Unit 4
Page 370-377
1887:
Defined acid:
Produces H+ in water
(acidic solutions)
Svante Arrhenius
H+ ions released are bonded to the water molecule to form H3O+ ions
H+ (aq) + H2O (l)
H2O + H2CO3
H3O+ (aq)
H3O+ +
HCO3-
Note:
H+ (aq)
+
H2O (l)
H3O+ (aq)
Is a reversible reaction: the double arrows
indicate that this reaction proceeds at equal
rates in each direction.
This reaction is in dynamic equilibrium;
hydronium ions are created as fast as they are
breaking down to produce hydrogen ions and
water.
1887:
Defined base:
Produces OH- in water
(alkaline solutions)
Svante Arrhenius
Bases are normally a cation (+) and a hydroxide ion.
K+OH- (s) + H2O (l)
Ba(OH)2 + H2O (l)
K+ (aq) + OH- (aq)
Ba+2 (aq) +
2 OH- (aq)
1887:
Neutralization Reaction
Svante Arrhenius
When equal amounts of H+ ions react with OH- ions, water is the
only product.
H3O+ (aq) + OH- (aq)
2 H2 O (l)
Johannes Brönsted
1923
Brönsted and Lowry independently
published their findings, which is known
as the Brönsted-Lowry theory or concept.
A. Hydrogen ions are protons
B. Acids are proton donors
C. Bases are proton acceptors
D. Acids increase [H+]
E. Bases decrease [H+]
Thomas Lowry
Johannes Brönsted
“Acids and bases are substances that
are capable of splitting off or taking up
hydrogen ions, respectively.”
Bases accept hydrogen ions. This is why NH3 is a base
under the Brönsted-Lowry theory but not under the Arrhenius
theory.
NH3
(Base)
+
H2O
NH4+
(Acid)
(Acid)
+
OH(Base)
When an acid donates a proton, it
is then able to accept a proton, so
it becomes a conjugate base.
When a base accepts a proton, it
is then able to donate a proton
and becomes a conjugate acid.
Now try your hand at identifying the
conjugate acid-base pairs in the equations
shown below.
acid
base
Cl-
+
H2O
NH3
Cl-
+
NH4+
NH3
OH-
+
NH4+
HCl
OH-
H2O
Proton
donor
+
+
Proton
acceptor
HSO4-
congugate
acid
H3 O +
H2O
HCl
congugate
base
+
H2SO4 +
+
Equations
Proton
acceptor
Proton
donor
Exercise:
Complete “Developing Skills”
on page 372 (C.5 – Acids and
Bases) to pass in.
1909
Developed pH Scale
(potential hydrogen)
Sören Sörensen
Based on the solution’s MOLAR concentration of the H+ ions.
Solution: homogeneous mixture of a solute
dissolved in a solvent
Acidic solution: hydronium ion concentration is
greater than hydroxide concentration
Basic (alkaline) solution: hydroxide ion
concentration is greater than the hydronium ion
concentration
Neutral solution: hydronium ion concentration
equals the hydroxide ion concentration
Hydronium and hydroxide ion concentrations
are measured in moles per liter, or molarity,
and its symbol is M
Ex:
2.0 moles of NaCl (116 g) dissolved in
Water to make 1.0 liters of solution has a
molarity of:
2.0 moles
= 2.0 M
1.0 L
Likewise:
0.16 moles of NaCl (116 g) dissolved in
Water to make 2.0 liters of solution has a
molarity of:
0.16 moles
= 0.080 M
2.0 L
Problem: Find molarity of 5 grams HCl
dissolved in water to make 250 mL of solution.
Solution:
5 g x 1mol = 0.14 mol
36 g
0.14 mol = 0.56 M
0.25 L
pH Scale
• We use this scale to measure the strength
of an acid or base.
• pH is defined as the –log[H+]
– (negative log of the hydrogen ion
concentration, in moles per liter)
• pH can use the concentration of
hydronium ions or hydrogen ions.
