Transcript Chapter 17:

Ch15. Acids
and Bases: A
Second Look
Brady & Senese, 5th
Ed
1
Arrhenius acid is a substance that produces H+ (H3O+) in water
Arrhenius base is a substance that produces OH- in water
4.3
A Brønsted acid is a proton donor
A Brønsted base is a proton acceptor
base
base
acid
acid
base
acid
conjugate
acid
conjugate
base
15.1
(a) Formic acid transfers a proton to a water molecule. HCHO2 is the acid and
H2O is the base. (b) When a hydronium ion transfers a proton to the CHO2- ion,
H3O+ is the acid and formate ion is the base.
• Identify the Conjugate Partner for Each
conjugate base
conjugate acid
HCl
NH3
HC2H3O2
CNHF
15.1. Brønsted-Lowry acids and bases exchange protons
6
Identify the conjugate pair for each acid/base:
Iodic acid
Nitrous acid
Acetic acid
Bisulfite ion
Hydrogen sulfate
Your Turn!
How many of the following pairs are
conjugate pairs:
i. HCN/CN- ii. HCl/Cl- iii. H2S/S2-
A. 1
B. 2
C. 3
D. None of them are conjugate
15.1. Brønsted-Lowry acids and bases exchange protons
8
• An amphoteric substances can act as either
an acid or base, also called amphiprotic,
and can be either molecules or ions
As an acid :

3

2
3
HCO (aq)  OH (aq)  CO (aq)  H 2O
As a base :

3

HCO (aq)  H 3O (aq)  H 2CO3 (aq)  H 2O
Your Turn!
How many of the following are amphoteric?
HC2H3O2 , H2PO42-, HSO4A. 1
B. 2
C. 3
D. None of these
15.1. Brønsted-Lowry acids and bases exchange protons
13
Strong Acid
Weak Acid
15.4
ACID-BASE THEORIES
Acetic acid is only 0.42% ionized when [HOAc] = 1.0 M.
It is a WEAK ACID
HOAc
WEAK A
+
H2 O
base
H3O+
acid
Because [H3O+] is small, this must mean
1.H3O+ is a stronger acid than HOAc
2.OAc- is a stronger base than H2O
+
OAc-
STRONG B
Identify The Preferred Direction Of The Following
• H3O+ (aq) + CO32-(aq)
↔
HCO-3(aq) + H2O (l)
• NH4+(aq) + HCO-3(aq)
↔
NH3(aq) + H2CO3(aq)
• Cl- + HCN(aq)
H2 O
HCN
↔
NH4
HCl(aq) + CN-(aq)
+
H2CO3
H 3 O+
Increasing Acid Strength
15.1. Brønsted-Lowry acids and bases exchange protons
17
15.4
Your Turn!
Which reaction is not written as in the preferred direction.
A.
B.
C.
D.
HC2H3O2 + H2AsO4- ↔H3AsO4 + C2H3O2H3AsO4 +H2AsO3- ↔ H2AsO4- + H3AsO3
HC2H3O2 + H2AsO3- ↔C2H3O2- + H3AsO3
All are preferred as written
H2O H3AsO3 HC2H3O2 H3AsO4
Increasing Acid Strength
15.1. Brønsted-Lowry acids and bases exchange protons
H3 O+
19
Periodic Trends Of Binary Acids (HnX )
• As we read left to right in a period, increasing
electronegativity of X makes the H-X bond
more polar
• Acid strength increases with increasing polarity
• As we read top to bottom in a group, the acid
strength increases due to increasing bond length
of the HX bond due to increased radius of the
anion, X
15.2. Strengths of Brønsted acids and bases follow periodic trends
20
Molecular Structure and Acid Strength
H X
H+ + X-
The
stronger
the bond
The
weaker
the acid
HF << HCl < HBr < HI
15.9
Learning Check
δ-
Which is a stronger
acid?
• H2S or H2O
δ-
δ+
δ+
δ-
• CH4 or NH3
δ+
δ+
• HF or HI
δ-
15.2. Strengths of Brønsted acids and bases follow periodic trends
δ+
δ-
22
Oxoacids ( A(O)m(OH)n)
• Increase in strength as the
electronegativity of the central
atom, A, increases
• Increase in strength as the
number of oxygen atoms on
(hence the oxidation state of)
the central atom, A, increases
• Electrical induction through
the central atom weakens
strength of the bond to H
δ+
δ
-
δ+
δ
-
15.2. Strengths of Brønsted acids and bases follow periodic trends
23
Learning Check
Which is a stronger
acid?
• H2SO4 or H3PO4
• HNO3 or H3PO4
• H2SO4 or H2SO3
• HNO3 or HNO2
15.2. Strengths of Brønsted acids and bases follow periodic trends
24
Anions Of Oxoacids Are Basic
• Oxygen atoms are electron withdrawing, thus the
charge on an anion is located on the lone oxygens
• The more oxygen atoms there are that share the same
charge, the less basic is the anion
• The stronger the base behavior of the anion, the
greater the strength of the conjugate acid
3O
share
the (-)
charge
-
2O
share
the (-)
charge
15.2. Strengths of Brønsted acids and bases follow periodic trends
25
Your Turn!
Which statement about acid strength is false:
A. Binary acids get stronger as the anion gets
larger
B. Ternary oxoacids get stronger as the
central atom increases in size
C. Ternary acid strength increases with
increasing oxidation state
D. None of these are false
15.2. Strengths of Brønsted acids and bases follow periodic trends
26
Rank by Acid Strength
• HI
HBr
• HClO4
HCl
HClO3
• HClO HBrO
HF
HClO2
HIO
HClO
Lewis Acid/Base Reactions
• Lewis acids accept an electron pair to form
coordinate covalent bonds
• Lewis bases donate lone pairs of electron to form
coordinate covalent bonds
• Neutralization is the formation of a coordinate
covalent bond between the donor and acceptor
15.3. Lewis acids and bases involve coordinate covalent bonds
28
Lewis Acids and Bases
+
F B
••
H
F
N H
F
H
acid
base
F
F B
F
H
N H
H
No protons donated or accepted!
15.12
• In general, most metal oxides react with
water to form bases, and nonmetal oxides
react with water to form acids
• In Section 5.5 metal oxides were called base
anhydrides and nonmetal oxides were called acid
anhydrides
• When cations dissolve in water, they form
species called hydrated ions
• Hydrated metal ions tend to be Brønsted
acids
• For the monohydrate of the metal ion Mn+
the equilibrium can be represented as

