chapter 7 acids and bases. arrhenius definitions - acids produce hydrogen ion in aqueous, and...
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Chapter 7
Acids and Bases
Acids and Bases
Arrhenius Definitions- Acids produce hydrogen ion in aqueous, and bases produce hydroxide ions.
Brønsted-Lowry Definitions- An acid is a proton (H+) donor, and a base is a proton acceptor.
Lewis Acid-Base Definitions- An acid as an electron pair acceptor and a base as an electron donor.
An Acid is Dissolved in Water
HA(aq)+H2O(l) H3O+(aq)+A-(aq)
acid base conjugate acid
conjugatebase
3[ ][ ] [ ][ ]
[ ] [ ]
: acid dissociation constant
a
a
H O A H AK
HA HA
K
Conjugate Acid-Base Pair
Conjugate acid-base pair consists of two substances related to each other by the donating and accepting of a single proton.
Water as an Acid and a Base
Water is the common amphoteric substance.Kw=[H3O+][OH-]=1×10-14 ion-product constant
pH=-log[H+]
2H2O(l) H3O+(aq)+OH-(aq)
Calculating the pH of Weak Acid Solutions
HF H++F-
1.0MBefore 0 0Equilibrium (1-x)M xM xM
24 2 4
2 4 4 2
2
7.2 10 7.2 10 ( 1)1
7.2 10 7.2 10 2.7 10
log(2.7 10 ) 1.57
a
xK x if x
x
x x
pH
The 5% Rule The validity of an approximation should
always be check. The 5% rule should be used to evaluate which
approximation are reasonable.
%.%.
.For HF,
%%[HA]
x
-
72100001
1072
5100
2
0
The approximation is acceptable.
The pH of a Mixture of Weak Acids
HNO2 is assumed to be the dominant producer
of H+.
HCN(aq) H+(aq)+CN-(aq) Ka=6.2X10-10
HNO2(aq) H+(aq)+NO2-(aq) Ka=4.0X10-4
H2O(l) H+(aq)+OH-(aq) Kw=1.0X10-14
4 2
2
[ ][ ]4 10
[ ]a
H NOK
HNO
HNO2(aq) H+(aq)+NO2-(aq)
5 0 0
5-x x x2 2
4 2
2
2 0
4 10 4.5 105 5
Using the 5% rule
4.5 10100% 100% 0.9%
[ ] 5
a
x xK x
x
x
HNO
2
-
- 2 -10
- -8
[ ] 4.5 10 1.35
To calculate [CN ]
[ ][CN ] (4.5 10 )[CN ]6.2 10
[HCN] 1
[CN ]=1.4 10 ( )
a
H pH
HK
M
Percent Dissociationamount dissociation
Percent Dissociation= 100%initial concentration
HC3H5O3 H++C3H5O3-
0.1 0 00.1-x x x
3
3 34
3.7% 100% 3.7 10 ( )0.1
(3.7 10 )(3.7 10 )1.4 10
0.1a
xx M
K
Polyprotic AcidsDiprotic Acids
H2CO3(aq) H+(aq)+HCO3-(aq)
HCO3-(aq) H+(aq)+CO3
2-(aq)
1
2
73
2 3
2113
3
[ ][ ]4.3 10
[ ]
[ ][ ]5.6 10
[ ]
a
a
H HCOK
H CO
H COK
HCO
The Concentration of Species of Polyprotic Acid As a Function of pH
21312
3
212
21312
3
311
21312
3
23
0
210
2
212
0
22
][][
][][
][
][][
][
1
aaa
aa
aaa
a
aaa
TTT
T
KKOHKOH
KKα
KKOHKOH
OHKα
KKOHKOH
OHα
ααα
C
][M , α
C
][HM , α
C
M][Hα
][M][HMM][HC
Polyprotic Acid- H3PO4
a3a2a1a2a12
a13
a3a2a13
a3a2a1a2a12
a13
a2a12
a3a2a1a2a12
a13
2a1
1
a3a2a1a2a12
a13
3
0
3210
T
34
3T
24
2T
421
T
430
34
244243T
KKK][HKK][HK][H
KKKα
KKK][HKK][HK][H
][HKKα
KKK][HKK][HK][H
][HKα
KKK][HKK][HK][H
][Hα
1αααα
C
][POα ,
C
][HPOα ,
C
]PO[Hα ,
C
]PO[Hα
][PO][HPO]PO[H]PO[HC
Calculate the equilibrium concentration of the different species in a 0.1 M phosphoric acid solution at pH=3.
(M)103.3103.30.1C][PO
103.3
(M)109.6109.60.1αC][HPO
109.6
(M)109.20.920.1αC]PO[H
92.0
(M)108.3108.30.1]PO[H
103.8
15143POH
-34
143
652POH
-24
52
21POH
-42
1
320POH43
20
43
43
43
43
C
Acid-Base Properties of Salts Salt that produce neutral solutions
Salts that consist of the cations of strong
bases and the anions of strong acids have no
effect on [H+] when dissolved in water.
The aqueous solutions of salts such as KCl,
NaCl, NaNO3 and KNO3 are neutral.
Acid-Base Properties of Salts Salt that produce basic solutionsC2H3O2
-(aq)+H2O(l) HC2H3O2(aq)+OH-(aq)
2 3 2
2 3 2
2 3 2 2 3 2
2 3 2 2 3 2
1410
5
[ ][ ]
[ ]
[ ][ ] [ ][ ]
[ ] [ ]
1 105.6 10
1.8 10
b
a b w
wb
a
HC H O OHK
C H O
H C H O HC H O OHK K K
HC H O C H O
KK
K
Acid-Base Properties of Salts Salt that produce acidic solutions
NH4+(aq) NH3(aq)+H+(aq)
14103
54
[ ][ ] 1 105.6 10
[ ] 1.8 10w
ab
NH H KK
NH K
Water Contributes to the Very Dilute H+ Concentration
HA(aq) H+(aq)+A-(aq)
H2O(l) H+(aq)+OH-(aq)
0
[ ][ ]
[ ][ ]
[ ]
[ ] [ ] [ ] charge balance
[ ] [ ] [ ] material balance
w
a
K H OH
H AK
HA
H A OH
HA HA A
0 0
0
2
2
0
[ ] [ ] [ ] [ ] [ ][ ]
[ ] [ ] [ ] [ ] [ ] [ ][ ]
[ ] [ ][ ][ ][ ]
[ ][ ] [ ]
[ ]
[ ]
[ ][ ]
[ ]
w
w
w
a
w
w
w
KH A OH A H
H
KHA HA A HA HA H
H
KH H
HH AK
HA KHA H
H
H K
H KHA
H
+ 2 + 2 + 2
+ 2 2
+ 20 0
0
Assume that the condition
[H ] [H ] [H ]
[ ] [ ][ ] [H ]
[ ] [H ] [ ][ ] [ ]
[ ]
w w
w
a
w
K K
KH H
H xK
HA HA xKHA H
H
If [H+]2>>Kw, the full equation reduces to
the typical expression for a weak acid.
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