Lecture5
Acid-Base,+pH+and+Buffer
Instructor:-Nguyen-ThaoTrang
Semester-I 201 20209-
Analytical Chemistry 1
Acid-Base*Definition
Definition*of*Arrehnius(Sweden,*1884):
Acid:
Acid*is*a*substance*that*can*release*proton*or*hydrogen*ion*when*
dissolved* in*water.
Must*have*H*in*formula.*The*general*formula* is*H
n
X.
Monoprotic:-one-acidic-proton-
HNO
3
Nitric* acid
HClHydrochloric*acid
HBr Hydrobromicacid
CH COOH*
3
Acetic*acid
2
HCl
H
+
+*Cl
-
Acid-Base*Definition
Definition*of*Arrehnius(Sweden,*1884):
Acid:
Diprotic:-two-acidic-protons
H
2
SO Sulfuric*acid
4
H
2
SO Sulfurous*acid
3
Triprotic:-three-acidic-protons
H
3
PO
4
Orthophosphoricacid*(Phosphoric*acid)
Base:
Any*substance*that*dissociates* in*H
2
O*to*give*OH
-
ions.
The*general*formula*is*M(OH)
n
:* NaOH,*KOH,*Ca(OH)
2 2
,*Mg(OH) ,*
Al(OH)
3
3
NaOH Na
+
+*OH
-
Acid-Base*Definition
Definition*of*Arrehnius(Sweden,*1884):
Strength*of*Acid/Base:
Acids*and*bases*are*commonly*classified*as*strong*or*weak,*depending*
on*whether*they*react*nearly*completely”*or*only*“partially”*to*
produce*H
+
or*OH
-
.
Example:*
In*a*6*M*solution*of*HCl,*99.996%*HClmolecules*reacts*with*water*to*form
H
+*
and*Cl
-
à HClis*a*strong-acid.
Acetic*acid*is*a*weak*acid*as*1%*CH
3
COOH*reacts*with*water*to*form*H
and*CH
3
COO CH COOH*is*a*
-
à
3
weak-acid.
Equilibrium*constant*or*dissociation*constant*K
a
(acid)*or*K
b
(base)*
reveals*if*an*acid/base*is*strong*or*weak.
4
Acid-Base*Definition
Definition*of*Arrehnius(Sweden,*1884):
Strength*of*Acid/Base:
Example:
5
CH
3
COOH*(aq)*****=****CH )**+****H
3
COO
-
(aq
+
( )aq
K
a
=
[CH
3
COO
-
]**[H
+
]
[CH
3
COOH]*
=**1.76*x*10
-5
pK
a
(*=*negative*of*log
10*
K
a
)*can*also*be*used:*
pK
a
=**- log
10
(1.76*x*10
-5
)**=**4.75
Acid-Base*Definition
Definition*of*Arrehnius(Sweden,*1884):
Strength*of*Acid/Base:
Example:
6
pK
b
(*=*negative*of*log
10*
K
b
)*can*also*be*used:*
pK
b
=**- log
10
(1.76*x*10
-5
)**=**4.75
NH
4
OH*(aq)*****=****NH )**+****OH
4
+*
(aq
-
(aq)
K
b
=
[NH
4
+
]**[OH
-
]
[NH
4
OH]*
=**1.76*x*10
-5
Acid-Base*Definition
Definition*of*Arrehnius(Sweden,*1884):
Strength*of*Acid/Base:
The*strongerthe*acid,*the*larger-the-K
a
(the*smaller*the*pK
a
):
7
HNO
2
(aq)*****=***NO
2
-
(aq)**+****H
+
(aq)******************** K
a
=*4
pK
a
=*3.34
CH
3
COOH*(aq)*****=****CH )**+****H
3
COO
-
(aq
+
(aq)************K
a
=*1.
