Unit 2
Acids and Bases
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http://www.cbhs.k12.nf.ca/adrianyoung/
1
Topics
 Properties
/ Operational Definitions
 Acid-Base Theories
 pH & pOH calculations
 Equilibria (Kw, Ka, Kb)
 Indicators
 Titrations
 STSE: Acids Around Us
2
Operational Definitions
 An
Operational Definition is a list of
properties, or operations that can be
performed, to identify a substance.
 See
p. 550 for operational definitions of
acids and bases
3
Operational Definitions
(Properties – see p. 550)
Acids
 pH < 7
 taste sour
 react with active
metals (Mg, Zn) to
produce hydrogen
gas
Bases
 pH > 7
 taste bitter
 no reaction with
active metals
 feel slippery
4
Operational Definitions
Acids
 blue litmus turns
red
 react with
carbonates to
produce CO2 gas
Bases
 red litmus turns
blue
 no reaction with
carbonates
5
Operational Definitions
Acids
 conduct electric
current
 neutralize bases to
produce water and
a “salt”
any ionic
compound
Bases
 conduct electric
current
 neutralize acids to
produce water and
a “salt”
any ionic
compound
6
7
8
Acid-Base Theories
1. Arrhenius Theory (p. 549 )
acid – any substance that dissociates
in water to produce H+ ions
ie. an acid must contain H+ ions
9
Arrhenius Theory
eg.
HCl(aq) →
H2SO4(aq) →
HSO4-(aq) →
10
Arrhenius Theory
base – any substance that dissociates
in water to produce OH- ions
ie. a base must contain OH- ions
11
Arrhenius Theory
eg. NaOH(aq)
Ca(OH)2(aq)
12
Arrhenius Theory
Which
Arrheniusbase?
acid?
Which is
is an
a Arrhenius
a) KOH
c) CH4
b) HCN
d) CH3OH
13
Limitations of Arrhenius theory (p.551)
1.
H+ cannot exist as an ion in water.
The positive H+ ions are attracted to the
polar water molecules forming
+
HYDRONIUM ions or H3O (aq)
H+(aq) + H2O(l) → H3O+(aq)
14
Limitations of Arrhenius theory
2.
CO2 dissolves in water to produce
an acid.
NH3 dissolves in water to produce a
base.
Neither of these observations can
be explained by Arrhenius theory
15
Limitations of Arrhenius theory
3.
Some acid-base reactions can occur
in solvents other than water.
Arrhenius theory can explain only
aqueous acids or bases.
16
Limitations of Arrhenius theory
Arrhenius theory is not able to
predict whether certain species are
acids or bases.
eg. NaHSO4 H2PO4- HCO34.
Arrhenius theory
needs some work
17
Acid-Base Theories
To be used when
Arrhenius is inadequate
2. Modified Arrhenius Theory (p. 552)
acid – any substance that reacts with
water to produce H3O+ ions
eg.
HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)
18
Modified Arrhenius Theory
base – any substance that reacts with water
to produce OH- ions
eg.
O(l) - →
NH3(aq) + HH+2-OH
pp. 558, 559 #’s 1, 3, 8, & 9
19
20
21
Acid-Base Theories
3. Brønsted-Lowry Theory (p. 553)
acid – any substance from which a
proton (H+) may be removed
ie. an acid is a substance that loses a
proton (H+)
22
Brønsted-Lowry Theory
base – any substance that can remove
a proton (H+) from an acid.
ie. a base is a substance that gains a
proton (H+)
In BLT , an acid-base reaction
requires the transfer of a proton
(H+) from an acid to a base.
23
Brønsted-Lowry Theory
base
eg.
conjugate
acid
HCN(aq) + NH3(aq) ←
→ CN-(aq) + NH4+(aq)
acid
conjugate
base
24
Brønsted-Lowry Theory
What is a conjugate acid-base pair??
(p. 554)
 Two particles (molecules or ions)
that differ by one proton are called a
conjugate acid-base pair.
 The conjugate base forms when an
acid loses a proton.
 The conjugate acid forms when a
base gains a proton (H+).
25
conjugate
Brønsted-Lowry Theory
acid
base
acid
conjugate
base
26
conjugate
Brønsted-Lowry
Theory
base
acid
eg.
NH3(aq) + H2O(l) ←
→ NH4+(aq) + OH-(aq)
acid
conjugate
base
←
H2O(l) + H2O(l) →
27
Brønsted-Lowry Theory
- an amphoteric substance can be
either an acid or a base
- usually these are negative ions that
contain at least one hydrogen atom
eg. H2O, HCO3-(aq), H2PO4-(aq)
