Download - 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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
Calorimetry 15
Ionization Constants of Acids and Basis
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
Calorimetry 17
Ionization Constants of Acids and Basis
( ) ( ) ( ) ( )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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
Calorimetry 18
Ionization Constants of Acids and Basis
( ) ( ) ( ) ( )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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
Calorimetry 19
Ionization Constants of Acids and Basis
( ) ( ) ( ) ( )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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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
Lec-25-Thermo2-Complete.ppt 11/1/2021
Thermochemistry
Chemical Reactions. Hess Law.
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