Acids & Bases
Lesson 2
Strong and Weak
(Bases)
Review of BronstedLowry Acids
FORMING HYDRONIUM
IONS
The proton (H+) has been transferred from
the HCl molecule to a water molecule.
form a hydronium (H3O+) ion and a Clion.
This type of reaction is called ionization
(because ions are being formed).
BRONSTED-LOWRY DEFINITION
OF ACIDS AND BASES
An acid is any substance which donates
(gives) a proton (H+) to another
substance.
A base is any substance which accepts
(takes) a proton from another substance.
A Bronsted Acid is a proton donor
A Bronsted Base is a proton acceptor
BRONSTED-LOWRY DEFINITION
OF ACIDS AND BASES
We see that the HCl is donating the proton and the water is accepting
the proton.
Therefore HCl is the Bronsted acid and H2O is the Bronsted base.
HCl + H2O
acid
base

H3O+ +
Cl-
BRONSTED-LOWRY DEFINITION
OF ACIDS AND BASES
Let’s look at another example:
NH3
base
+
H2 O
acid

NH4+
acid
+
OH-
base
Bronsted Bases
bases
Increasing strength
acids
Weak Bases
SO32NH3
CNCO32-
PO43-
Strong Bases- yellow
Oxide O2-
Amide NH2-
The two Bronsted strong bases. They are found on the right bottom area of the acid
chart -p 334
O2-
H+
+
H2O

OH-
NH2-
H+
+
H2 O

NH3
Again, all bases make OH-
 means strong
+
+
OH-
OH-
Bronsted Weak Bases
F-
H+
+
H2 O
⇄
HF
+
OH-
H2O
⇄
HC2O4-
+
OH-
⇄
H2CO3
+
OH-
H+
C2O42-
+
H+
HCO3-
+
H2 O
Arrhenius bases
Strong Bases- yellow
LiOH
NaOH
KOH
RbOH
CsOH
FrOH
Ra(OH)2
Sr(OH)2
Ba(OH)2
Arrhenius Bases
Weak Basesmost other metals
Be(OH)2
Zn(OH)2
AgOH
Arrhenius Bases
Strong Bases
Strong bases completely ionize in water to produce OHThere are 11 Arrhenius strong bases - Group I and Group II hydroxides
LiOH

Li+
+
OH-
KOH

K+
+
OH-
Sr(OH)2

Sr2+
+
2OH-
Ra(OH)2

Ra2+
+
2OH-
Note all bases make OH-
 means strong
Arrhenius Weak Bases
A weak base is one that partially reacts with water to produce OH-.
Be(OH)2(s)
⇄
Be2+
+
2OH-
Zn(OH)2(s)
⇄
Zn2+
+
2OH-
H+
C6H5O73-
NH3
+
H2O
⇄
HC6H5O72-
+
OH-
H+
+
H2O
⇄
NH4+
+
OH-
Whether you consider Arrhenius or BronstedLowry, BASES act the same way: They all make
OH- by accepting a proton from water!
Weak bases
• At equilibrium, use
• Use Kb
– Base ionization constant
Write the Kb expression for the following 3 weak bases
[HF][OH-]
Kb
=
[F-]
[HC2O4-][OH-]
Kb
=
[C2O42-]
[H2CO3][OH-]
Kb
=
[HCO3-]
AMPHIPROTIC SUBSTANCES
Some substances (ex: H2O) are capable of
acting as an ACID (when surrounded by a
stronger base) OR acting as a BASE (when
surrounded by a stronger acid).
Substances that act as acids or bases are called
amphiprotic.
AMPHIPROTIC SUBSTANCES
Other amphiprotic substances:
• H2O
• H2PO4• HCO3+ H+
• Example:
H3PO4
- H+
H2PO4-
HPO42-
AMPHIPROTIC SUBSTANCES
In general, amphiprotic substances…
Have a negative charge and
An easily removable hydrogen.
AMPHIPROTIC SUBSTANCES
Identify the acid and base in the reactants of the following reactions:
H2S + HCO3-  H2CO3 + HS-
NH4+ + H2O 
H3O+ + NH3
HCOOH + HSO3-  H2SO3 + HCOO-
CONJUGATE ACID-BASE
PAIRS
Conjugate acid – the species with one more proton (ex.
HIO3)
Conjugate base - the species with one less proton (ex.
IO3-)
*There will always be 2 conjugate pairs
POLYPROTIC ACIDS
The formula of an acid tells us how many protons (H+) the acid can
donate.
An acid that can supply:
– ONE proton (ex: HCl)
– TWO protons (ex: H2SO4)
– THREE protons (ex: H3PO4)
– More than ONE proton
= monoprotic acid
= diprotic acid
= triprotic acid
= polyprotic acid
WRITE THE BRONSTED-LOWRY ACID-BASE
EQUILIBRIA WHEN THIS PAIR IS MIXED:
HCN and F-
Homework:
Hebden: pg 115 #10, pg 117 #11-12,
pg 119 #13-14, pg 121 #16-19