28
Brønsted-Lowry Theory
p.557
#’s 1 – 7
p. 558
#’s 8, 9
p. 559
#’s 2, 4-7, 10,11
29
Strength of Acids and Bases
A
strong acid is an acid that ionizes
or dissociates 100% in water
eg. HCl(aq)→ H+(aq) + Cl-(aq)
 Strong
acids react 100% with water
(BLT)
eg. HCl(aq) + H2O(l) → H3O+(aq) + Cl-(aq)
30
Strength of Acids and Bases
Strong acids produce more H+ ions
OR more H3O+ ions than weak acids
with the same molar concentration
NOTE: The equilibrium symbol,  , is
NOT used for strong acids because
there is NO REVERSE REACTION.
31
Strength of Acids and Bases
A
weak acid is an acid that ionizes
or dissociates LESS THAN 100%
eg. HF(aq)  H+(aq) + F-(aq)
 Weak
acids react less than 100%
with water
eg. HF(aq) + H2O(l)  H3O+(aq) + F32
Strength of Acids and Bases
Weak acids produce fewer H+ ions
OR fewer H3O+ ions than strong acids
with the same molar concentration
 For
weak acids, an equilibrium is
established between the original acid
molecule and the ions formed.
DO NOT confuse the terms strong and
weak with concentrated and dilute.
33
Strength of Acids and Bases
eg. Classify the following acids:
 0.00100 mol/L HCl(aq)
strong and dilute
 12.4 mol/L HCl(aq)
strong and concentrated
 10.5 mol/L CH3COOH(aq)
weak and concentrated
34
Strength of Acids and Bases
monoprotic – acids that contain or lose
one proton
diprotic – acids that contain or lose two
protons
polyprotic –
35
Strength of Acids and Bases
A
strong base is a base that dissociates
100% in water, or reacts 100% with
water, to produce OH- ion.
 The only strong bases are hydroxide
compounds of most Group 1 and Group
2 elements
eg. NaOH(s) → Na+(aq) + OH-(aq)
Ca(OH)2(s) → Ca2+(aq) + 2 OH-(aq)
36
Strength of Acids and Bases
A
weak base is a base that reacts less
than 100% in water to produce OH- ion.
eg. S2-(aq) + H2O(l)  HS-(aq) + OH-(aq)
37
Writing Acid-Base Equations (BLT)
Step 1: List all the molecules/ions present
in the solution
 ionic compounds form cations and anions
 strong acids exist as hydronium ion and
the anion (conjugate base)
 for weak acids use full formula of the
compound (i.e. un-ionized molecule)
 always include water in the list.
38
Writing Acid-Base Equations (BLT)
Step 2: Identify the strongEST acid and
the strongEST base from Step 1.
Step 3: Write the equation for the reaction
by transferring a proton from the strongest
acid to the strongest base.
39
Writing Acid-Base Equations (BLT)
 Step
4: Determine the type of reaction
arrow to use in the equation.
Stoichiometric (100%) reactions occur
between:
Hydronium (H3O+) and bases stronger
than nitrite (NO2-)
hydroxide (OH-)and acids stronger than
hypochlorous acid (HOCl)
40
Writing Acid-Base Equations (BLT)
Step 5: Determine the position of the
equilibrium by comparing the strengths of
both acids in the equation.
The favoured side is the side with
the weaker acid!
41
Writing Acid-Base Equations (BLT)
Sample problems:
 Write the net ionic equation for the acidbase reaction between:
- aqueous sodium hydroxide (NaOH(aq)) and
hydrochloric acid (HCl(aq)).
42
species present
Na+(aq) OH-(aq)
H3O+(aq) Cl-(aq) H2O(l)
strongest acid
strongest base
H3O+(aq) + OH-(aq)
H2O(l) + H2O(l)
OR
H3O+(aq) + OH-(aq)
→
2 H2O(l)
43
Writing Acid-Base Equations (BLT)
Sample problems:
 Write an equation for the acid-base
reaction between nitrous acid (HNO2(aq))
and aqueous sodium sulfite (Na2SO3(aq)).
44
species present
HNO2(aq)
Na+(aq) SO32-(aq)
SA
H2O(l)
SB
HNO2(aq) + SO32-(aq)  NO2-(aq) + HSO3 - (aq)
Weaker Acid
Stronger Acid
Products favored
45
Write the Net Ionic Equation for each
aqueous reaction below:
1. Na2CO3(aq) and CH3COOH(aq)
2. NH3(aq) and HNO2(aq)
3. HNO3(aq) and RbOH
4. H2SO4(aq) and K3PO4(aq)
5. HF(aq) and NH4CH3COO(aq)
6. CaCl2(aq) and PbSO4(aq)
p. 564 #’s 10 &11
46
species present
Na+(aq) CO32-(aq) CH3COOH(aq) H2O(l)
SB
CH3COOH(aq) + CO32-(aq)
Stronger Acid
SA