M (H2O)n  H 2 O
MOH (n1)  H3O
The metal ion
makes the
hydrogen on
the water
more acidic.
• The charge density of a cation is its charge
divided by its volume
• The higher the charge density, the better a
cation is at drawing electron density from a
O-H bond and the more acidic it is
• Within a given period, the cation size increases, and
the charge density decreases, from top to bottom
• As a result, the most acidic hydrated cations are
found at the top of a group
• As the cation charge increases, it becomes
more acidic
• Water undergoes self-ionization or
autoionization making it a weak electrolyte



H 2O  H 2O H3O  OH

• This equilibrium is described by the ion
product of water




Kw  [H3O ][OH ]  [H ][OH ]
The Ion Product of Water
H2O (l)
H+ (aq) + OH- (aq)
[H+][OH-]
Kc =
[H2O]
[H2O] = constant
Kc[H2O] = Kw = [H+][OH-]
The ion-product constant (Kw) is the product of the molar
concentrations of H+ and OH- ions at a particular temperature.
At 250C
Kw = [H+][OH-] = 1.0 x 10-14
[H+] = [OH-]
Solution Is
neutral
[H+] > [OH-]
acidic
[H+] < [OH-]
basic
15.2
Learning Check
Complete the following with the missing data
[OH-]
[H3O+]
pH
3.2 × 10-3 M
2.3 × 10-5 M
1.5 × 10-2 M
2.55 × 10-6 M
8.50
15.5. pH is a measure of the acidity of a solution
38
Learning Check
• Complete the following with the missing
data
pH
pOH
[H3O+]
2.33
1.89
15.5. pH is a measure of the acidity of a solution
39
What is the [H3O+] in a HCl solution that
has a concentration of 5.9E-2M?
At 250C
Kw = [H+][OH-] = 1.0 x 10-14
a) 5.9E-2 M
b) 3.0 E-2 M
c) 1.0 E-2 M
d) 1.2 E-1 M
e) 1.23 M
What is the pH of a solution that has a
[OH-] = 1.2×10-3 M?
A.
B.
C.
D.
2.92
11.1
1.20
None of the above
15.5. pH is a measure of the acidity of a solution
41
At 250C
Kw = [H+][OH-] = 1.0 x 10-14
What is [OH-] in a Ba(OH)2 solution
that has a concentration of 3.3E-2M?
a) 1.7 E-2 M
b) 3.3 E-2 M
c) 6.6 E-2 M
d) 1.2 E-1 M
e) 0.23 M
Indicators Help Us Estimate pH
15.5. pH is a measure of the acidity of a solution
45
pFUNCTIONS
• pX = - log X
• pH = - log [H+]
• pOH = - log [OH ]
• pK = -log K
The pH of rainwater collected in a certain
region of the northeastern United States on
a particular day was 4.82. What is the H+
ion concentration of the rainwater?
pH = -log [H+]
15.3
The OH- ion concentration of a blood sample
is 2.5 x 10-7 M. What is the pH of the blood?
15.3
Strong Acids Ionize 100% in Water
• As the substances are placed into water,
they form H3O+ .
• The H3O+ formed by the acid suppresses
water’s ionization. (if [acid] > 10-7 M)