pK
a
=*4.75
HCN*(
aq)*****=***CN
-
(aq)**+****H
+
(aq)***************************
pK
a
=*9.21
stronger
weaker
Acid-Base*Definition
8
Acid-Base*Definition
9
Acid-Base*Definition
Definition*of*Arrehnius(Sweden,*1884):
Strength*of*Acid/Base:
The*strongerthe*base,*the*larger-the-K
b
(the*smaller*the*pK
b
):
10
stronger
weaker
PO
4
3-
(aq)**+**H
2
O*(l)***=***HPO
4
2-
(aq)**+****OH
-
(aq)********K
b
=*4.5*x
pK
b
=**1.34
NH
4
OH*(aq)*****=***NH
4
+
(aq)**+****OH
-
(aq)********************K
b
=
pK
b
=**4.74
Acid-Base*Definition
Definition*of*Arrehnius(Sweden,*1884):
Strength*of*PolyproticAcid/Base:
Example:*oxalic*acid*is*diprotic*and*phosphate*is*tribasic:
11
K
a1
(or*K
1
)*refers-to-the-acidic-species-with-
the-mostprotons.
K
b1*
refers-to-the-basic-species-with-the-least
number-of-protons.
Acid-Base*Definition
Definition*of*Arrehnius(Sweden,*1884):
Strength*of*PolyproticAcid/Base:
12
Acid-Base*Definition
Definition*of*Arrehnius(Sweden,*1884):
Limitations:
Many*substances*dissolve*in*water*to*give*acidic*or*basic*solutions* but*
do*not*have*the*classical*acid*or*base*formulas.
Example:*A*solution*of*K
2
O*in*H O*is*identical*with*a*solution*of*KOH
2
H
2
O.*However*,*according*to*the*Arrhenius* system*K
2
O*cannot*be*
classified*as*
a*base*in*that*it*does*not*have*OH
-
s*in*its*formula.
13
Acid-Base*definition
Definition*of*Lowry*(England)*or*Bronstedworking*
independently*in*1923*(Bronsted-Lowry*definition):
Acid*is*a*material*that*donates*a*proton.
Base*is*a*substance*that*accepts*a*proton.
The*BronstedLowry*definition*can*be*extended*to*gas*or*nonaqueous
solvent:
14
HCl
H
+
+*Cl
-
NH NH
4
+
3
+*H
+
OH
-
+*H
+
H
2
O**
NH
3
+*H
+
NH
4
+
Acid-Base*definition
Bronsted-Lowry*definition:
Acid:*must*have*a*H*in*its*formula.
Base:*must*have*a*lone*pair*of*electrons*to*form*a*covalent*bond*with*
H
+
The*product*formed* when*an*acid*gives*up*a*proton*is*a*potential*
proton*acceptor*and*is*called*the*conjugate-base-of*the*parent*acid.
The*product*formed* when*a*base**accepts*a*proton*is*a*potential*
proton*donator*and*is*called*the*conjugate-acid-of*the*parent*base.
NH
4 3
+
/NH :*a-conjugate-acid/base-pair.
15
NH NH
4
+
3
+*H
+
(Acid)
(Conjugate-base)
NH NH
3
+*H
+
4
+
(Base)
(Conjugate-acid)
Acid-Base*definition
Bronsted-Lowry*definition:
General:
Combination*of*2*reactions:*neutralization-reaction
Direction-of-the-reaction-depends-on-which-acid-donates-proton-stronger-
or-which-base-accepts-proton-stronger.
Example:
16
Acid-Base*definition
Bronsted-Lowry*definition:
Proton*H
+*
does*not*exist*by*itself*in*water.**It*exists*in*the*form*of*H
3
(a*proton*covalently*binds*to*a*water*molecule).
H
5
O
2
+
cationis*another* simple*species.
Hydroxyl*ion*OH
-
also*exists*in*the*form*of*H
3
O
2
-
(OH
-
.H
2
O)
17
Acid-Base*definition
Bronsted-Lowry*definition:
Species*possess* both*acidic*and*basic*properties:*amphiprotic.
Example:
H
2
PO
4
-
acts*as*a*base:
H
2
PO
4
-
acts*as*an*acid:
Amino*acids*are*amphiproticcompounds:*rearrange*to*form*
zwitterionicspecies*that*bears*both*a*positive*and*a*negative*charge.
H
2
O,*methanol,*ethanol*are*common*amphiproticsolvent.
