No Oxygen, No Oxidation?
Date: November 2006
Does removing oxygen always prevents an oxidation reaction?
Not necessarily. The "old" definition of "oxidation" meant the
reaction with oxygen. As understanding developed, that definition
was generalized to mean the loss of electrons by a substance, or
equivalently, an increase in the valence state/number of a
substance. So, for example, the reaction of aluminum:
Al(metal) + 3 H2O + OH(-1) = (Al[OH]4)(-1) + 3/2 H2
is considered to be an oxidation reaction, even though there is no
oxygen directly involved. Here the presence/absence of oxygen (O2)
plays no role.
There are many other examples that could be given.
While elemental oxygen (O2) itself is a great oxidant, it is not
always required for oxidation. There are plenty of molecules that
contain oxygen that can oxidize things like bleach (sodium
hypochlorite) and many different peroxides, including hydrogen
peroxide (H2O2). An oxygen atom is not always required for
oxidation either. If you consider organometallic reactions, then
all kinds of molecules undergo oxidative addition during catalytic
cycles that don't contain oxygen.
Bacteria are also in the same boat. They can undergo aerobic (with
oxygen) or anaerobic (without oxygen) oxidation. Though during
these processes different products are formed depending on whether
oxygen is present or not.
No, not always, Henrietta.
Elements F, Cl, Br are also strong oxidizers, as is Oxygen.
Many compounds can be oxidizers, some with no oxygen.
The other half of the reaction is the substance being oxidized.
If it is extremely eager to be oxidized,
then unexpected things like water (H2O) or Nitrogen (N2) can be the
Those are weak oxidizers, but they are sometimes enough.
Reactive metals like Sodium (Na) are like that, when they are pure
An oxidation reaction is when one compound more-or-less steals electrons
Or when two molecule-parts bond together, but we believe from experience,
or from knowing our "oxidation numbers", that one part
tends to hold a little more than it's 50% share of the bond electrons.
Any time two parties to a reaction are unequal in the degree they like
chemists usually say that one oxidizes the other.
Oxidation does not mean reacting with oxygen. Oxidation means that a
substance loses at least one electron in the process of the
reaction. This also means that at least one substance gains at least
one electron and is reduced.
For example in this reaction of iron and chlorine gas to make
Fe(s) + Cl2(g) --> FeCl2(s)
The solid iron and the chlorine in the chlorine gas molecule both
have an oxidation state of zero (0). In iron(II) chloride, however,
the oxidation state of iron is (2+) and the oxidation state of
chlorine is (1-). This means that iron, in going from zero to 2+,
lost 2 electrons; the chlorine in going from zero to 1- gained 1
electron (remember that electrons are negatively charged). This also
tells us that 2 chlorines are needed for every one iron since this
is the only way that all the electrons get transferred.
So, as you will notice, no oxygen was involved in this reaction.
Greg (Roberto Gregorius)
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Update: June 2012