acid ose bases
TRANSCRIPT
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Acidez e Basicidade
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A substncia pode ser classificada como umcido or umabase.
A palavracidovem do Latim acere que significa azedo. Umabase um lcali, a qual derivada da palavra rabe al-quali.
A presena de cidos e bases reconhecida desde a
antiquidade.Lavoisier props que o oxignio seria o elemento dos quais oscidos seriam originados.
Liebig (Alemanha) props que um cido contm hidrognio.
Muitas reaes qumicas so classificadas como reaescido-base; no so necessariamente neutralizaes.
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Teoria cido-Base
Como voc descreveriaumcidoou umabase
? Questes semelhantes aparecem desde suadecrio no sc. XIII.
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cidos
Produzem ons H+ (como H3O+) em gua
Produzem tambm um on negativo (-)
Sabor azedo
Corroem metais
Reagem com bases formando sais e gua
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Bases
Produzem ons OH- em gua
Sabor amargo, adstringente
So eletrlitos
Textura de sabo, escorregadios
Reagem com cidos para formar sais e
gua
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Acid-Base Theory Arrhenius
(1884) Arrhenius defined the
terms acid and base as
follows: An acidis a substance
that produces H+ ionswhen dissolved in water
A baseis a substancethat produces OH- ionswhen dissolved in water
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Acid-Base Theory - Arrhenius
HNO3(l) H+(aq) + NO3
-(aq)H2O
Ca(OH)2(s) Ca2+(aq) + 2OH-(aq)H
2
O
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Problems with Arrhenius
theoryLike Daltons atomic theory, Arrhenius theory have problems today:Acidity did not show in other solvent. What is the solvent role?
Some salts produce acidic or basic solutions, not neutral. Why?
Which is the base, NH3 or NH4OH? Is OH
really the only base?How can H+ be stable? Are proton donated?
Some chemists want students to learn Brnsted-Lowry theory of acidsand bases before learning Arrhenius theory, because the former ismore general.
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Brnsted-Lowry Theory Brnsted and Lowry came up with an
alternative definition for acids andbases
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Brnsted-Lowry Theory
In the Bronsted-Lowry definition:
An acidis a substance from which aproton can be removed (proton donor)
A baseis a substance that can remove a
proton from an acid (proton acceptor)
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HCl + H2O H3O
+
+ Cl
-
acid base
Bronsted-Lowry Concept
Example:
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NH3 + H2O NH4+
+ OH-
base acid
Bronsted-Lowry Concept
Example:
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HCl + H2O H3O+ + Cl-
acid base
Bronsted-Lowry Concept
NH3 + H2O NH4++ OH-
base acid
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Brnsted-Lowry Concept
Bronsted-Lowry Concept (J. Brnsted,T. Lowry, 1923)
Acid-base reaction involves proton transfer
Acid: proton donor Base: proton acceptor
does not have to have OH- in formula
Water is amphoteric
Acids & bases can be molecules or ions
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Brnsted-Lowry Concept
NH4
+ + H2O H
3O+ + NH
3
baseacid
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NH3 + H2O NH4++ OH-
base acid
Brnsted-Lowry Concept
acid base
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NH3 + H2O NH4++ OH-
base acid
Brnsted-Lowry Concept
acid base
Acid-base conjugate pairs
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HA + B- A- + BH
acid baseconjugate
baseconjugate
acid
Brnsted-Lowry Theory
+H+-H+
acid = proton donor
base = proton acceptor
Conjugate Acid = Base + Proton
Conjugate Base = Acid - Proton
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Acid-Base Strength
Strong acids
Ionize completely in water
HX + H2O H3O+ + X-
Weak acids
Ionize only partially in water
HX + H2O H3O+ + X-
Common weak acids: Numerous moleculesand certain cations (well learn more about
these later)
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Strong Acids
HX H3O+ X-
Before
Equilibrium
At Equilibrium
100 %
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Common Strong Acids (Know these!)