18
Acid-Base*definition
Bronsted-Lowry*definition:
Autoprotolysis:* self-ionization*(autoionization)*to*form*a*pair*of*ionic*
species
The*extent*of*these*reactions*is*very*small.*The*autoprotolysisconstant-
(equilibrium*constants)*for*H
3
O
+
/OH
-
pair*is*1×10
-14*
M*at*25
o
C.
19
Acid-Base*definition
Bronsted-Lowry*definition:
Autoprotolysisconstant*for*H
2
O*has*the*special*symbol*K
w
K
w
=*1.0110
-14*
or*pK
w
=*14
K
w
depends*on*temperature
20
pH
An*approximate*definition*of*pH+is*the*negative*logarithm*of*
the*H*concentration.
Relationship*between*pH*and*K
w
21
Acidity/basicity*of*a*solution
Solution*is*acidic+if*[H
+
]*>*[OH
-
].
Solution*is*basic+if*[H
+
]*<*[OH
-
].
At*25
o
C,*an*acidic*solution*has*a*pH*below*7*and*a*basic*
solution*has*a*pH*above*7.
pH*generally*falls*in*the*range*0*to*14,*these*are*not*the*limi
of*pH.
22
Acidity/basicity*of*a*solution
What*is*pH*of*pure*water?
If*x*=*the*molarity*of*[H
+
]*,***
23
H
2
O** =**H K
+
+*OH
-
w
=*[H
+
]*[OH
-
]***=****10
-14
x********
x x
2 =*********
10
-14
Therefore,*****x*=*[H
+
]**=**[OH ]**=**10
- -7
M in*pure*water*at*25°C.
Acidity/basicity*of*a*solution
The*degree*of*acidity*or*basicity*of*a*solution*is*measured*on
the*pH*scale
24
[H
+
] pH***********pOH
1*M 0*************14
10
-2
M 2 12
10
-4
M 4 10
10
-6
M 6 8
10
-8
M 8 6
10
-10
M 10 4
10
-12
M 12 2
10
-14
M 14 0
low*pH**or*high*pOH*is*strongly*acid
pH**7*is*a*neutral*solution
high*pH**or*low*pOHis*strongly*base
Acidity/basicity*of*a*solution
25
Acid-Base*definition
Bronsted-Lowry*definition:
Relative*strength*of*acids/bases:* Strong-acids-will-yield-weak-conjugate-
bases-and-weak-acids-will-give-strong-conjugate-bases.-
26
Acid-Base*definition
Bronsted-Lowry*definition:
Relationship*between* K
a
and*K
b*
for*a*conjugate*acid/base*pair:
For*a*diprotic*acid*or*triproticacid:
27
Acid-Base*definition
Definition*of*J.N.*Lewis:
Acid*is*a*substance*that*accepts*an*electron*pair*in*forming*a*
coordinate*covalent*bond.
A*Lewis*acid*must*have*a*fairly*low*energy*vacant,*or*easily*vacated*
orbital.
Example:
BF
3
(vacant*2p*orbital* on*B)
Ni
2+
(vacant*3d,*4s,*and*4p*orbitals)
SO
3
(the*S—O*π*bond*is*easily*broken)
Acid*are*electrophiles*because* they*react*readily*with*electron*pair*
donors.
28
Acid-Base*definition
Definition*of*J.N.*Lewis:
Base*is*a*substance*that*donates*an*electron*pair*in*forming*a*
coordinate*covalent*bond.
A*Lewis*base*must*have*a*lone*pair*of*electrons the*same*as*a*
Bronsted-Lowry*base.
Example:
The*base*is*a*nucleophile.
29
O-H
-
..
..
:
N-H
H
H
: O-H
H
:
..
I
..
..
:+++:
..
:S+:
..
2-
hydroxide++++ iodide+++ ammonia+++++++++ water++++++++++++++s
Acid-Base*definition
Definition*of*J.N.*Lewis:
The*Lewis+definitiongeneralizes*the*acid/base*concept:
Every*Arrhenius* acid/base*is*also*a*Lewis*acid/base.
