Lecture 30 - organic.cc.stonybrook.edu

Lec-25-Thermo2-Complete.ppt 11/1/2021

Thermochemistry

Chemical Reactions. Hess Law.

Calorimetry 1

1

Lecture 30Acids and Bases

Chapter 14

Office Hours:

Monday 2:30 PM – 4:30 AM Zoom meeting Accessed

From Blackboard.

Fernando Raineri

Chemistry Department Room 519

1


Thermochemistry


Calorimetry 2

Acid-base reactions involve two conjugate acid-base pairs

2

H+ donor H+ acceptor

CH3COOH + H2O → H3O+ + CH3COO–

Acid Base New acid New base

Pair 1

Pair 2

H2O + NH3 → NH4+ + OH–

Acid Base New acid New base

Note: water can act as either an acid or a base - amphoteric

2


Thermochemistry


Calorimetry 3

Autoionization of Water

32 H O O2H O H+ −+

( )3

141.0 10 atH O H 25O CwK −+ − = =

In pure water:3H O OH+ −=

1

3

741.0 10m

H O Ool

1.0 10L

H wK −+ −−= = == 3

Kw is referred to as the ionization product of

water. Notice that it is an equilibrium constant.

2

3 3H O H Ow wK K+ + ==

3


Thermochemistry


Calorimetry 4

The pH Scale

3pH log H O , pOH log OH−+ = −= −

( )

( )

3

7

7

pOH

pH log H O log 1.0 1

log OH log 1.0 0

7

7

0

1− −

+ −

=

= − = − =

− = − = 4

In pure water

1

3

7OH 1.0 10 mH o O l−+ − −= =

It is convenient to measure the concentrations

of hydronium and hydroxide ions in solution

using the alternative scales:

( ) ( )p loan gything anything= −

4


Thermochemistry


Calorimetry 5

•The pH of a solution is defined as pH = -log[H3O+]

5

( ) ( ) ( )0logl 1o log 10 0g 10 ,x x = ==

( ) ( ) ( )

( ) ( )

log log log

log log log

ab a b

aa b

b

= +

= −

( ) pH

3 3pH log H O H O 10

log( ) 10 yy x x

+ + − = − =

= =

5


Thermochemistry


Calorimetry 6

[H3O+] pH

1 0

10-1 1

10-2 2

10-3 3

10-4 4

10-5 5

10-6 6

10-7 7

10-8 8

Increasing [H3O+] Increasing pH

6

( ) ( )3pH log H O , log 10 x x−+ −− ==

6


Thermochemistry


Calorimetry 7

7

3 , pOH log OH l g H O Hp o −+ = − = −

3

141.0 1 H0 O OHwK −− += =

If we know the concentration of one of the ions

(hydronium or hydroxide) we can find the

concentration of the other species (both in neat water

and in solution)

4

3

1

OHH O

OH

1.0 10wK −

−

+

−

=

=

3 3

14

H OOH

1.0 10

H O

wK+ +

−− = =

7


Thermochemistry


Calorimetry 8

8

3 , pOH log OH l g H O Hp o −+ = − = −

3

141.0 1 H0 O OHwK −− += =

If we know the pH (or pOH) we can find the pOH (or pH)

(both in neat water and in solution).

14.0pHpK 14.0

14p

pOHpOHH

pH0H .pOw

= −= = +

= −

( ) ( )

( ) ( )

14

pK 14.0

3

H pOp H

l log OH

log log

og H O

1.0 10

w

wK

+

−

− − −

=

= +

− −

8


Thermochemistry


Calorimetry 9

More explicitly

7

7

3

7 7

3

7

3

mol molAcid pH 7 H O 10 ; O

molNeutral pH 7 H O OH 10

mol molBasic pH 7 OH 10 ; H O 1

H

0

10+ − −

− − + −

+

−

− −

= =

=

9

In terms of pH, for aqueous solutions at 25°C

Acid solution pH

Neutral solut

Basic s

io

ol

n pH

ut

7

H 7

7

ion p

=

9


Thermochemistry


Calorimetry 10

10

Strong Acids: They fully dissociate in water

( ) ( ) ( ) ( )

3

00

0

2

initial

HA H O

HA 0

0 Hequil

AH

HA A

0

O

.

aaq aq q+−+ → +

In particular 3 0H O HA+ =

Strong Bases: They fully dissociate in water

( ) ( ) ( )

0

00

initial

NaOH Na

0

equ

OH

il NaO

NaO 0

NaOH

H

H. 0

a aqaqq − +→ +

This implies 0

OH NaOH− =

10


Thermochemistry


Calorimetry 11

The pH Scale

3

NaOH Na OH

OH 10 mol/L

+ −

− −

+

=

What is the pH of a 10–3 mol L–1 NaOH(aq)

solution?

11

( )

1411

3 3 3

11

3

10H O OH H O 10 mol/L

10OH

pH log H O log 10 11

ww

KK

−+ − + −

−−

+ −

= = = =

= − = − =

11


Thermochemistry


Calorimetry 12

Weak Acids and Bases: Ionization Constants

The magnitude of the ionization constants of acids and

bases give us an immediate idea of their strength.