HCl
HBrHI
HNO3
HClO4H2SO4
Acid-Base Strength
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Weak Acids
HX
Before
Equilibrium At Equilibrium
< 100 %
X-
HX
H3O+
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Acid-Base Strength
Strong bases
Ionize completely in water
NaOH Na+ + OH-
Common strong bases: MOH and M(OH)2, M2O
and MO (M = Grp IA and IIA metals)
Weak bases
Ionize only partially in water
B + H2O BH+ + OH-
Common weak bases: NR3 (R = H or other
chemical species) and certain anions
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Relative Acid-Base Strength
Strongest HClO4 ClO4-
WeakestAcids H2SO4 HSO4
-
Bases
HCl Cl-
HNO3 NO3-H3O
+ H2O
HSO4- SO4
2-
HNO2 NO2-
HClO ClO-
Weakest NH4+ NH3 Strongest
Acids H2O OH-
Bases
OH- O2-
Strongestacid that can
exist in H2O
Strongest base that
can exist in H2O
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More on Neutralization
Reactions
Acids react with bases:
acid + base salt + water
or
acid + base salt
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Autoionization of Water
In pure water, the following equilibrium
occurs: H2O + H2O H3O+ + OH-
The equilibrium constant for this reaction
is the ion product, Kw:
Kw = [H3O+][OH-]
[H3O+] = [OH-] = 1 x 10-7 M (25 C)
so
Kw = 1 x 10-14 (25 C)
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The pH scale
Srensen introduced the pH scale in 1909 using the symbol pH.
Thep is from the German wordpotenz, power of (10).
pH = log [H+]; [H+] = 10 pH
pOH = log [OH]; [OH] = 10 pOH
pK= log [K]; K= 10 pK
pH = 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9[H+] = 0.79 0.63 0.50 0.40 0.32 0.25 0.20 0.16 0.13
(not linear; need not copy, figure out yourself)
For aqueous solution;
Kw = [H+] [OH]
- log Kw = pH + pOH = 14 only at 25oC
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pH meter and pH electrodesThe pH meter is based on the principle to bediscussed in electrochemistry.
This topic is also related to the equilibriumconstant Kand Gibbs free energy, G.
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Brnsted-Lowry definition
HA + H2O A
+ H3O+
A + H2O HA + OH
Ka =[H3O+][A]
[HA]
Kb(A) =Kw
Ka(HA)=
[HA][OH][A]
Ka = 105 pKa =K
a= 108 pK
a=
e.g., CH3CO2H Ka = 1.8 x 105
Kb = 5.6 x 1010
CH3CO2
pKa = 4.7 pKb = 9.3
HCN Ka = 7.9 x 1010 Kb = 1.3 x 105 CNpKa = 9.1 pKb = 4.9
pKa = log Ka
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pKa = - log Ka
Strong acid = large Ka = small pKa
Weak acid = small Ka = large pKa
pKa
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HClO4 ClO4
_stronger
weaker
ACIDSTRENGTH
weaker
stronger
BASESTRENGTH
ACID CONJ. BASE Ka pKa
10 -1010
10-5
10-10
10 50-50
5
-
10
10 16
-16
OH O
CH3
C OH
O
CH3
C O
O
CH3
CH3 CH
3CH
2
_
CH3CH
2O-H CH
3CH
2O
Relative Acid and Base Strengths
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Strong AcidConjugate base is weak
pKa is small
Weak Acid
Conjugate base is strong
pKa is large
Acid Strengths
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Polyprotic acidsPolyprotic acids such as sulfuric and carbonic acids have more than
one hydrogen to donate.H2SO4 = H
+ + HSO4 Ka1 very large completely ionized
HSO4
= H+ + SO4
2 Ka2 = 0.012
H2CO3 = H+
+ HCO3
Ka1 = 4.3e-7HCO3
= H+ + CO3
2 Ka2 = 4.8e-11
Ascorbic acid (vitamin C)is a diprotic acid, abundant
in citrus fruit.
Others:H2S, H2SO3, H3PO4, H2C2O4(oxalic acid)
C
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Aspartame -
nutrasweet
A dipeptide methyl ester :L-aspartyl-L-phenylalanine methyl esterC14H18N2O5 (molar mass = 294.31)
Aspartame has two ionizable protons
1965. Jim Schlatter synthesizedit and discovered it sweet leavingno bitter after tast.
C ff i
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CaffeineCaffeineC8H10N4O2 is a weakbase, (pH of 1% soln 6.9)
taste bitter smell like tea, acardiac stimulant, (boost ofenergy), mild diuretic, addictive,operates using the samemechanisms that amphetamines,cocaine and heroin use tostimulate the brain.
Crystals are hexagonal prisms bysublimation, mp 238C. Sublimes178. Fast sublimation is
obtained at 160-165 under1 mm Hg pressure.