Every*Bronstedacid/base*is*also*a*Lewis*acid/base.
Example:**A*strong*acid*reacts*with*a*strong*base
30
Least general definition Most general definition
Arrhenius Bronsted-Lowry Lewis
H
+
(aq) + :OH
-
(aq) = H
2
O(l)
Electron pair acceptor electron pair donor

Preview text:

Analytical Chemistry 1 Lecture5 Acid-Base,+pH+and+Buffer Instructor:-Nguyen-ThaoTrang Semester-I 2019-2020 Acid-Base*Definition
• Definition*of*Arrehnius(Sweden,*1884): Acid:
– Acid*is*a*substance*that*can*release*proton*or*hydrogen*ion*when* dissolved* in*water. HCl H+ +*Cl-
– Must*have*H*in*formula.* The*general*formula* is*HnX.
– Monoprotic:-one-acidic-proton- • HNO3 Nitric*acid • HClHydrochloric*acid • HBr Hydrobromicacid • CH3COOH*Acetic*acid 2 Acid-Base*Definition
• Definition*of*Arrehnius(Sweden,*1884): Acid:
– Diprotic:-two-acidic-protons • H2SO4 Sulfuric*acid • H2SO3 Sulfurous*acid
– Triprotic:-three-acidic-protons
• H3PO4 Orthophosphoricacid*(Phosphoric*acid) Base:
– Any*substance*that*dissociates* in*H2O*to*give*OH- ions. NaOH Na+ +*OH-
– The*general*formula*is*M(OH)n:*NaOH,*KOH,*Ca(OH)2,*Mg(OH 2 ) ,* Al(OH)3 3 Acid-Base*Definition
• Definition*of*Arrehnius(Sweden,*1884): Strength*of*Acid/Base:
– Acids*and*bases*are*commonly*classified*as*strong*or*weak,*depending*
on*whether*they*react*nearly*“completely”*or*only*“partially”*to* produce*H+ or*OH-. – Example:*
• In*a*6*M*solution*of*HCl,*99.996%*HClmolecules*reacts*with*water*to*form
H+*and*Cl- à HClis*a*strong-acid.
• Acetic*acid*is*a*weak*acid*as*1%*CH3COOH*reacts*with*water*to*form*H and*CH -
3COO à CH3COOH*is* a*weak-acid.
– Equilibrium*constant*or*dissociation*constant*Ka (acid)*or*Kb (base)*
reveals*if*an*acid/base*is*strong*or*weak. 4 Acid-Base*Definition
• Definition*of*Arrehnius(Sweden,*1884): Strength*of*Acid/Base: – Example: CH +
3COOH*(aq)*****=****CH3COO- (a ) q **+****H (a ) q [CH K 3COO-]**[H+] a = [CH3COOH]* =**1.76*x*10-5
pKa (*=*negative*of*log10*Ka)*can*also*be*used:*
pKa =**- log10(1.76*x*10-5)**=**4.75 5 Acid-Base*Definition
• Definition*of*Arrehnius(Sweden,*1884): Strength*of*Acid/Base: – Example: NH +* - 4OH*(aq)*****=****NH4 (a ) q **+****OH (aq) [NH +]**[OH-] K 4 b = [NH4OH]* =**1.76*x*10-5
pKb (*=*negative*of*log10*Kb)*can*also*be*used:*
pKb =**- log10(1.76*x*10-5)**=**4.75 6 Acid-Base*Definition
• Definition*of*Arrehnius(Sweden,*1884): Strength*of*Acid/Base: stronger
– The*strongerthe*acid,*the*larger-the-Ka (the*smaller*the*pKa): HNO -
2 (aq)*****=***NO2 (aq)**+****H+ (aq)******************** Ka =*4 pKa =*3.34 CH +
3COOH*(aq)*****=****CH3COO- (a )
q **+****H (aq)* ***********Ka =*1. pKa =*4.75
HCN*(aq)*****=***CN- (aq)**+****H+ (aq)*************************** pKa =*9.21 weaker 7 Acid-Base*Definition 8 Acid-Base*Definition 9 Acid-Base*Definition
• Definition*of*Arrehnius(Sweden,*1884): Strength*of*Acid/Base:
– The*strongerthe*base,*the*larger-the-Kb (the*smaller*the*pKb): stronger PO 3- 2-
4 (aq)**+**H2O*(l)***=***HPO4 (aq)**+****OH- (aq)********Kb =*4.5*x pKb =**1.34 NH +
4OH*(aq)*****=***NH4 (aq)**+****OH- (aq)********************Kb = pKb =**4.74 weaker 10 Acid-Base*Definition
• Definition*of*Arrehnius(Sweden,*1884):
Strength*of*PolyproticAcid/Base:
– Example:*oxalic*acid*is*diprotic*and*phosphate*is*tribasic:
Ka1(or*K1)*refers-to-the-acidic-species-with- the-mostprotons.
