Several concepts of acid-base theory: The Arrhenius concept The Bronsted-Lowry concept The Lewis...
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Transcript of Several concepts of acid-base theory: The Arrhenius concept The Bronsted-Lowry concept The Lewis...
Acidsand
Bases
Several concepts of acid-base theory:
The Arrhenius concept
The Bronsted-Lowry concept
The Lewis concept
Arrhenius Acids & BasesAccording to the Arrhenius concept of acids and bases
An acid is a substance that, when dissolved in water, increases the concentration of Hydrogen ion, H+(aq)
The H+(aq) ion, called the Hydrogen ion is actually chemically bonded to water, that is, H3O+, called the Hydronium ion
The Hydronium Ion (H3O+) actually exists as a hydrogen
bonded H9O4+ cluster.
)(3)(2)( aqlaq OHOHH
According to the Arrhenius concept of acids and bases
A base is a substance that when dissolved in water increases the concentration of the hydroxide ion, OH-(aq)
HBr(aq) H+(aq) + Br-(aq)Acid + H2O Anion + H3O+
HA + H2O A– + H3O+
HC2H3O2(aq) + H2O H3O+(aq) + C2H3O2
−(aq)
a. Metal HydroxidesNaOH(s) Na+
(aq) + OH–(aq)
Mg(OH)2(s) Mg2+(aq) + 2OH–
(aq)
b. Molecular bases: NH3(aq)+H2O NH4
+(aq)+ OH-
(aq)
Arrhenius acid-base neutralization Acid and base combine to produce water and salt.
Ex. HCl(aq) + NaOH(aq) H2O + NaCl(aq)
H3O+(aq) + Cl–(aq) + Na+(aq) + OH–(aq)
2H2O + Cl–(aq) + Na+(aq)
However, many reactions resemble this without forming H3O+ or OH– in solution!!!
5
Gas Phase Acid-Base• Not covered by Arrhenius definition
Ex. NH3(g) + HCl(g) NH4Cl(s)
Brønsted-Lowry Definition• Acid = proton donor• Base = proton acceptor
Allows for gas phase acid-base reactions
HCl + H2O H3O+ + Cl–
HCl = acid : Donates H+ Water = base : Accepts H+
7
Bronsted-Lowry Acids & Bases
Amphoteric Species: can act as either as acid or base
WaterWater can react as an acid by donating a proton to a base
Water can react as a base by accepting a proton from an acid
+ -3 2 4N H (a q ) + H O (l) N H (a q ) + O H (a q )
H+
+2 3H F (a q ) + H O (l) (a q ) + H O (a q )F
H+
Learning Check
• Write a reaction that shows that HCO3– is a
Brønsted acid when reacted with OH–
• HCO3–(aq) + OH–(aq)
• Write a reaction that shows that HCO3– is a
Brønsted base when reacted with H3O+(aq)
• HCO3–(aq) + H3O+(aq)
9
H2CO3(aq) + H2O(ℓ)
H2O(ℓ) + CO32–(aq)
Lone pair binds H+
Lone pair binds H+
Conjugate Acid-Base
Pair
H
H
Learning Check
conjugate base
conjugate acid
HCl
NH3
HC2H3O2
CN–
HF
12
Identify the Conjugate Partner for Each
Cl–
NH4+
C2H3O2–
HCNF–
)()(3)(2)( aqaqlaq AOHOHHA
][
][][ 3
HA
AOHKa
The concentration of water is almost constant.
Ka very large Ka very small
solutes
ElectrolytesNon-
electrolytesSolute undergoes Dissociation
strong weakcomplete
dissociationpartial dissociation
NaCl / H2OHCl / H2O
HAc / H2O
Sugar / H2O
)()()(2
aqaqOH
s ClNaNaCl )()()(2
aqaqOH
l AcHHAc
)()(3)(2)( aqaqlaq AOHOHHA
Relative Strengths of
Acid and Bases
As acid strength decreases, base
strength increases; the weaker the acid,
the stronger its conjugate base.
As base strength decreases, acid
strength increases; the weaker the base,
the stronger its conjugate acid.
Position of Acid-Base Equilibrium
• Acetic acid (HC2H3O2) is weak acid– Ionizes only slightly in water
HC2H3O2(aq) + H2O(ℓ) H3O+(aq) + C2H3O2–(aq)
weaker acid weaker base stronger acid stronger base
• Hydronium ion– Better H+ donor than acetic acid – Stronger acid
• Acetate ion – Better H+ acceptor than water – Stronger base
• Position of equilibrium favors weaker acid and base19
Calculate [H+] or [OH-] as required for each of the following solutions at 25oC, and state whether the solution is neutral, acidic, or basic
MH
HOHH
OHHKw
713
132
13
100.3100.1][
100.1][][][
100.1][][
As T is increased, the product concentration increases,Reaction shifts to the right,
Forward reaction proceeds faster,Forward reaction is endothermic.
