Lecture 9

Acid-Base Equilibria -I

Strong acids:

HCl HBr HIHNO3 H2SO4 HClO4

O NO

OH ClO O

O

HO

(HO)nE=O

Weak acid

(HO)nEO2

Strong acid

S

O

O

OH

HO

We titrate with strong acids ONLY!

O NO

OH

O NO

O

_

Concentrations below 10-8 Mdo not affect the pH value

10-6 -10-8 require specialtreatment (in practice, this will be still affected mostly by CO2 and NH3 from air)

Only concentrationsabove 10-6 M may changethe pH value – and only inabsence of other acids or bases

Conclusion: we cannottitrate low concentrationsof acids or bases in water

Weak acids and bases:

Ka is a small number (<< 1)

][

]][[

HA

AHKa

NHH

H + H+N+

HH

H

H

NHH

H + H2O N+

HH

H

H

+ OH-

acid base

base acid

][

]][[

HA

AHKa

][

]][[

A

OHHAKb

HA A + H+

A + H2O HA + OH-

HA A + H+

A + H2O HA + OH-+

H2O H+ + OH-

KaKb =

][

]][[

][

]][[

A

OHHA

HA

AH

= [H+][OH-]

pH value of weak acid solution (C> 10-3 M)

The main source of H+ is HA, not H2OHA A + H+

[A] = [H+]

][

]][[

HA

AHKa

[HA] = C - [H+]

A small number

C

HHKa

]][[

[H+]2 = KaC

CKH a ][

)][(][ HCKH aCheck the resulting [H+] and correct if necessary

Any acid is “strong” at very low concentration – but it does notchange the pH!

pH value of weak base solution (C> 10-3 M)

The main source of OH- is A, not H2O

[HA] = [OH-]

][

]][[

A

HAOHKb

[A] = C - [OH-]

A small number

C

OHOHKb

]][[

[OH-]2 = KbC

CKOH b ][

)][(][ OHCKOH bCheck the resulting [OH-] and correct if necessary

A + H2O HA + OH-

KaKb =[H+][OH-]=10-14

pH value of mixture of a weak base and weak acid (C> 10-3 M)

NHH

H + H+N+

HH

H

H

HA A + H+

][

]][[

HA

AHKa

][

][][

A

HAKH aAt high concentrations,

[A]=CA and [HA]=CHA

A

HAa C

CKH ][

acid

basea C

CpKpH log

pH value of mixture of a weak base and weak acid (C> 10-3 M)

A

HAa C

CKH ][

acid

basea C

CpKpH log

buffer - pH does not change when you varyconcentration a little

In practice, one buffer pair (acid+ conjugate base) determines thepH value of the whole multicomponent mixture

acid

basea C

CpKpH log

A complex mixture:

NHH

H + H+N+

HH

H

H

0.1 M 0.1 M

Harris, p. AP12

pKa=9.24

pH= 9.24

][

][log

HA

ApKpH a

All other species in the system(indicator, any admixtures, …)will follow the pH determinedby the buffer

Enzyme activity as a functionof pH

Effect of [A]/[HA] on pH or

Effect of pH on [A]/[HA]

)(

)(

)(

)(_

pHd

Cd

pHd

CdcapacityBuffer AB

pK=5

pH calculations: pH = -log[H+]

KaKb =[H+][OH-]=10-14

K>1 Strong acid or strong base

[H+]=Cacid [OH-]=Cbase

K<<1 Weak acid or weak base

CKOH b ][CKH a ][

A mixture of a weak base and weak acid

acid

basea C

CpKpH log