7
Acid
0
Base
pH Scale
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 515
14
pH Scale
Acidic
Neutral
Basic or
Alkaline
pH of Common Substances
Timberlake, Chemistry 7th Edition, page 335
pH and pOH of Common Substances
More acidic
More basic
pH
NaOH, 0.1 M
Household bleach
Household ammonia
Lime water
Milk of magnesia
Borax
Baking soda
Egg white, seawater
Human blood, tears
Milk
Saliva
Rain
Black coffee
Banana
Tomatoes
Wine
Cola, vinegar
Lemon juice
Gastric juice
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
[H1+]
[OH1-]
1 x 10-14
1 x 10-13
1 x 10-12
1 x 10-11
1 x 10-10
1 x 10-9
1 x 10-8
1 x 10-7
1 x 10-6
1 x 10-5
1 x 10-4
1 x 10-3
1 x 10-2
1 x 10-1
1 x 100
1 x 10-0
1 x 10-1
1 x 10-2
1 x 10-3
1 x 10-4
1 x 10-5
1 x 10-6
1 x 10-7
1 x 10-8
1 x 10-9
1 x 10-10
1 x 10-11
1 x 10-12
1 x 10-13
1 x 10-14
pOH
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Acid – Base Concentrations
concentration (moles/L)
10-1
pH = 3
pH = 11
OH-
H3O+
pH = 7
10-7
H3O+
OH-
OH-
H3O+
10-14
Timberlake, Chemistry 7th Edition, page 332
[H3O+] > [OH-]
[H3O+] = [OH-]
acidic
solution
neutral
solution
[H3O+] < [OH-]
basic
solution
pH
pH = -log [H+]
pOH = -log [OH-]
pH + pOH = 14
Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 285
Self-Ionization Of Water
Even the purest of water conducts electricity. This is
due to the fact that water self-ionizes, that is, it
creates a small amount of H3O+ and OH-.
H2O + H2O
H3O+ + OHIn pure water, the concentration of H+ is always equal
to the concentration of OH-.
This equilibrium is very important because it creates
a concentration of H+ whose negative logarithm is
7, the pH of a neutral solution.
pH and pOH
• pH = - log[H3O+]
[H3O+] = 10-pH
pOH = - log[OH-]
[OH-] = 10-pOH
• pH + pOH = 14.00
• neutral solution: [H3O+] = [OH-] = 10 –7 M pH = 7.0
acidic solution: [H3O+] > 10-7 M
pH < 7.0
[H3O+] < 10-7 M
pH > 7.0
basic solution:
Practice Problems:
Calculate the pH and pOH of:
3.2 x 10-6 M HCl
pH = 5.5
2.5 x 10-8 M NaOH
pOH = 7.6
Calculate the molarity of:
HNO3 pH = 5.75
1.8 x 10-6 M
5 g HNO3 in 800 mL
solution.
0.099 M
HCl pOH = 11.3
0.002 M
pOH = 8.5
pH = 6.4
Strengths of Acids and Bases
Strong acid:
Completely ionizes to produce H+
Weak acid:
Only partly ionizes
Strong base:
Completely dissolves to produce OH-
Weak base:
Only partly dissolves
Strong Acids
LiOH
NaOH
KOH
RbOH
CsOH
*Ca(OH)2
*Sr(OH)2
*Ba(OH)2
lithium hydroxide
sodium hydroxide
potassium hydroxide
rubidium hydroxide
cesium hydroxide
calcium hydroxide
strontium hydroxide
barium hydroxide
* These bases completely dissociate in solutions of 0.01 M
or less. The other bases make solutions of 1.0 M and are
100% dissociated at that concentration. There are other
strong bases than those listed, but they are not often
encountered.
HCl
HNO3
H2SO4
HBr
HI
HClO4
hydrochloric acid
nitric acid
sulfuric acid
hydrobromic acid
hydroiodic acid
perchloric acid
Strong Bases
Common Weak Acids
Acid
Formula
Formic
HCOOH
Acetic
CH3COOH
Trichloroacetic
CCl3COOH
Hydrofluoric
HF
Hydrocyanic
HCN
Hydrogen
sulfide
H2S
Water
H2O
Conjugate acids
NH4+
of weak bases
Common Weak Bases
Base
Formula
ammonia
NH3
trimethyl
ammonia
N(CH3)3
pyridine
C5H5N
ammonium
hydroxide
NH4OH
water
H2O
HS- ion
HS-
conjugate bases e.g.:
of weak acids
HCOO-
Strength vs. Concentration
Acids and bases:
Strong acids and bases can be concentrated or dilute
Weak acids and bases can be concentrated or dilute
Strength is how well the acid or base produces ions
Strong = complete ionization
Weak = partial ionization
Concentration is how many ions are in a volume
Concentrated
= many ions in a volume
Weak
=
few ions in a volume
Your turn!
1. Complete exercise C.9 (Strong vs
Concentrated) found on pages
376-377 (Due next class for a Lab
grade)
2. Complete pH and pOH worksheet
(Due next class for a homework
grade.)