CH3COO-(aq) + HCO3-(aq)
Weaker Acid
Products Favoured
47
species present
NH3(aq)
HNO2(aq)
SB
HNO2(aq) + NH3(aq)
Stronger Acid
H2O(l)
SA
-(aq) + NH4 + (aq)
NO
 2
Weaker Acid
Products favored
48
species present
H3O+(aq)
NO3-(aq)
Rb+(aq) OH-(aq)
H2O(l)
strongest acid
strongest base
H3O+(aq) + OH-(aq)
H2O(l) + H2O(l)
OR
H3O+(aq) + OH-(aq)
→
2 H2O(l)
49
species present
+
3+
HSO
K
PO
H3O (aq)
4 (aq)
(aq)
4 (aq)
SA
H3O+(aq) + PO43-(aq)
H2O(l)
SB
H2O(l) + HPO42-(aq)
50
species present
+
NH
HF(aq)
CH
COO
4 (aq)
3
(aq)
SA
HF(aq) + CH3COO-(aq)
Stronger Acid
H2O(l)
SB

F-(aq) + CH3COOH(aq)
Weaker Acid
Products favored
51
strongest acid
species present
Ca2+(aq) Cl-(aq) Pb2+(aq) SO42-(aq) H2O(l)
strongest base
H2O(l) + SO42-(aq)
Weaker Acid
Reactants favored

HSO4-(aq) + OH-(l)
Stronger Acid
52
NO!!
Products are NOT always favoured
Try these:
CH3COOH(aq) + NH4F(aq)
HCN(aq) + NaHS(aq)
53
Acid-Base Calculations
Kw
Ka
Kb
[H3O+]
pH
[OH-]
pOH
54
Kw (Ionization Constant for water)
 With
very sensitive conductivity testers,
pure water shows slight electrical
conductivity.
PURE WATER MUST HAVE
A SMALL CONCENTRATION OF
DISSOLVED IONS
55
Kw
Auto-Ionization of water
H2O(l) + H2O(l)
Kw ==

H3O+(aq) + OH-(aq)
[H3O+] [OH-]
[H2O] [H2O]
56
Kw
In pure water at 25 °C;
[H3O+] = 1.00 x 10-7 mol/L
[OH-] = 1.00 x 10-7 mol/L
Calculate Kw at 25 °C.
57
H2O(l) + H2O(l)

H3O+(aq) + OH-

What happens if we add OHions (NaOH(aq)) to water?
shift to the left
[H3O+] ?
[OH-] ?

