1

Water is polar and forms H-bonds with itself

responsible for high boiling point of water

Water forms H-bonds with polar and ionic substances

responsible for solubility of ions and molecules

Water: The Medium of LifeOH H

H

OH H

OH

H OH

HO H H

O

H

Substance MW BP (°C)Water (H2O) 18.02 100.0Ammonia (NH3) 17.03 -33.4Methane (CH4) 16.04 -161.5

H OH

RO

H

HO

H

H

O HR

CR

HO

H

HO

H

ONa+

H

OH

HO

H

HO

H

Cl-

HOH

H O H

HOH

Ion-dipole interactions

dipole-dipole interactions

2

Equilibrium Reactions

Reactions do not go to completionRate of forward reaction equals rate of reverse reaction

Equilibrium reactions can be described by a mathematical equation

A + B C + D

]][[]][[BADCKeq

reactants

products The greater degree of the forward reactionthe bigger Keq

At equilibrium, the rate of forward = rate of reverse so ratio of products:reactants isconstant – therefore, Keq is constant.

3

Water self-dissociates

H2O H+ + OH-

Hydrogen ionoften called a

proton

Hydroxide ion(base)

H+ + H2O H3O+

Hydronium ion(acid)

equilibriumconstant

@25 C, pure H2Ois 55.5 M - constant

][]][[

2OHOHHKeq

weq KOHHOHK ]][[][ 2

14101 xKw 7101][][ xOHH

For pure water at 25 °C, 1 atm:

Ion-productconstant ofwater

x(x)=1x10-14

x2=1x10-14

x=1x10-7

4

At 25 C, 1atm, Kw always = 10-14 so you can always calculate H+ or OH- concentrationif given the other

Example:Aqueous solution has H+ concentration of 10-2 M, what is OH- concentration?

Kw=1x10-14=[H+][OH-]=1x10-2 [OH-] therefore, [OH-] = 1x10-12

5

Water is amphoteric (10.4) Can act as anacid or base

Is water an acid or base in the following reactions?

H3PO4 (aq) + H2O(l) H2PO4- (aq) + H3O+ (aq)

F- (aq) + H2O(l) HF (aq) + OH- (aq)

NH4 +

(aq) + H2O(l) NH3 (aq) + H3O+ (aq)

Base

Base

Acid

Brønstead-Lowry definitions

Acid - substance that gives H+ to another molecule or ion

Base - substance that accepts H+ from an acid (uses lone pair electrons)

6

Measuring Acidity A solution’s acidity depends on the concentration of H3O+

The higher the conc of H+, the more acidic the solution

pH scale gives a convenient comparison of acidityH+ concentration ranges over 14 fold magnitude

which solution is more acidic?

[H3O+] = 9.0 x 10-8 M or [H3O+] = 3.5 x 10-7 M

which solution is more acidic?

pH = 7.05 or pH = 6.46

acidity basicityneutralpH

[H3O+]

0 1 2 3 4 5 6 8 9 10 11 12 13 147

1 10-1 10-2 10-3 10-4 10-5 10-6 10-8 10-9 10-10 10-11 10-12 10-13 10-1410-7

]log[ 3 OHpH

H-A + H2O H3O+ + A-

figure 10.2 on page 276

7

pH example problems (use equation):

1. The H+ concentration in coffee is about 1 x 10-5 M. What pH is this?

2. Soft drinks usually have a pH of approximately 3.1. What is the [H3O+] in a soft drink?

3. A cleaning solution was found to have [OH-]=1 x 10-3 M. What is the pH?

A logarithim contains the same number of digits to the right of the decimal point that theoriginal number has. An antilogarithm contains the same number of digits that the

original number has to the right of the decimal point.

assign application on page 280 of text for reading

8

Strong acids give up hydrogens easily and produce weakconjugate basesDissociation - splitting of an acid into hydrogen ion and an anionStrong acids dissociate “100%”

Weak acids don’t give up hydrogens easily and produce strongconjugate basesWeak acids dissociatemuch less than “100%”

Acids Produce Conjugate Bases (4.12, 6.10, Chap.10)

H-A + H2O H3O+ + A-

If this is a strongacid because itgives up a protonreadily...

…then this is a weakbase because it haslittle affinity for aproton.

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

acid baseconjugate

acidconjugate

base

H-A + H2O H3O+ + A-

If this is a weakacid because itgives up a protonwith difficulty...

…then this is a strongbase because it hasa high affinity for aproton.

acid

base

conjugate

acid

conjugate

base

9

Dissociation of a weak acid is an equilibrium rxnGeneral rxn for dissociation of a weak acid:

Equilibrium constant reveals extent of rxn (7.7, 10.7-10.8)

Acid Dissociation Constant, Ka

H-A + H2O H3O+ + A-

][][]][[

eactantrctantreaproductproductKeq

Write equilibrium equation for the dissociation of a weak acid:

][[]][[

2

3

OH]HAOHAKeq

Concentration of water is essentially constant so a newconstant is defined - the acid dissociation constant, Ka

aeq KHAOHA

OHK

][]][[

][ 32

10

Weak acids have Ka << 1

pKa scale gives a convenient comparison of acid strength

If the acid doesn’t dissociate very much, HA is large.The larger the denominator, the smaller Ka.

