Oxidation-Reductions:
Part I: Oxidation Numbers
(from Chapter 6 Sec 1
of Jespersen 6TH ed)
Dr. C. Yau
Spring 2013
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What are oxidation-reduction reactions?
"Oxidation-reduction" reactions are
commonly called "redox" reactions.
Remember:
Acid-base rxns involve a transfer of H+ or
proton.
Redox rxns involve a transfer of electrons.
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Oxidation Numbers
To understand redox, first you must learn
to assign oxidation numbers.
Oxidation numbers are hypothetical
charges assigned to each atom.
Each atom, even in molecular substances
that have no ions, is assigned a charge.
Note: The charge is hypothetical.
Oxidation numbers is only for "book
keeping" to keep track of electrons.
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Assigning Oxidation Numbers
LEARN THESE RULES IN ORDER!
1. Oxidation # of any free element is zero. e.g.
H in H2, P in P4 are assigned zero.
2. Oxid # of simple monatomic ions is the
charge of the ion.
e.g. In MgCl2, Mg is +2. Cl is -1.
In SnS, Sn is +2 and S is -2.
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Assigning Oxidation Numbers
3. In its cmpd, F is assigned -1.
4. In its cmpds, H is +1 unless it is bonded
to a metal, where it is -1 (as in hydride H-)
e.g. In HCl, H is +1 and Cl is -1.
In MgH2, Mg is +2 and H is -1.
5. In its cmpds, O is -2 unless it is a
peroxide. eg. in MgO, Mg is +2, O is -2.
In hydrogen peroxide (H2O2), H is +1 and
O is -1.
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Assigning Oxidation Numbers
6. Oxid # of others are generally calculated from knowing that
the sum of all charges must add up to charge of substance.
For example,
In HClO, sum of charges = 0
+1 ? -2
H is assigned +1, O is assigned -2. Cl is calculated to be +1
in order for the sum to be zero.
In the chlorite ion, ClO2-, sum of charges = -1
? -2
Oxygen is assigned -2, what must Cl be in order for net
charge to be -1?
Ans. Cl = +3
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Example 6.2
Titanium oxide, TiO2, is a white pigment used
in making paint. A now outmoded process
of making TiO2 from its ore involved
Ti(SO4)2.
What is the oxidation number of titanium in
Ti(SO4)2?
Example
Determine the oxidation numbers of atoms in
oxygen difluoride.
When there is conflict, apply the rules starting
from the top (Rule #1 and down).
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Example 6.4
The air bags used as safety devices in
modern autos are inflated by the very rapid
decomposition of the ionic compound
sodium azide, NaN3. The rxn gives
elemental sodium and gaseous nitrogen.
What is the average oxidation # of the
nitrogen in sodium azide?
How can we have a fractional charge?
Remember these are hypothetical charges
only.
Do Pract Exer 3, 4, 5 p.181
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Assign Oxidation #
Do not confuse oxidation # with overall
charge. We assign oxidation #’s to EACH
element, and it is the charge per atom.
a) SO42O is -2 and S is +6
b) NH4+
For the rest, jot down
what you think the
c) CoBr3
answers are and then
d) Fe(NO3)2
check them at my
e) CrClO3
CHEM 121 webpage
under ANSWERS.
f)
V2(OH)3
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Oxidation # is to identify oxidation & reduction.
Redox MUST have a change in oxidation numbers in at least
two of the atoms.
One MUST increase (oxidation).
LEARN
The other MUST decrease (reduction).
THESE
TERMS
H2 + Cl2
2 HCl
0
0
+1 -1
Oxid # of H increased from 0 to +1 (oxidation)
Oxid # of Cl decreased from 0 to -1 (reduction)
We say H2 is oxidized and Cl is reduced.
We say H2 has undergone oxidation and
Cl has undergone reduction.
An oxidizing agent causes others to be oxidized and is itself
reduced. (Cl is reduced and therefore it is the oxidizing
agent.)
A reducing agent causes others to be reduced and is itself
oxidized. (H is oxidized and therefore it is the reducing
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agent.)
Recognizing Redox Rxns
Example 6.5 p. 220
Is the following a redox rxn? If so, identify
the substance oxidized and the substance
reduced as well as the oxidizing and
reducing agents.
2KCl + MnO2 +2H2SO4
K2SO4 + MnSO4 + Cl2 + 2H2O
Do Pract Exer 8, 9, 10, 11 on p. 221.
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Recognizing Redox Rxns
Is the following a redox rxn?
Na2CO3 + 2HCl
2NaCl + CO2 + H2O
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