Does Kw change?
LCP:


GET
REAL!!
58
Kw = [H3O+] [OH-]
1.00 x 10-14 = [H3O+] [OH-]
[H3O+]
0.00357 M
2.04 x
10-12
8.00 x 10-16
1.50 mol/L
[OH-]
2.80 x 10-12
4.89 x 10-3 mol/L
12.5 M
6.67 x 10-15
59
Calculations with Kw (p. 564 – 566)
For strong acids and strong bases, the
[H3O+] and [OH-] may be calculated
using the solute concentration.
eg. What is the [H3O+] in a 2.00 mol/L
solution of HNO3(aq)?
Ans: 2.00 mol/L
[OH-] = ???
60
Calculations with Kw
eg. What is the [OH-] in a 2.00 mol/L
solution of NaOH(aq)?
Ans: 2.00 mol/L
eg. What is the [OH-] in a 2.00 mol/L
solution of Ca(OH)2(aq)?
Ans: 4.00 mol/L
[H3O+] = ???
61
Calculations with Kw
eg. What molar concentration of
Al(OH)3(aq) is needed to obtain a
[OH-] = 0.450 mol/L?
Ans: 0.150 mol/L
62
What is the [H3O+] and [OH-] in:
[H3O+]
[OH-]
1.0 x 10-8
1.0 x 10-6
5.00 x 10-14
0.200
1.50
6.67 x 10-15
1.0 x 10-2
1.0 x 10-12
63
solute
[H3O+]
[OH-]
0.680 mol/L HCl(aq)
0.680
1.47 x 10-14
1.50 M NaOH
6.67 x 10-15
1.50
0.0500 M Ca(OH)2(aq)
1.00 x 10-13
0.100
0.450 mol/L HClO4(aq)
_____
0.450 M
2.22 x 10-14
____ mol/L Mg(OH)2(aq) 2.00 x 10-14 0.500 mol/L
0.250
p. 566
#’s 12 - 15
64
what
factor does
thep. 568)
pH By
and
pOH
(See
+
[H3O ] change when the
pH value changes by 1?
by 2?
65
pH
The [H3O+] changes by a
and pOH
p. 568)
factor
of 10 (10X)(See
for each
pH changes of 1.
66
pH and pOH FORMULAS
pH = -log [H3O+]
pOH = -log [OH-]
[H3O+] = 10-pH
[OH-] = 10-pOH
67
pH and pOH
eg. What is the pH of a 0.0250 mol/L
solution of HCl(aq)?
What is the pOH of a 0.00087 mol/L
solution of NaOH(aq)?
What is the pH of a 1.25 mol/L
solution of KOH(aq)?
68
Significant digits in pH values?
The number of significant digits in a
concentration should be the same as the
number of digits to the right of the
decimal point in the pH value.
eg. In a sample of OJ the
[H3O+] = 2.5 × 10−4 mol/L
pH = 3.60
(See p. 568)
69
[H3O+]
[OH-]
pH
pOH
0.0035
1.2 x 10-5
4.68
9.15
8.33 x 10-15
-1.10
70
[H3O+]
[OH-]
pH
0.0035
2.9 x 10-12
2.46
1.2 x 10-5
9.08
2.1 x 10-5
4.8 x 10-10
4.68
1.4 x 10-5
7.1 x 10-10
4.85
1.20
8.33 x 10-15
13
7.9 x 10-16
-0.079
-1.10
8.3 x
10-10
pOH
11.54
4.92
9.32
9.15
14.079
15.10
71
pH, pOH and Kw
p. 569 #’s 16 – 19
p. 572 #’s 20 – 25
Examine #23. Where is the energy
term in this equation?
H2O(l) + H2O(l)  H3O+(aq) + OH-(aq)
72
Dilutions
When a solution is diluted the
number of moles does not change.
OR
ninitial = nfinal
CiVi = CfVf
73
eg. 400.0 mL of water was added to 25.0
mL of HCl(aq) that had a pH of 3.563.
Calculate the pH of the resulting solution.
[H3O+]
 dilution formula
 calculate pH
 calculate
74
Before dilution:
[H3O+] = 10-3.563
= 2.735 x 10-4 mol/L
After dilution:
(2.735 x 10-4) (25.0 mL) = (Cf)(425.0 mL)
[H3O+] = 1.609 x 10-5
pH = -log (1.609 x 10-5)
= 4.793
p.574
#’s 26 - 29
75