]log[ HpH

]log[ OHpOH

aa KpK log

As [H+] , pH

As [OH-] , pOH

As Ka , pKa - This means that strong weak acids (thosewith large Ka because HA is small) have low pKa and weakweak acids (those with small Ka because HA is largehave high pKa

H-A + H2O H3O+ + A-

][]][[ 3

HAOHAKa

11

Weak Acid Equation Ka pKa

acetic acid HC2H3O2 H+ + C2H3O2- 1.8 x 10-5 4.74

ammonium ion NH4+ H+ + NH3 5.6 x 10-10 9.25

benzoic acid C6H5CO2H H+ + C6H5CO2- 6.4 x 10-5 4.19

carbonic acid (1) H2CO3 H+ + HCO3- 4.3 x 10-7 6.37

carbonic acid (2) HCO3- H+ + CO3

-2 5.6 X 10-11 10.2

chlorous acid HClO2 H+ + ClO2- 1.2 x 10-2 1.91

formic acid HCHO2 H+ + CHO2- 1.8 x 10-4 2.74

hydrocyanic acid HCN H+ + CN- 6.2 x 10-10 8.21hydrofluoric acid HF H+ + F- 7.2 x 10-4 2.14

hypobromous acid HOBr H+ + OBr- 2 x 10-9 7.7hypochlorous acid HOCl H+ + OCl- 3.5 x 10-8 6.46hypoiodous acid HOI H+ + OI- 2 x 10-11 9.7

CH3CH(OH)CO2H

H+ + CH3CH(OH)CO2-

nitrous acid HNO2 H+ + NO2- 4.0 x 10-4 2.4

phenol HOC6H5 H+ + OC6H5- 1.6 x 10-10 8.8

phosphoric acid (1) H3PO4 H+ + H2PO4- 7.25 x 10-3 2.14

phosphoric acid (2) H2PO4- H+ + HPO4

-2 6.31 x 10-8 7.2

phosphoric acid (3) HPO4-2 H+ + PO4

-3 3.98 x 10-13 12.4

lactic acid 1.38 x 10-4 3.86

Acid Dissociation Constants of Common Weak Acids

12

Ka and pKa example problems (use equation):1. Place the following acids in order of increasing strength based on their pKa values:

Acid pKaH2CO3 6.37 NH4

+ 9.25HF 3.45CH3COOH 4.75H3PO4 2.12

2. At equilibrium, the conjugate base of formic acid has a concentration of .0029 M. What is the concentration of formic acid present at equilibrium?

3. At equilibrium, an unknown weak monoprotic acid has a concentration of 0.1 M and the concentration of conjugate base is .0037 M. What is its pKa and what is the acid?

13

Acid-Base Titration (10.15) Used to determine concentration of an acid by addition of

known amounts of base (usually NaOH)using a color indicator, base is added until neutralization is complete.Volume and molarity of base is used to determine original concentration of acid

Example: To determine the concentration of the acid in an old bottle of HCL whose label had become unreadable, a titration was carriedout. What is the HCL concentration if 58.4 ml of 0.250 M NaOH was required to titrate a 20.0 ml sample of the acid?

14

Used to identify an unknown weak acid through its pKa

pH is monitored as known volumes of base are added to the acid solution.

Titration Curve

Discuss starting pH, equivalence point, and buffer region

Equivalence point

Half-equivalence point

50.00 ml(50.00/2) ml = 25.0 ml

15

Mathematical relationship of pH and pKaHenderson-Hasselbalch equation:

equation relates pH and Ka

][]][[ 3

HAOHAKa

When A-=HA, pH=pKa - very important for buffering

H-A H3O+ + A-

Rearrange for [H3O+]:

Take the negative log of both sides:

Split up right side of equation (be sure to distribute the negative sign) and rearrange:

Recall:log(xy) = log(x) + log(y)andlog(x/y)=log(x) - log(y)

][][][ 3

AHAKOH a

)][][log(]log[ 3

AHAKOH a

][A)][log(log HAKpH a

)][][log(

AHApKpH a )

][][log(

HAApKpH a

ORWhen A-=HA, pH=pKa

16

Experimentally determining identity of acid through pKa

Step 1Step 2

Step 1: find the equivalence pointStep 2: Take 1/2 volume needed to reach equivalence point - here, [HA]=[A-]Step 3: determine pH at where [HA]=[A -]

Step 3

pH = pKa at 1/2 equivalence point

Titration of ________________ with NaOH

17

diprotic acids have biphasic curves

2nd Equivalence point

1st half-equivalence1st Equivalence point

2nd half-equivalence

18

Draw a model titration curve for a triprotic acid

19

Buffers maintain pHBuffers and pKa (10.12)

