# 22.1: Oxygen in Reactions

We worry a lot about our smiles. Over the years teeth do discolor some, so the procedure of teeth bleaching has become more and more popular. Best done in a dentist's office, various chemical preparations containing peroxides are used to whiten teeth. Less effective, but easier to use are "teeth-whitening" toothpastes (also containing peroxides) that promise to give you a brighter smile and improve your life in every way.

## Oxygen in Reactions

Many elements simply combine with oxygen to form the oxide of that element. The heating of magnesium in air allows it to combine with oxygen from the air to form magnesium oxide.

$2 \ce{Mg} \left( s \right) + \ce{O_2} \left( g \right) \rightarrow 2 \ce{MgO} \left( s \right)$

Compounds can also react with oxygen, possible creating oxides of more than one element. When methane burns, carbon dioxide and water are produced.

$\ce{CH_4} \left( g \right) + 2 \ce{O_2} \left( g \right) \rightarrow \ce{CO_2} \left( g \right) + 2 \ce{H_2O} \left( g \right)$

Carbon dioxide is an oxide of carbon, while water is an oxide of hydrogen. Early scientists viewed oxidation as a process in which a substance was reacted with oxygen to produce one or more oxides. In the previous examples, magnesium and methane are being oxidized.

Oxidation is also defined as a loss of hydrogen atoms. In the following equation, ethanol is oxidized to acetaldehyde by the loss of two hydrogen atoms:

$\ce{CH_3CH_2OH} \rightarrow \ce{CH_3CHO} + 2 \ce{H} \: \text{atoms}$

Oxidation does not necessarily require heating. Iron that is exposed to air and water slowly oxidizes in a process commonly known as rusting. Bleaches contain various compounds such as sodium hypochlorite $$\left( \ce{NaClO} \right)$$, which releases oxygen that oxidizes stains. Hydrogen peroxide $$\left( \ce{H_2O_2} \right)$$ releases oxygen as it spontaneously decomposes. It acts as a bleach and an antiseptic that kills bacteria by oxidizing them.

The chemical reaction that is the opposite of oxidation is called reduction. Following from the notion that oxidation was originally though to mean only the addition of oxygen, reduction was thought to be only the removal of oxygen from a substance. Many naturally occurring metal ores are present as oxides. The pure metals can be extracted by reduction. Iron is obtained from iron (III) oxide by reacting with carbon at high temperatures.

$2 \ce{Fe_2O_3} \left( s \right) + 3 \ce{C} \left( s \right) \rightarrow 4 \ce{Fe} \left( s \right) + 3 \ce{CO_2} \left( g \right)$

The removal of oxygen from the $$\ce{Fe_2O_3}$$ means that it is being reduced to $$\ce{Fe}$$. Note that an oxidation process is simultaneously occurring. The carbon reactant is being oxidized to $$\ce{CO_2}$$. This is an important concept. Oxidation and reduction must happen together. Neither can happen alone in a reaction.

Reduction can also be considered as a gain of hydrogen. The reverse of the ethanol $$\rightarrow$$ acetaldehyde reaction shown above is a reduction reaction:

$\ce{CH_3CHO} + 2 \ce{H} \: \text{atoms} \rightarrow \ce{CH_3CH_2OH}$

## Summary

• Oxidation and reduction are defined in terms of reactions with oxygen.
• Oxidation and reduction reactions must occur together.

## Contributors

• CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.