Aqueous Reactions and
Solution Stoichiometry
(continuation)
1. Electrolytes and non-electrolytes
2. Determining Moles of Ions in Aqueous
Solutions of Ionic Compounds
3. Acids and Bases
4. Acid Strength
5. Neutralization Reaction
Electrolytes and non-electrolytes
•Substance, which dissolve to give a solution
that conduct electricity due to the ions they
contain are called electrolytes
•Those substances that give aqueous solutions
which do not conduct electricity because the
solute remains molecular are called –
non-electrolytes
Ionic Compounds in Water (Electrolytes)
•The conduction process in the solution is due to the free
movement of ions when the compound dissolves in water
There are two ways to provide mobile ions for conducting
purposes.
1. Dissociation of Ionic Compounds
These ions are not the result of a chemical reaction, they
are the result of a dissociation of the compound into ions
that compose the solid.
+
−
NaCl ( s ) ⎯⎯
⎯→ Na ( aq) + Cl ( aq)
H 2O
2. Ionization of Polar Covalent Molecular Substances
In the case when dissolves the covalent
compound no ions are formed, the molecules
just disperse throughout the solventNon-electrolyte is formed
sugar ( s ) ⎯⎯
⎯→ sugar ( aq)
H 2O
Strong and Weak Electrolytes
Strong electrolyte – A substance which completely
dissociates (1) or ionizes (2) in water.
∆EN=2.1
∆EN=0.9
+
−
NaCl ( s ) ⎯⎯
⎯→ Na ( aq) + Cl ( aq)
H 2O
HCl + H 2O ⎯
⎯→ H 3O + + Cl −
(1)
(2)
H+ ions in water = hydronium ion (H3O+)
3H2O*H or H7O3+
Weak electrolyte: A substance which partially
ionizes when dissolved in water.
−
CH 3CO2 H + H 2O ⇔ CH 3CO2 + H 3O +
Acetate-anion Hydronium-cation
< 5% ionization
Problem:
How many moles of each free ion are formed when
75.0 mL of 0.56 M scandium bromide is dissolved in water?
ScBr3 (s)
H2O
Sc+3(aq) + 3 Br -(aq)
Converting from volume to moles:
0.56 mol ScBr3
1
L
x
= 0.042 mol
Moles of ScBr3 = 75.0 mL x 3
1
L
10 mL
3 mol Br Moles of Br - = 0.042 mol ScBr3 x 1 mol ScBr3
= 0.126 mol Br -
0.042 mol Sc+3 are also present.
Arrhenius Acid - substance which contain and
releases hydrogen cations=protons (H+) in water
Examples:
Hydrochloric Acid
HCl
Nitric Acid
HNO3
Acetic Acid
CH3CO2H
Sulfuric Acid
H2SO4
Notice that sulfuric acid can provide two H+’s
– Diprotic acid,
the other acids can provide only one H+
– Monoprotic acid.
Arrhenius Bases – substances, which produces
hydroxide ions on ionization in water
NH3 + H2O NH4+ + OHAmmonia
Ammonium Hydroxide anion
H2O
NaOH
Na+ + OH-
The limitations on Arrhenius definitions
1. Specific to one particular solvent-water.
2. The need for Hydroxide bases
Brønsted/Lowry theory
Acids: proton donors
- HCl, HF, H2CO3
Bases: proton acceptors - OH-, NH3, N2H4
This theory focuses attention on the ability of acids and
bases to participate in the transfer of proton from donor
molecule or ion to the acceptor molecule or ion.
These definitions are not as restrictive as Arrhenius’
definitions.
1. No need for water although it can be present, it need
not be.
2. Bases do not have to be Hydroxide compounds.
3. The theory makes no distinction between elements
and compounds
However, one restriction still remaining is the need for
a protic acid.
Compounds that do not contain OH- ions may
also be bases
CaO (s)+H2O (l) → Ca(OH)2 (aq)
Ca 2+ + O 2- +H2O (l) → Ca 2+ + 2OH- (aq)
O2- +H2O (l) → 2OH- (aq) net ionic eq.
The strong ability of O2- anion to attract protons
accounts for the observation that aqueous solutions of
ionic oxides do not exist: all soluble oxides
immediately form hydroxides when they dissolve.
