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Name: Henrietta
Status: student
Grade: 9-12
Location: N/A
Country: N/A
Date: November 2006

Question:
Does removing oxygen always prevents an oxidation reaction?



Replies:
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.

Vince Calder


Henrietta,

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.

Matt Voss


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 oxidizer. Those are weak oxidizers, but they are sometimes enough. Reactive metals like Sodium (Na) are like that, when they are pure elements.

An oxidation reaction is when one compound more-or-less steals electrons from another. 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 electrons, chemists usually say that one oxidizes the other.

Jim Swenson


Henrietta,

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 iron(II) chloride:

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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