( ) ( ) ( ) ( )2 3HHA O A aaq qH O aq aq+ −+ +

Acid Ionization Constants:

( )

3 acid ionization,

constantH

H OpK log

A

Aa aa KK

−+

= − =

Weak acids have

( )weak acids ,0 p 01 KaaK 12

12


Thermochemistry


Calorimetry 13

Relative Strength

of Acids and Bases

2 3HA H OH O A−++ +

3[H O[ ]]

[ H ]

A

AaK

+−

=

13

ionization

constant

of the acid

aK

=

2H O H + A A OH− −+

13


Thermochemistry


Calorimetry 14

Ionization Constants of Acids and Basis

When a solution of acetic acid is prepared 0.01 mol L–1

4 1

3H O 4.15 10 mol L+ − − = 14

For acetic acid

53 AcO

1.H

8 10cO

H O

AaK

−+

− =

=

3

HAcO

CH COOH

14


Thermochemistry


Calorimetry 15


15

3 3at equilibrium

initial concentration 0

H O H O

ac

degree of

i id HAonization

+ +

= ==

percentage

ionizat0

ion10

=

4 12

1

4.15 10 mol L4.15 10

0.01mol L

%ionization 100 4.15%

− −−

−

= =

= =

For acetic acid at [HAcO]0 = 0.01 mol L–1

Both α and the percentage

ionization depend on the

concentration of the acid.

15


Thermochemistry


Calorimetry 16

16

pH of solutions of acetic

acid a function of the

analytical concentration

of the acid [HAcO]0 .

2 3HAcO H O AcO H O− ++ +

Weak acids are

increasingly ionized

as the analytical

concentration of the

acid decreases.

16


Thermochemistry


Calorimetry 17


( ) ( ) ( ) ( )2 BHB aq OH aqH O aq+ −+ +

Base Ionization Constants:

( )

OH

B

Base Ionization

Constan

BB

H

tbK

−+

=

17

( ) ( ) ( ) ( )2 43 NNH aq OH aqHH O aq+ −+ +

17


Thermochemistry


Calorimetry 18


( ) ( ) ( ) ( )2 BHB aq OH aqH O aq+ −+ +

( )

( ) ( )B

B pK B BOH H

oB

l g,b b bK K

−+ = −

=

( ) ( ) ( ) ( )2 3HHA O A aaq qH O aq aq+ −+ +

( )

( ) ( )3

,HA pK HA log HAAH O

HAa a aK K

−+

= − =

18

18


Thermochemistry


Calorimetry 19


( ) ( ) ( ) ( )HA pK A 4HA pK A K, p 1a aw wb bK K K− −= + = =

( ) ( ) ( ) ( ) ( )

( ) ( ) ( ) ( ) ( )2 3

2

HA aq H O aq HAA aq

A aq OH H aq

H O

O a AHA q b

a

K

K−

− − −

++ +

+ +

( ) ( )

3H O HAHA

HA

A OHA

Aa bK K

− −

−

−

+

=

19

( ) ( ) ( )2 3 a2 qH O q OH O H a wK+ −+

( ) ( ) conjugate baseacid A,HA baK KK K −= =

19


Thermochemistry


Calorimetry 20

Acid pKa Conjugate Base

HClO4 –10 ClO4–

HI –10 I–

H2SO4 –10 HSO4–

HBr – 9 Br–

HCl – 7 Cl–

HNO3 – 2 NO3–

H3O+ – 1.74 H2O

HSO4–

+ 1.99 SO42–

H3PO4 + 2.12 H2PO4–

HF + 3.17 F–

CH3CO2H + 4.75 CH3CO2–

NH4

++ 9.24 NH3

H2O +15.74 OH–

CH3OH +18 CH3O–

HC≡CH +25 HCC–

NH3 +34 NH2

–

CH2=CH2 +36 CH2CH–

CH3–CH3 +42 CH3CH2

–

More

acidic

More

basic

20

pK loga aK= −

20


Thermochemistry


Calorimetry 21

Acid pKa Conjugate Base

HClO4 –10 ClO4–

HI –10 I–

H2SO4 –10 HSO4–

HBr – 9 Br–

HCl – 7 Cl–

HNO3 – 2 NO3–

H3O+ – 1.74 H2O

HSO4–

+ 1.99 SO42–

H3PO4 + 2.12 H2PO4–

HF + 3.17 F–

CH3CO2H + 4.75 CH3CO2–

NH4

++ 9.24 NH3

H2O +15.74 OH–

CH3OH +18 CH3O–

HC≡CH +25 HCC–

NH3 +34 NH2

–

CH2=CH2 +36 CH2CH–

CH3–CH3 +42 CH3CH2

–

More

acidic

More

basic

Weak

Acids

In H2O

Strong

Acids

in H2O

Non

Acids

in H2O

Weak

Bases

in H2O

Non

Bases

in H2O

Strong

Bases

in H2O

21

pK loga aK= −

21


Thermochemistry


Calorimetry 22

Acid-Base Calculations

( ) ( ) ( ) ( )

3

0

2

0

HClO aq H O aq

HCinit.

chan

lO 0

HClO

C

ge

eq

lO aq

0

l.

O

ui

H

x xx

x x x

− +

−

+

−

+

concentrations at equilibru

23

0 0

m

H O

HClO HClO HClO

ClOa

x x

x x

xK

− +

−=

−==

Hypochlorous acid HClO has an ionization constant of

Ka=3.2×10–8 in water at 25°C. What is the pH of a

solution that is 0.35 mol L–1 in HClO?

22

22


Thermochemistry


Calorimetry 23

Acid-Base Calculations

2

0 0HClO HClO

ax

x xK

x

−=

Assuming that [HClO]0>>x (because Ka is so

small) we can approximate

( ) ( )8 4 1

0HClO 0.35 1.063 10 mo.2 1 l L0 ax K − − −==

This agrees with the exact calculation (without

neglecting x in the denominator).

( )3pH log H O log 3.9x+ = − = − = 23

4

30

mol molHClO 0.35 H O 1.06 10+ − = =

Indeed

23

Lecture 30 - organic.cc.stonybrook.edu

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