is administered. How that the medicine is administered. How does this method increase the does this method increase the absorption? absorption?

Documents

1 CHAPTER 16 Acid – Base Equilibria. 2 Properties of Aqueous Solutions of Acids & Bases n Acidic properties taste sour change the colors of indicators