19.1
Oxidation-reduction reactions, or
redox reactions, are reactions in
which electrons are transferred from
one atom or molecule to another.
Many of the equations you are already
familiar with are redox reactions.
Consider the following redox reaction:
Mg + S
MgS
Recall that Mg has 2 valence
electrons and S has 6.
Mg
x
xx
S
xx
x
You can think of it as the sulfur
taking the two electrons from the Mg
when it bonds.
x
xx
Mg S
xx
x
Mg forms 2+ ions and
S forms 2- ions.
Oxidation numbers are a way of
tracking changes in the possession of
electrons.
Table 19.2 on page 686 has a list of
rules; you already know most of these.
Rules 3 and 4 are new to you.
You can put these in your notes later.
Consider the oxidation numbers of
the atoms:
0
0
+2
-2
Mg + S yields MgS
Because Mg has lost electrons
(negative charges), its oxidation
number has increased from 0 to +2.
The Mg has been oxidized.
0
0
+2
-2
Mg + S yields MgS
Because S has gained electrons, its
oxidation number has dropped from 0
to -2.The sulfur has been reduced.
The reducing agent supplies electrons
to the substance being reduced.
0
0
+2
-2
Mg + S yields MgS
Mg is the supplier or reducing agent.
The oxidizing agent accepts electrons
from the substance being oxidized.
0
0
+2
-2
Mg + S yields MgS
S gains electrons, so it’s the oxidizing agent.
There is a really good
Table (Table 19.1) on
p. 683 in your book.
Put it into your notes.
•Loses e•Ox. #
Mg is oxidized
Reducing agent
0
+2 -2
0
Mg + S yields MgS
•Gains e•Ox. #
S is reduced
Oxidizing agent
This silly mnemonic may help you:
LEO the lion says GER
Loss of Electrons is Oxidation
Gain of Electrons is Reduction
Not all reactions are redox reactions;
the oxidation numbers must change.
+1
-1
+1
-2
+1
-2
-1
HCl + H2O yields H3O+ + Cl-
None of the oxidation numbers have
changed – not a redox reaction.