Microsoft PowerPoint - ch07Jwu-Ting Chen
Department of Chemistry, National Taiwan University Taipei, Taiwan 106, jtchen@ntu.edu.tw
What is acid? What is base?
Are acids and bases two classes of substances that have opposite properties, and their character could be canceled off by meeting each other?
Background
Acids are a class of substances that taste sour
By tastes
Bases (alkalis) are a class of substances that taste bitter
HCl was called salt spirit HCl
By Alchemist Experience
Minerals without metal often give acidic solutions

Theory of Ionization
HCl(aq) → H+ (aq) + Cl(aq)
NaOH(aq) → Na+ (aq) + OH
Electrolytes & Non-electrolytes
Brønsted-Lowry concept
Conjugate acid-base pair
Arrhenius vs Brønsted-Lowry concepts
acid conjugate
base conjugate
base conjugate
Self- Ionization of Water
H3O is hydrated proton, and called hydronium ion (H3O)
Actually, either H+ or OHis hyrdated
The chemistry of aqueous solution is unique
2H2O(l) H3O+ (aq) + OH-
[H2O]
Kw = [H3O][OH], and is called ion product constant Kw = [H3O][OH,
At 25, [H3O] = [OH] = 1.0×10-7 M; Kw = 1.0×10-14
Self ionization of water is in chemical equilibrium
in ethanol
Acid-Base in Other Solutions
Ethoxide is a stronger base than water, and protonated ethanol is a stronger acid than water.
2EtOH EtO- + EtOH2 +
in ammonia
+ > H3O+
O
N
O
O
H
O
N
O
O
O
N
O
H
O
N
O
The higher oxidation state of the central atom, the stronger acid it will be.
perchloric acid
Acid Strength
Ka acid dissociation constant, indicating the extent of dissociation of acids Ka
the equilibrium of strong acid goes toward its complete dissociation the weak acid reaches its equilibrium with partial dissociation pH = log [H+] the pH scale of aqueous solutions is <14 pH<14
pH meterSaturated Calomel Electrode (SCE) Hg + Cl- = ½ Hg2Cl2 + e-
Acid Strength pH of strong acid, [H] = [HA]0 Leveling Effect
HClO4 H2SO4 HCl HBr HI HNO3
H2O H+ + A-
All strong acids or bases show the same strength in the same solvent. In aqueous solution, the strongest acid allowed is H+, the strongest base allowed is OH-. H+OH–
Acid Strength
[HA]0 >> x, x = [H+] = (Ka[HA]0)1/2
x2
C C Cα α α α
α α α = = =
α =
,
Example:Example: Calculate the pH of a solution that contains 1.00 M HCN (6.2×10-10) and 5.00 M HNO2 (4.0×10-4) and calculate [NO2
-] and [CN-]. HCN H+ + CN- Ka=6.2×10-10
HNO2 H+ + NO2 - Ka=4.0×10-4
H2O H+ + OH- Kw=10-14
-+ + 2 4 2
−× = = + 2[H ]
[CN ] 1.4 10 M
−
−
× × = ≈
∴ = ×
Nitrous acid is stronger than cyanic acid to the 6th order of magnitude, it will dominate [H+].
[H+] = [NO2 -] = 4.5x10-2 M pH = 1.35
The dissociation of cyanic acid is suppressed by nitrous acid.
The pH of very dilute acid ([H+] < 1.0x10-6 M)
,
+ -
charge balance: [H+] = [OH -] + [A-] ……(eq 4)
……(eq 1)
……(eq 2)
+ - -
[H ] K[A ]=[H ]-
[H ] [HA]=[HA] -[A ]
K =[HA] -([H ]- ) [H ]
K[H ]([H ]- ) [H ] K[H ]K = K [H ] -K[HA] -([H ]- ) [HA] -( )[H ] [H ]
[H ]if [H ] K K [HA] [H ]
∴
− =
>> = −
learn mass and charge balance
learn how to solve the unknowns
learn how to do approximation
learn the meaning of approximation
if [HA]0 >> ([H+]2 – Kw)/[H+] [H+] = (Ka[HA]0 + Kw)1/2
Polyprotic (Polybasic) Acids
,
charge balance: [H3O+] = [OH -] + [HA-] + 2[A2-] ……(eq 4)
…(eq 1)
…(eq 2)
Unknowns: [H+], [OH-], [H2A], [HA-], [A2-] Knowns: CH2A, K1, K2, Kw
H2A + H2O H3O+ + HA-
HA- + H2O H3O+ + A-2
+
+= −
[HA-]>>[A2-]
[H3O+] ≈ [HA-]
2
2
+ 3 1 H A
if C >>[H O ]
[H O ]= K C
,
a -2 8 4
[H ]=1.9 10 pH=0.72
[H ][HPO ]K 6.2 10 [H PO ]
[H ] [H PO ]
[H ][PO ] 0.19[PO ]K 4.8 10 6.2 10[HPO ]
[PO ]=1.6 10
,
H2SO4 H2O HSO4
2-+ 2- -2 3 4
4- 4
≈ ×
a proper assumption.
- 2-+ 3 4 4
2-+ 3 4
- 2- 4 4
2 - 2- 4 4
[H O ]- C [SO ]
[H O ]= 0.0100 + [SO ]
[H O ][SO ] (0.0100 [SO ])[SO ]K [HSO ] (0.0100 [SO ])
[SO ]=4.5 10 M [H
=
+ = =
−
× +
metal oxides : CaO, BaO
metal hydrides : NaH, KH
metal amides : NaNH2, LiNMe2, NaOMe
weak acid salts : NaOAc, NaCN, Na2CO3
N
amphetamine
dopamine
Salts
Neutral Salts: salts formed from strong acid & strong base are always of neutral salts, pH = 7.
HCl(aq) + NaOH(aq) NaCl(aq) + H2O
Acidic Salts: salts formed from strong acid & weak base
HCl + NH3 NH4Cl
Ka = [NH3][H3O+]
[H3O+] = (Ka[NH4 +])1/2 = (Ka[NH4Cl])1/2
= 2.4x10-5 M
Mass balance: CNaHA = [H2A] + [HA-] + [A2-]
HA- + Na+NaHA
[HA-]
[HA-]
[H2A] = [A2-] + [OH-] - [H3O+]
3 - 1
K [HA ] K [HA ] [OH ]-[H O ] K [OH ] [H O ]
[H O ][HA ] KK [HA ] [H O ] K [H O ] [H O ]
[HA ][H O ] ( +1)=K [HA ]+K K
K K [HA ]+K K[H O ]= [HA ]+K
= +
= + −
[H O ]= K K 1 1pH= pK + pK 2 2
if both Kb”, K2 are small [HA-] ≈ CNaHA
and K1 << [HA-]= CNaHA
NH4 + + OAc-NH4OAc
Charge balance: [NH4
+] + [H3O+] = [OH-] + [OAc-]
- + 2 + + 2w 3
- ++ 2 w
+ + w a 4 b
+ - a b 4 NH OAc
K [OAc ][H O ] K[NH ] -[H O ]= - [H O ] K K [H O ]
K [OAc ][H O ][NH ] -[H O ] = -K K K
K[OAc ](1+ )[H O ] [NH ] K K K
K K ([NH ]+K )[H O ]= K (K [OAc ])
K , K are small, [NH ]=[OAc ] F
[H
K KO ]= K
>
HA/NaA H2A/ NaHA or H2A/Na2A NaHA/Na2A
1
2
[A ]pH=pK +log [HA ]
or
COOH
Method 1. +
[H O ]=6.5 10 M pH=3.19
−= ×
0.2 =2.89+log 0.1