Constant pH crucial for proper organ functionenzymesprotein structurerespiration

20

A buffer is a solution of a weak acid and its conjugate baseweak acid can neutralize moderate amounts of added baseconjugate base can neutralize moderate amounts of added acid

Example above: Mixture of CH3COOH and CH3COO-

CH3COOH + OH- CH3COO- + H2O pH rises slightly

CH3COO- + H3O+ CH3COOH + H2O pH lowers slightly

Buffer capacity is maximal when conc. of weak acid = conc of conjugate base

Buffer capacity is maximal when pH=pKa

21

Major Physiological Buffers (10.13) dihydrogen phosphate-hydrogen phosphate buffer system

carbonic acid – bicarbonate buffer system

H2PO4- + H2O HPO4

-2 + H3O+ pKa = 7.2

Most important intracellular buffer. Phosphate concentration too low extra-cellular to be a sufficient buffer

H2CO3 + H2O HCO3-2 + H3O+ pKa = 6.37

Most important buffer in blood. But blood pH is 7.4 so this buffer is near the end ofthe buffer range of this system.

Allows more buffering effect for added acid than for added base – excessive acid production is more common in the body than excessive loss of acid or addition of base.

Carbonic acid is unstable and breaks down into CO2 and H2O

H2CO3 CO2 + H2Omakes blood pH intimately related to breathing rates

22

CO2 + H2O H2CO3 HCO3-2 +

H3O+

][]][[

2

33

COOHHCOK a

][][][3

23

HCOCOKOH a

Increase here causes lower pHdecrease here causes higher pH

23

1. Stroke and heart attack patients often experience acidosis. Explain in detail.

2. Although not recommended, some sprinters hyperventilate for 30 seconds just prior to a race. Why is this supposed to work?

3. Asthma and emphysema patients are at risk for developing acidosis. Explain in detail.

4. Why does exposure to high altitudes temporarily induces mild alkalosis?

Acidosis Questions. Explain in detail with structures and equations where applicable.

Lack of blood flow causes lack of oxygen and increased production of lactic acid whichaccumulates in blood causes drop in pH.

Difficulty breathing causes lack of oxygen and increased production of lactic acid whichaccumulates in blood causes drop in pH.

Similar to hyperventilation. The atmosphere causes respiration to increase which causesmore carbon dioxide to be expired. This causes a shift towards H2CO3 lowering the conc.of H+ and increasing pH.

Sprinting produces excess lactic acid due to insufficient oxygen. To counteract the lacticacid, sprinters may hyperventilate to induce a temporary state of alkalosis.

24

Amines Are Organic Bases Amines are ammonia derivatives

Amines are weak basesAmmonia and amines are weak bases because its lone pairs can accept

a proton from an acid or water

H N

H

H

ammonia

R1 N

H

H

Primary amine(1° amine)

R1 N

H

R2

Secondary amine(2° amine)

R1 N

R3

R2

Tertiary amine(3° amine)

+ H3O+ + H2O

1° amine

acid ammoniu

mion

R1 N

H

H R1 N

H

H

H

Ammonium ions have pKa ~10 (Theyare 50% unprotonated around pH 10)RNH3

+ + OH- RNH2 + H2OpH = 4 - protonated pH = 7 - protonated pH = 11 - unprotonated

General Structure:

25

Carboxylic Acids Are Organic Acids

Common Name Formal Name pKaFormic acid(ants) Methanoic acid 3.75

Acetic acid (vinegar) Ethanoic acid 4.76

Butyric acid(rancid butter) Butanoic acid 4.81

Caproic acid(goat smell) Hexanoic acid 4.84

HC OH

O

CH3C OH

O

H3C(H2C)2C OH

O

H3C(H2C)4C OH

O

Examples:O

COHR

General Structure

H3C(H2C)3C OH

O

+ H2O H3C(H2C)3C O

O

+ H3O+

Pentanoic acid Pentanoate ion

Carboxylic acids have pKa ~4-5 (Theyare 50% unprotonated around pH 4-5)

pKa of pentanoic acid is 4.82pH = 4 - mostly

protonatedpH = 7 - mostly unprotonated

26

Using pKa charts, draw the predominant structure for the following parent compounds at the pH indicated

CH2CH3C OH

O

C

O

OH

C

O

OH

pka1=2.9pka2=5.4

pka=4.87Propanoic acid

Phthalic acid

H

NH3C H

CH3

Dimethylammonium ion pka=10.72

H

NH H

Cyclohexylammonium ion pka=9.3

H

NH3C CH3

CH3

Trimethylammonium ion pka=9.7

Parent Compound pKa Structure at pH:

N C C

C

OH

OH

H

H

H

H3

Alanine

pH = 7

pH = 4

pH = 7

pH = 11

pH = 6

pH = 7