Kb1*refers-to-the-basic-species-with-the-least number-of-protons. 11 Acid-Base*Definition
• Definition*of*Arrehnius(Sweden,*1884):
Strength*of*PolyproticAcid/Base: 12 Acid-Base*Definition
• Definition*of*Arrehnius(Sweden,*1884): Limitations:
– Many*substances*dissolve*in*water*to*give*acidic*or*basic*solutions*but*
do*not*have*the*classical*acid*or*base*formulas.
– Example:*A*solution*of*K2O*in*H2O*is*identical*with*a*solution*of*KOH
H2O.*However*,*according*to*the*Arrhenius* system*K2O*cannot*be*
classified*as*a*base*in*that*it*does*not*have*OH-’s*in*its*formula. 13 Acid-Base*definition
• Definition*of*Lowry*(England)*or*Bronstedworking*
independently*in*1923*(Bronsted-Lowry*definition):
– Acid*is*a*material*that*donates*a*proton. HCl H+ +*Cl- NH + 4 NH3 +*H+
– Base*is*a*substance*that*accepts*a*proton. OH- +*H+ H2O** NH + 3 +*H+ NH4
– The*Bronsted–Lowry*definition*can*be*extended*to*gas*or*nonaqueous solvent: 14 Acid-Base*definition • Bronsted-Lowry*definition:
– Acid:*must*have*a*H*in*its*formula.
– Base:*must*have*a*lone*pair*of*electrons*to*form*a*covalent*bond*with* H+ NH + 4 NH3 +*H+ (Acid) (Conjugate-base)
– The*product*formed*when*an*acid*gives*up*a*proton*is*a*potential*
proton*acceptor*and*is*called*the*conjugate-base-of*the*parent*acid. NH + 3 +*H+ NH4 (Base) (Conjugate-acid)
– The*product*formed*when*a*base**accepts*a*proton*is*a*potential*
proton*donator*and*is*called*the*conjugate-acid-of*the*parent*base. – NH + 4 /N 3
H :*a-conjugate-acid/base-pair. 15 Acid-Base*definition • Bronsted-Lowry*definition: – General:
– Combination*of*2*reactions:*neutralization-reaction
• Direction-of-the-reaction-depends-on-which-acid-donates-proton-stronger-
or-which-base-accepts-proton-stronger. – Example: 16 Acid-Base*definition • Bronsted-Lowry*definition:
– Proton*H+*does*not*exist*by*itself*in*water.**It*exists*in*the*form*of*H3
(a*proton*covalently*binds*to*a*water*molecule). – H +
5O2 cationis*another* simple*species.
– Hydroxyl*ion*OH- also*exists*in*the*form*of*H - 3O2 (OH-.H2O) 17 Acid-Base*definition • Bronsted-Lowry*definition:
– Species*possess* both*acidic*and*basic*properties:*amphiprotic. – Example: • H - 2PO4 acts*as*a*base: • H - 2PO4 acts*as*an*acid:
– Amino*acids*are*amphiproticcompounds:*rearrange*to*form*
zwitterionicspecies*that*bears*both*a*positive*and*a*negative*charge.