A common way to express acidity and basicity is with pH
pH = - log [H3O+] or [H3O+]=10-pH
Acidity and Basicity
The # of sig figs in the concentration
= the number of decimal places in pH
In an acidic solution,
[H3O+]= 1.25 x 10-4 M at 25 oC
pH =-log (1.25 x 10-4)= -(-3.903) = 3.903
UA GenChem
Your Turn!What is the pH of Black Coffee, [H3O+]= 1.0 x 10-5 M?
What is the H3O+ concentration in sea water if pH=8.30?
What is the OH¯ concentration in acid rain if pH=5.25?
[H3O+], [OH-] and pH
General conclusion @25ºC Basic solution pH > 7 Neutral pH = 7
Acidic solution pH < 7
Other pX ScalesIn general pX = -log X
and so pOH = - log [OH-]
Which solution is more basic, one that has a pH of 5.5 or one with a pOH of 8.5?
If Kw = [H3O+] [OH-] = 1 x 10-14
Taking the log of both sides
-log (10-14) = - log [H3O+] + (-log
[OH-])
pKw = 14 = pH + pOH
The pH of a sample of human blood was measured to be 7.41 at 25oC. Calculate pOH, [H+], and [OH-] for the sample.
33 NOHHNO
OHHNOsolution HHH23][][][
0.1 M 0.1 M< 10-7 M
3][][ HNOsolution HH
acidstrongHNOsolution HHHpH ]log[]log[]log[3
pH =-log (0.1)= -(-1) = 1
ClHHCl
OHHClsolution HHH2][][][
1x10-10 M 1x10-10 M
= 10-10 M
OHsolution HH2][][
OHsolution HHpH2]log[]log[
pH ≈-log (1x10-7)= -(-7) ≈7
≈ 10-7 M
slightly smaller
Calculate the pH of 1.0 M HF solution.
OHHFsolution HHH2][][][
FHHFco
chang
e
ceq
molxx 242 107.2102.700.1
%7.2%1001
107.2%100
2
oc
x < 5%
For any weak acid: HA → H+ + A-
check
If < 5%
pH =-log(x)=-log (2.7x10-2)= 1.57
autoionization of water neglected
Calculate the pH of a solution that contains 1.00 M HCN (Ka=6.2×10-10) and 5.00 M HNO2 (Ka= 4.0×10-4). Also calculate the concentration of cyanide ion (CN) in this solution at equilibrium.
co
chang
e
ceq
Mx
x
Kcx ao
2
4
105.4
100.45
35.1)105.4log(105.4][ 22 pHMxH
co
chang
e
ceq
][
][][
HCN
CNHKa
MCN
CN 8
2
10 104.1][00.1
][105.4102.6
FHHF
Calculate the pH of 1.0 M HF solution.
co1 0 0
chang
e
-x x x
ceq1-x x x
amount dissociated of
HF = x%100%
oc
x
co1.0 M 0.1
x 2.7x10-
2
0.0081
a 2.7% 8.1%
AcHHAc
Calculate the percent dissociation of 1.0 M and 0.10 M
HAc solution.
coco 0 0
chang
e
-x x x
ceqco-x x x
amount dissociated of
HAc = x
%100% oc
x
co1.0 M 0.1
x 4.2x10-
3
1.3x10-
3
a 0.42% 1.3%
Ka(Hac)=1.8x10-5
ao
oo
a Kcxc
x
xc
xK
22
In a 0.100 M aqueous solution, lactic acid is 3.7% dissociated. Calculate the value of Ka for this acid.
AHHAco
1 0 0
chang
e
-x x x
ceq1-x x x
%100% oc
x
xco %100
%
0037.0%100
%7.31.0 x
4
22
104.10037.01.0
0037.0
xc
xK
o
a
OHNaNaOH aq )(
OHNaOHsolution OHOHOH2][][][
0.05 M 0.05 M< 10-7 M
NaOHsolution OHOH ][][
basestrongNaOHsolution OHOHOHpOH ]log[]log[]log[
pOH =-log (0.05)= -(-1.3) = 1.3
Calculate the pH of 5.0x10-2 M NaOH solution.
pH =14-pOH=14-1.3 = 12.7
NH3(aq) + H2O NH4+
(aq) + OH-(aq)
Weak Bases•Partial Ionization•Small Kb
Bases
Calculate the pH of 15.0 M NH3 solution (Kb=1.8x10-5).
NH3(aq) + H2O NH4+
(aq) + OH-(aq)
co15 0 0
change -x x x
ceq15-x x x
2522
106.1108.11515
xx
x
xK b
8.1)102.1log(log]log[ 2 xOHpOH solution
pH =14-pOH=14-1.8 = 12.2
Calculate the pH of 1.0 M methylamine solution (Kb=4.38x10-4)
Check the 5% rule!
triprotic acid
Calculate the pH of a 5.0 M H3PO4 solution and the equilibrium concentrations of the species H3PO4, H2PO4
-, HPO42- and PO4
3-.