Many organic bases areinteresting drugs, and theirchemistry is fascinating.
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Problems with B-L theory
The theory works very nicely in all protic solvent, but fails toexplain acid-base behaviour in aprotic solvents and non-
solvent situations.A more general concept on acid and base was proposed byG.N. Lewis at about the same time Bronsted-Lowry theorywas proposed.
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Evolution ofthe acid-base
conceptyear thinker Acid Base acid-base reaction
1884 Arrhenius ionize ionize H+ + OH= HOHH+ OH
1923 Bronsted- Proton proton HA + B = HB + A
Lowry Donor acceptor conjugation
1923 Lewis electrophil nucleophil E + Nu = E:Nu
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Proposed another method of defining acidsand bases. Lewis acid - Any substancethat can accept a pair of nonbondingelectrons.
aka: electron-pair acceptorLewis base - Any substance that candonate a pair of nonbonding electrons.
aka: electron-pair donor
BF3 + :NH3 F3B:NH3
1923 - G.N. Lewis
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Lewis Acid-Base theory
B: electron-pair donorBASE
A ACIDelectron-pair acceptor
Some Lewis Acids: Fe3+ BF3 H3O+
Some Lewis Bases: NH3 H2O
more general than Bronsted theory
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Electronegativity
and Size Effects
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C N O F
P S Cl
Br
I
Se
Si
electronegativity increases acidityincreased
increased
size
increases
acidity
Sect. 7.5: Electronegativity and Size
Effect of Atomic Size on Acidity
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Effect of Atomic Size on Acidity
R C
O
OH
R C SH
O
R C SH
S
HOH
HSH
HSeH
HTeH
HF
HCl
HBr
HI
increasing
atom size pKa Values
3.5
-7
-9
-10
16
7
4
3
5
F- Cl- Br- I -
1.36 A 1.81 A 1.95 A 2.16 A
Effect of Electronegativity on Acidity
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CH4
NH3
H2O
HF
RCH3
RNH2
ROH
R C
O
CH3
R C NH2
O
R C OH
O
>45
34
16
3.5
45
35
18
20
15
5
Effect of Electronegativity on Acidityincreasing
electronegativity pKa Values
CH
H
H
H N
H
H O F
_ _ _ _
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Resonance Effects
Resonance in the Acetate Ion
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CH3 C O
O
H-H+
CH3 CO
O
CH3 C
O
O
base
_
_
acetate ion
Resonance in the Acetate Ion
acetic acid
equivalent structures
charge on oxygens
Phenolate ion Resonance
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Phenolate ion Resonance
O
-
_
_
_
_
_
O O
OOO
non-equivalent structures
charge on carbon and oxygen
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Nitrophenols
Placing a nitro group on the benzene ringof a phenol increases its acidity.
The effect is largest when the nitro group
is placed in an ortho or a para position onthe ring, and considerably smaller for the
meta position.
Multiple nitro groups at the ortho and parapositions can increase the pKa of a phenol
to the point that it becomes a very strong
acid.
K V l f Nit h l
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OH
O2N
NO2
OH
NO2
OH
NO2
OHNO2
OH OH
NO2O2N
NO2
10
7.2
7.3
0.4
pKa Values of Nitrophenols
9.3 4.0
Resonance in p Nitrophenol
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_Resonance inp-Nitrophenol
_
_
_
_
O
N O2
O
N O2
O
NO O
O
N O2
O
N O2
O
NO O
_
_ _+
extra
structure
When in an ortho
orpara position
a nitro group canparticipate in
resonance.
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Take Away Lession:
Resonance strengthens acidResonance weakens base
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Alpha Hydrogen
compounds
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Acid pKa
CH3CH2CH2CH2-H
H
H
H
H
R H
O
OCH2CH3H
O
CH3H
Acid pKa
O
OCH2CH3CH3CH2O
O
H
~50
44
25
24
20
13
O
CH3H3C
O
H
9
10H-CH2NO2CNH 25
CNNC
11H
NO2O2N
4H
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Inductive Effects
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Resonance operates through thep
bonding system. It doesnt drop off
with distance.
Inductive effect operates through thesigma bonding system. It drops off with
increasing distance.