• The pH can be calculated from the
concentration of H3O+ produced by the
strong acid
• The reaction of strong acids occurs
irreversibly, so we show the reaction with a
→ instead of using a double arrow
15.6. Strong acids and bases are fully dissociated in solution
49
Learning Check
What is the pH of 0.1M HCl
15.6. Strong acids and bases are fully dissociated in solution
50
Learning Check
What is the pH of 0.1M HCl
HCl(aq) + H2O(l) →H3O+(aq) + Cl-(aq)
0.1
N/A
0
0
I
-0.1
-0.1
0.1
0.1
C
0
N/A
0.1
0.1
end
pH = -log(0.1) = 1
15.6. Strong acids and bases are fully dissociated in solution
51
Strong Bases Dissociate 100% In Water
• They are strong electrolytes that form OHwhen dissolved
• pOH can be calculated from the [OH-] from
the solution
• Water’s contribution is negligible if the
base is sufficiently concentrated [OH-]>107M
15.6. Strong acids and bases are fully dissociated in solution
52
What is the pH of 0.5M Ca(OH)2?
15.6. Strong acids and bases are fully dissociated in solution
53
What is the pH of 0.5M Ca(OH)2?
Ca(OH)2(aq) →
0.5
0
-0.5
+0.5
0
0.5
pOH = -log(1.0) =0
pH = 14
Ca2+(aq) + 2OH-(aq)
0
I
+ 0.5×2
C
1.0
E
15.6. Strong acids and bases are fully dissociated in solution
54
Your Turn!
What is the pH of a 0.50M solution of NaOH?
A. 0.500
B. 0.301
C. 13.7
D. None of these
15.6. Strong acids and bases are fully dissociated in solution
55
What is the pH of an NaOH
solution that has a concentration
of 3.3E-3 M?
a) -2.48
b) 2.48
c) 11.52
d) 1.2 E+1,
e) 12.52,
What is the final pH, if 200 mL of
3.3E-2 M HNO3 is diluted with
water to a final volume of 328 mL?
a) -1.27
b) 2.0E-2
c) 5.4E-2
d) 1.27
e) 1.69
What is the pH of a 3.3E-2 M
Ba(OH)2 solution?
a) -1.48
b) -1.18
c) 1.18
d) 12.52
e) 12.82
In a pure aqueous solution of
NaOH, the pH was measured to be
13.41. What is the molar
concentration of NaOH?
a) 3.9E-14 M
b) 5.7E-1 M
c) 0.26 M
d) 0.48 M
e) 2.6E13 M
What is the final molarity of
[H3O+], if 100 mL of 7.0E-2 M
HCl is diluted with 236 mL of
distilled water?
a) 3.0E-4 M
b) 2.1E-2 M
c) 3.0E-2 M
d) 1.7E-1 M
e) 2.4E-1 M
Exactly 100 mL of 0.20 M NaOH
was mixed with 125 mL of 0.20 M
HCl. What is the molarity of the
NaCl formed?
a) 1.6E-1 M
b) 1.8E-1 M
c) 1.8E-2 M
d) 8.9E-2 M
e) 0.20 M
Exactly 100 mL of 0.20 M NaOH
was mixed with 125 mL of 0.20
M HCl. What is the pH of this
mixture ?
a) 0.70
b) 0.80
c) 1.65
d) 1.80
e) 7.80
Exactly 100 mL of 0.20 M NaOH
was mixed with 118 mL of 0.30 M
HCl.
What is the pH of this mixture?
a) 0.80
b) 1.15
c) 2.13
d) 3.12
e) 12.45