4243 POHHPOH
co5 0 0
chang
e
ceq13
22
1 109.1105.755
xx
x
xK a
72.0)109.1log(log]log[ 1 xHpH
][109.1][ 1
42
HMxPOH
2
442 HPOHPOHco 0.19 0.19 0
change -y +y +y
ceq 0.19-y 0.19+y y
yy
yyK a
)19.0(
)19.0(2
2
2
4
8 ][102.6 aKHPOy
3
4
2
4 POHHPOco 6.2x10-8 0.19 0
change -z +z +z
ceq 6.2x10-8-z 0.19+z z
][106.119.0
102.6108.4
108.4)102.6(
)19.0(
3
4
198
13
13
83
POMz
z
zzK a
442 HSOHSOH1 M 1 M
2
44 SOHHSOco 1 1 0
change -x +x +x
ceq 1-x 1+x x
1 M
2
2 102.1)1(
)1(
x
xxK a
2102.1)1(
)1( x
0)012.1log()1log(]log[ xHpH
442 HSOHSOH0.01 M 0.01 M
2
44 SOHHSOco 0.01 0.01 0
change -x +x +x
ceq 0.01-x 0.01+x x
0.01 M
2
2 102.1)01.0(
)01.0(
x
xxK a
2102.1)01.0(
)01.0( x
84.1)0145.0log()0045.001.0log(]log[ HpH
Calculate the pH of a 0.01 M H2SO4.
Approximation?
%120%10001.0
012.0%100
oc
x
Solve quadratic equation: Mx 3105.4
saltscation anion
acidic
1+ and 2+cations
)(2
)(2
)()(
,
,,
aqaq
aqaq
CaMg
KNa
conjugate acid of weak bases
NH4+
3324 NHOHOHNH
No effect on pH
3+ cationsacidic
)(3
)(3 , aqaq FeAl
OHOHOHAlOHOHAl 3
2
522
3
62 )()()(
As Size of Cation, AciditySmaller, more concentrated charge: Means pull of e
density from O—H bond: acidity of H+
saltscation anion
basic
conjugate base of strong acid
Cl-, Br-, I- , NO3-
OHHAcOHAc 2
No effect on pH
conjugate base of weak acidAc- , PO4
3-
][
][][
Ac
OHHAcKb
OHHAcOHAc 2
][
][][
Ac
OHHAcKb
OHAcOHHAc 32
][
][][ 3
HAc
AcOHKa
][
][][
][
][][ 3
Ac
OHHAc
HAc
AcOHKK ba
][][ 3
OHOHKK ba wK
Acid dissociation
constantx
constant for conjugate
base
Water ionization constant
=
OHHFOHF 2
co 0.3 0 0
change -x +x +x
ceq 0.3-x +x +xx
x
F
OHHFKb
3.0][
][][ 2
11
4
14
104.1102.7
100.1
a
wb K
KK
611 100.2104.130.0 x
69.5)100.2log(log]log[ 6 xOHpOH solution
pH =14-pOH=14-5.69 = 8.31
neutralanionKcationKif
basicanionKcationKif
acidicanionKcationKif
ba
ba
ba
)()(
)()(
)()(
10106.5 aK10106.5 bK
neutral
10106.5 aK5106.1 bK
basic
5104.1 aK13
422
103.8)(
SOHK
KK
a
wb
acidic
Binary Acids = HnX
X = Cl, Br, P, As, S, Se, etc.
1. Acid strength from left to right
within same period (across row)– Acid strength as electronegativity of X
Ex. HCl is stronger acid than H2S which is
stronger acid than PH3
– or PH3 < H2S < HCl
Trends in Binary Acid Strength
Binary Acids = HnX
X = Cl, Br, P, As, S, Se, etc.
•X: group VIIA, from bottom to top :• I, Br, Cl, F
•Electronegativity inccreases•Bond polarity higher
•Bond stronger (also because size smaller)
•More difficult to break the bond •Dissociation less, acid weaker
Trends in Oxoacid Strength
Oxoacids (HnYOm)
– Acids of H, O, and one other element– HClO, HIO4, H2SO3, H2SO4, etc.
1. Acids with same number of O’s and differing Ya. Acid strength from bottom to top within
group• HIO4 < HBrO4 < HClO4
b. Acid strength from left to right within period• H3PO4 < H2SO4 < HClO4
• as electronegativity of central atom 55
why
OClHHOCl
not ClHOHOCl
or ClHOHOCl
Positive ClPositive O
NaHOHONa Electronegativity of atom attached to O
Trends in Oxoacid Strength
Oxoacids (HnYOm)2. For same Y
– Acid strength as number of O’s • H2SO3 < H2SO4
• More oxygens, remove more electron density from central atom, weakening O—H bond and make H more acidic
57
Your Turn!Which of the following is the strongest organic acid?
OHC
O
CH
H
H
OHC
O
CF
H
H
OHC
O
CBr
H
H
OHC
O
CI
H
H
OHC
O
CCl
H
H
A B C
D E
oxidesOxides of nonmetals Oxides of metals
Strong covalent bond NM-O
acidicAcid anhydride
basicbase anhydride
Ionic bond M-O