Types of Inductive Effects
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Cl
d- d+
C CH3
d-d+
C
ELECTRON
WITHDRAWING
GROUPS
ELECTRON
DONATING
GROUPS
F, Cl, Br, NO2 , NR3+ R, CH3
Types of Inductive Effects
electronegative elements
take electron density
from carbon. This
strengthens the acidity.
alkyl groups donate electron
density to carbon. This weakens
the acidity.
Inductive Effects on
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Cl C C C
d- d+ d- d+ d- d+O
O
Inductive Effects on
Haloacids
The effect diminishes with distance
- it carries for about 3 bonds.
Cl COd+d-Chlorine helps
to stabilize -CO2-
by withdrawing
electrons
O
Inductive Effects
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Inductive Effects
I CH2COOH
Br CH2COOH
Cl CH2COOH
F CH2COOH
CH3 COOH
Cl CH2 COOH
Cl CH COOH
Cl
Cl C COOH
Cl
Cl
3.13
2.87
2.81
2.66
4.75
2.81
1.29
0.65
pKa Valuesincreasing
electronegativity
multiple
substituents
Inductive Effects
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CH3CH2CH2 COOH
COOHCH2CH
Cl
CH3
COOHCH
Cl
CH3CH2
COOHCH2CH2CH2
Cl
COOHH
COOHCH3
COOHCH3CH2
COOHCH3CH2CH2
COOHCCH3
CH3
CH3
Inductive Effects
pKa Values
3.75
4.75
4.87
4.81
5.02
4.8
4.5
4.0
2.9
distance
Alkyl groups release electrons.
This decreases acidity
When the chlorine atom is moved
further away from the carboxyl group,
acidity decreases
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Hybridization Effects
Effect of Hybridization
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Effect of Hybridization
C C HH
C C HH
H H
C HH
H
H
sp3
sp2
sp
ca. 50
35
25
sp orbitals are more electron
withdrawing than sp3 orbitals
(sp orbitals have more s
character).
sp > sp2 > sp3
pKa values
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Solvent Effects
Solvation Effects
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CO
O
-H
OH
H
OH
H
OH
H
O H
H
O H
Solvation Effects
4.75
4.87
4.81
Notice that these are all similar
COOHCH3
COOHCH3CH2
COOHCH3CH2CH2
COOHCCH3
CH3
CH3
5.02
.but this one has a larger pKa
This is probably a solvation effect.
Solvation lowers the energy of the ion.
The bulky
t-butyl group
is not as well
solvated.
Stronger
Acid
Weaker
Acid
unbranched
steric hindrance
CO
O -H
OH
H
OH
H
OHO
H
H
H
O H
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Intermolecular Attractions
hydrogen bonding
dipole-dipole attractionsLondon forces or van der Waals
attractions
Solubility
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SUMMARY
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Electronegativityof atom bearing acidic hydrogen; more
electronegative = more acidic
Size of atom bearing acidic hydrogen; larger = more acidic.
Resonance: greater charge delocalization in conjugate base
= more acidic.
alpha-hydrogens: carbonyls, nitro, cyano, etc. Greater
charge delocalization in conjugate base = more acidic.
Inductive effect: electronegative atoms withdraw electrons
making the acid more acidic.
Hybridization; more s-character = more acidic
Classification of Weak and Strong Acids
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40 20 10 5 0
HCl
HBr
HIR CH3 C C
H
H
H
R
R NH2
H2SO4
HClO4
HNO3
weak acids strong acids
by Functional Group
pKa
inorganic
acids
oxyacids
carboxylic
acids
nitrophenols
phenols
b-diketonesalcohols
ketones
amides
alkynes
alkenes
amines
alkanes
C C HR
OH
R
O
CH2 R
OR C
O
OH
nitrophenols
di- and tri-R C CH3
O
RC
NH2
O
R OH
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Electron-Withdrawing Effects
Strengthen Acids
R C
O
O
C
O
O
O R S
O
O
O
Any effect that bleeds electron density away from
the negatively-charged end of the conjugate base willstabilize (lower the energy) of the conjugate base and
make it a weaker base.
The parent acid will be a stronger acid.
- -
- -
-
(-) (-)
C l
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Electron-Donating Effects
Weaken Acids
R C
O
O
C
O
OO R S
O
O
O
Any effect that pushes extra electron density towardthe negatively-charged end of the conjugate base will
destabilize (increase the energy) of the conjugate base
and make the base stronger. The parent acid will be
weaker
- -- -
-
-
(-) (-)
Conversely ..