– H2O,*methanol,*ethanol*are*common*amphiproticsolvent. 18 Acid-Base*definition • Bronsted-Lowry*definition:
– Autoprotolysis:* self-ionization*(autoionization)*to*form*a*pair*of*ionic* species
– The*extent*of*these*reactions*is*very*small.*The*autoprotolysisconstant-
(equilibrium*constants)*for*H3O+/OH- pair*is*1×10-14*M*at*25oC. 19 Acid-Base*definition • Bronsted-Lowry*definition:
– Autoprotolysisconstant*for*H2O*has*the*special*symbol*Kw
– Kw =*1.01*×10-14*or*pKw =*14 – Kw depends*on*temperature 20 pH
• An*approximate*definition*of*pH+is*the*negative*logarithm*of* the*H*concentration.
• Relationship*between*pH*and*Kw 21 Acidity/basicity*of*a*solution
• Solution*is*acidic+if*[H+]*>*[OH-].
• Solution*is*basic+if*[H+]*<*[OH-].
• At*25oC,*an*acidic*solution*has*a*pH*below*7*and*a*basic* solution*has*a*pH*above*7.
• pH*generally*falls*in*the*range*0*to*14,*these*are*not*the*limi of*pH. 22 Acidity/basicity*of*a*solution • What*is*pH*of*pure*water?
– If*x*=*the*molarity*of*[H+]*,*** H + 2O** =**H +*OH- Kw =*[H+]*[OH-]***=****10-14 x********x x2 =*********10-14
Therefore,*****x*=*[H+]**=**[OH-]**=**10-7M in*pure*water*at*25°C. 23 Acidity/basicity*of*a*solution
• The*degree*of*acidity*or*basicity*of*a*solution*is*measured*on the*pH*scale [H+] pH***********pOH 1*M
0*************14 low*pH**or*high*pOH*is*strongly*acid 10-2M 2 12 10-4M 4 10 10-6M 6 8 pH**7*is*a*neutral*solution 10-8M 8 6 10-10M 10 4 10-12M 12 2 10-14M 14 0
high*pH**or*low*pOHis*strongly*base 24 Acidity/basicity*of*a*solution 25 Acid-Base*definition • Bronsted-Lowry*definition:
– Relative*strength*of*acids/bases:* Strong-acids-will-yield-weak-conjugate-
bases-and-weak-acids-will-give-strong-conjugate-bases.- 26 Acid-Base*definition • Bronsted-Lowry*definition:
– Relationship*between* Ka and*Kb*for*a*conjugate*acid/base*pair:
– For*a*diprotic*acid*or*triproticacid: 27 Acid-Base*definition • Definition*of*J.N.*Lewis:
– Acid*is*a*substance*that*accepts*an*electron*pair*in*forming*a* coordinate*covalent*bond.
– A*Lewis*acid*must*have*a*fairly*low*energy*vacant,*or*easily*vacated* orbital. – Example:
• BF3 (vacant*2p*orbital*on*B)
• Ni2+(vacant*3d,*4s,*and*4p*orbitals)
• SO3 (the*S—O*π*bond*is*easily*broken)
– Acid*are*electrophiles*because*they*react*readily*with*electron*pair* donors. 28 Acid-Base*definition • Definition*of*J.N.*Lewis:
– Base*is*a*substance*that*donates*an*electron*pair*in*forming*a* coordinate*covalent*bond.
– A*Lewis*base*must*have*a*lone*pair*of*electrons*à the*same*as*a* Bronsted-Lowry*base. – Example: H .. .. .. .. 2- : N-H : O-H :S+: :O-H- .. : I..+++: .. H H hydroxide++++ iodide+++ ammonia+++++++++ water++++++++++++++s – The*base*is*a*nucleophile. 29 Acid-Base*definition • Definition*of*J.N.*Lewis:
– The*Lewis+definition generalizes*the*acid/base*concept:
• Every*Arrhenius* acid/base*is*also*a*Lewis*acid/base.
• Every*Bronstedacid/base*is*also*a*Lewis*acid/base. Least general definition Most general definition Arrhenius Bronsted-Lowry Lewis
– Example:**A*strong*acid*reacts*with*a*strong*base H+(aq) + :OH-(aq) = H2O(l) Electron pair acceptor electron pair donor 30