Download Redox #2 Oxidation Numbers

Redox 2: Oxidation Numbers
Redox reactions are all about electrons being transferred from one substance to
another, so it would be useful if we had a system for keeping track of what gains
and what loses electrons, and how many electrons are involved.
We do - our record-keeping system is called Oxidation Numbers
Oxidation Numbers
A positive or negative number assigned to an atom in a molecule or
ion that reflects a partial gain or loss of electrons
What connections do you make with other chemistry concepts
you have studied?
Knowing the oxidation number of each individual element in a molecule will be a
key step in our understanding of redox reactions. Common oxidation states can be
found in Appendix A pg 856 Merrill Chemistry. Fortunately there are a series of
rules that we can use to determine oxidation numbers:
Example(s)/Explanation
Rules:
1. Pure elements have an oxidation number of 0
2. If the compound is an ionic compound, the oxidation number
for each element is the ion’s charge
3. The oxidation number of hydrogen in a compound is +1
4. The oxidation number of oxygen in most compounds is –2
(peroxides are the exception; in peroxides oxygen has an
oxidation number of –1)
5. The sum of the oxidation numbers in a compound is zero.
6. The sum of the oxidation numbers in a polyatomic ion is equal
to the ion charge.
It is important to note that oxidation number always refers to each individual
atom in the compound, not to the total for that element. For example in H2O - the
total positive "charge" for both hydrogen atoms will be +2 (which balances with
the -2 from oxygen), but EACH HYDROGEN has an oxidation number of +1.
Practice Assigning Oxidation Numbers
Oxidation Numbers for each Element
a.
SnCl4
Sn
Cl
b.
Ca3P2
Ca
P
c.
SnO
Sn
O
d.
Ag2S
Ag
S
e.
HI
H
I
f.
N2H4
N
H
g.
Al2O3
Al
O
h.
S8
S
i.
HNO2
H
j.
O2
O
k.
H3O+
H
O
l.
ClO3-
Cl
O
m. S2O32-
S
O
n.
KMnO4
K
Mn
O
o.
(NH4)2SO4
N
H
S
N
O
2. Determine the oxidation number of carbon in each of the following compounds:
a. methane, CH4
b. formaldehyde, CH2O
c. carbon monoxide, CO
d. carbon dioxide, CO2
O