Ch. 20
Oxidation-Reduction
Reactions
Types of Reactions
• There are many different types of reactions:
1.Redox
2.Acid-Base
3.Precipitation
20.1 Oxidation and Reduction
• Oxidation- Reduction Reaction:
reaction in which electrons are
transferred from one atom to
another
• AKA- Redox Reaction
Oxidation
The roaring fire
shown to the right is
an example of the
rapid oxidization of
the hydrocarbons
making up the wood
and the reduction of
the Oxygen gas from
the air.
The, very rusty,
Iron hammer is
also being
oxidized by the
Oxygen in the
air, but at a much
slower rate than
the burning
wood.
How do Oxidation and Reduction
Differ?
• Oxidation- loss of electrons
• OIL – Oxidation is Loss
• Ex: Na → Na+ + e-
Reduction
• Reduction- gain of electrons
• RIG – Reduction is Gain
• Ex: Cl + e- → Cl -
OIL RIG
• Oxidation Is Loss
• Reduction Is Gain
Changes in Oxidation number
•
Oxidation number – is the number of
electrons lost or gained by the atom
when it forms ions.
•
•
When an atom or ion is reduced, the
oxidation number is lowered.
Oxidation numbers keep track of the
movement of electrons in a redox
reaction.
Oxidation numbers are written with
the positive or negative sign
BEFORE the number. (+ 3, - 2)
Ionic charge is written with the sign
AFTER the number. (3+ , 2-)
Oxidation # in Redox Reactions
• When an atom is oxidized, its
oxidation number increases
• When an atom is reduced, its
oxidation number decreases
Oxidation # in Redox Reactions
• The Magnesium has been oxidized.
• The Hydrogen has been reduced.
• The Chlorine stayed the same.
Applications of redox chemistry
Removes tarnish
from metal
objects such as
silver.
• Chlorine bleach
whitens clothes
by oxidizing dyes,
stains, and other
materials that
discolor clothes.
• .
Hydrogen peroxide
is an antiseptic
because it oxidizes
biomolecules of
germs. It also
lightens hair
because it oxidizes
the dark pigment of
the hair.
• OxiClean contains
Hydrogen peroxide
which is a strong
oxidizing
substance which
will "bleach" the
stains away.
Assigning Oxidation
Numbers
The oxidation number of an uncombined
element is zero.
Includes diatomic molecules, subscripts
don’t matter.
• Examples:
• Fe: oxidation number = 0
• H2 : oxidation number = 0
• S8: oxidation number = 0
The oxidation number of a
monatomic ion is equal to the
charge on the ion.
• Examples:
• Oxidation number of S2- is -2.
Oxidation number of Al3+ is +3.
Oxidation number of Ca2+ is+2.
The oxidation number of the more
electronegative atom in a molecule or
complex ion is the same as the charge
it would have if it were an ion.
•
Ex: SiCl4 ; Cl is -1
Hydrogen (H) has two possible oxidation
numbers:
• +1 when bonded to a nonmetal
• -1 when bonded to a metal
The oxidation number of fluorine (F) is
always -1
• The sum of the oxidation numbers of
all atoms in a neutral compound = 0.
• Multiply subscripts by oxidation
numbers, then add.
• Ex:
•
NaCl (+1) + (-1) = 0
CaBr2 (+2) + 2(-1) = 0
• The sum of the oxidation numbers of
all atoms in a polyatomic ion = the
charge on the polyatomic ion (look on
polyatomic sheet).
• Multiply subscripts by oxidation
numbers, then add.
• Ex: NH4+1 (-3) + 4(+1) = +1
Assigning Oxidation Numbers
Let's start with Carbon
Dioxide, CO2.
1. Neutral compound
charge = 0
2. Oxygen gets its standard
charge of -2 for each
oxygen atom.
3. In order to tally to zero,
we MUST assign Carbon
the oxidation number of +4.
Neutral compound charge = 0
2. Chlorine is the most
electronegative element
charge of -1 for each Chlorine
atom.
3. both Hydrogen atoms are
given an oxidation number of
+1.
4. In order to tally to zero, we
MUST assign Carbon the
oxidation number of 0.
Polyatomic
• 1. This ion has a
charge = 2• 2. Oxygen gets its
standard charge of
-2 for each oxygen
atom.
• 3. In order to tally to
zero, we MUST
assign Sulfur the
oxidation number of
+6.
Oxidation Number in Redox
Reactions
• 1. When an atom is oxidized, its
oxidation number increases.
• 2. When an atom is reduced, its
oxidation number decreases.
Acid-Base Reactions
• In an acid-base reaction, an acid and a base
combine to produce a salt and water.
• Recall: all acids start with H
Example of Acid-Base
Reaction
• HCl(aq) + NaOH (aq)  NaCl (aq) + H2O (l)
• Acid + Base
 salt
+ water
• How to identify acid-base?
• All acids start with H
• All acid-base produce water as a product
Precipitation reactions
• In a precipitation reaction, two aqueous
solutions are combined to form a solid
precipitate, which settles out on standing.
• 2FeCl3(aq) + Cu(SO4) (aq)  Fe2(SO4)3 (aq) + 3CuCl2(s)
• How to identify precipitation reaction
• There is a small (s) on the product side
of the reaction
How to identify a precipitate if
not told
• Use your solubility chart!!!!!!