This theory introduced the concept of
conjugate acid- base pairs
•A conjugate base is the remainder
of the original acid, after it donates
it’s hydrogen ion
•A conjugate acid is the particle
formed when the original base gains
a hydrogen ion
H2O + NH3 (aq) →NH+4 (aq) + OH-(aq)
acid
base
acid
base
NH3 is a base and NH 4 + is its conjugate acid.
Water is an acid and OH- is its conjugate base.
HCO3- (aq) + H2O (l) ' H3O+ (aq) + CO32- (aq)
Acid(1)
Base (2)
+H+
Conj. Acid (2)
Conj. Base (1)
-H+
Conjugates
Conjugatesdiffer
differby
byaaproton
proton
EVERY
-versa.
versa
EVERYacid
acidhas
hasaaconjugate
conjugatebase
baseand
andvice
vice-versa.
Lewis Acids and Bases
Gilbert Lewis defined these in an even less
restrictive manner:
Acid- acceptor of a lone pair electrons
Base- donor of a lone pair electrons
In this set of definitions there is no longer a need
for a protic acid. In other words only electron
exchange must occur.
Arrhenius, Brønsted/Lowry and Lewis definition sets are
NOT contradictory.
e pair acceptor
e pair donor
Lone pairs
+
HCl
acid, H+ donor
+
H2O
base, H+ acceptor
H3O+
Cl−
e pair donor e pair acceptor
+
NH3
base, H+ acceptor
+
H2O
acid, H+ donor
NH4+
A Proton donor is the same as an electron acceptor.
A Proton acceptor is the same as an electron donor.
OH−
This definition is more broad than the Bronsted-Lowery definition. Obviously,
H+ is an acid and OH- is a base under either definition, since for an proton to
bind to a base, it must accept a pair of electrons
However, the Lewis definition extends beyond just the proton. For example,
many metal ions can act as Lewis acids when they form complex ions
Note, that there are no protons in this reaction, but it is still an acid/base reaction.
Strong and Weak Acids and Bases
The strength of acids and bases are concerned
with the ionization (or dissociation) of the
substance, not its chemical reactivity
Strong acids and bases are strong electrolytes.
Weak acids and bases are weak electrolytes
Example:
Hydrofluoric acid (HF) is a weak acid, but it is very
chemically reactive.
- this substance can’t be stored in glass bottles because it
reacts with glass (silicon dioxide)
Binary acids are acids where the proton is bonded to
any element other than oxygen
Oxyacids contain three elements (oxygen, hydrogen,
and something else). They are ternary compounds
with the general formula:
HxEyOz
They have a E-O-H group that supplies the proton:
HNO3 nitric acid, HC2H3O2 acetic acid, H2SO4 sulfuric acid
H2CO3 carbonic acid, H2CrO4 chromic acid
HClO
HClO2 HClO3 HClO4
The larger the number of non-hydrogenated oxygens, the
stronger the acid
Naming of Acids
12/7/2003
23
Common Strong Bases
Lithium Hydroxide
Sodium Hydroxide
Potassium Hydroxide
Rubidium Hydroxide
Cesium Hydroxide
Calcium Hydroxide
Strontium Hydroxide
Barium Hydroxide
LiOH
NaOH
KOH
RbOH
CsOH
Ca(OH)2
Sr(OH)2
Ba(OH)2
Unlike weak acids, weak bases (Ammonia and organic
bases) DO NOT dissociate. They undergo hydrolysis
reactions.
Weak bases only partially undergo hydrolysis to produce
hydroxide. The conjugate acids of weak bases are weak
acids! Weak bases also react completely if titrated with a
B- + H 2O ⇔ BH + OH strong acid!
H2O + NH3 (aq) →NH4+ (aq) + OH-(aq)
Strong Bases
12/7/2003
Strong Acids
25
Neutralization Reaction
-Reaction between an acid and a base , which produces
water and a salt”
The generalized reaction between an Acid and a Base is:
HX(aq) + MOH(aq)
Acid + Base
MX(aq) + H2O(l)
Salt + Water
HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq)
Salt – typically ionic compound whose cation comes from
a base and anion from an acid.
Net Ionic Equation
H+(aq) + OH-(aq) → H2O(l)
An Aqueous Strong Acid-Strong Base
Reaction on the Atomic Scale
Acid + Base
Salt + H2O