Oxidation-Reduction Reactions

Oxygen in Reactions

Define oxidation and reduction.
Describe oxidation-reduction reactions involving oxygen.

Figure 22.1

Before (top) and after (bottom) teeth bleaching.

Are our teeth sparkling white? Are they attractive?

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 the teeth. Less effective, but easier to use are “teeth-whitening” tooth pastes (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 (see video below).

$2text{Mg}(s)+text{O}_2(g) rightarrow 2text{MgO}(s)$

https://www.youtube.com/embed/NnFzHt6l4z8

Click on the image above for more content

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

$text{CH}_4(g)+2text{O}_2(g) rightarrow text{CO}_2(g)+2text{H}_2text{O}(g)$

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:

$text{CH}_3text{CH}_2text{OH} rightarrow text{CH}_3text{CHO} + 2text{H atoms}$

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 (NaClO), which release oxygen that oxidizes stains. Hydrogen peroxide (H ₂ O ₂ ) 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 thought 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.

$2text{Fe}_2text{O}_3(s)+3text{C}(s) rightarrow 4text{Fe}(s)+3text{CO}_2(g)$

The removal of oxygen from the Fe ₂ O ₃ means that it is being reduced to Fe. Note that an oxidation process is simultaneously occurring. The carbon reactant is being oxidized to 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 → acetaldehyde reaction shown above is a reduction reaction:

$text{CH}_3text{CHO} + 2text{H atoms} rightarrow text{CH}_3text{CH}_2text{OH}$

Summary

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

Practice

Answer the questions at the following link:

http://www.chemguide.co.uk/inorganic/questions/q-redoxdefs.pdf

Review

Questions

Give a definition of oxidation in terms of oxygen.
Give a definition of reduction in terms of oxygen.
Characterize the following reaction in terms of the definitions of oxidation and reduction: $text{CH}_4(g)+2text{O}_2(g) rightarrow text{CO}_2(g)+2text{H}_2text{O}(g)$

oxidation: A process in which a substance was reacted with oxygen to produce one or more oxides or a reaction involving the loss of hydrogen.
reduction: Removal of oxygen from a substance or the addition of hydrogen to a substance.

Redox Reactions and Ionic Compounds

Define oxidation and reduction in terms of electron transfers.
Write half-reactions for an oxidation-reduction process.
Give examples of redox reactions.

Nitric acid is created by the oxidation of ammonia

How do you make nitric acid?

Nitric acid has many uses in the manufacture of fertilizers and explosives. Most nitric acid is manufactured from ammonia using a three-step process. The ammonia is oxidized to HNO ₃ through the formation of several nitrogen oxides, finally resulting in the acid.

Redox Reactions and Ionic Compounds

In the course of a chemical reaction between a metal and a nonmetal, electrons are transferred from the metal atoms to the nonmetal atoms. For example, when zinc metal is mixed with sulfur and heated, the compound zinc sulfide is produced. Two valence electrons from each zinc atom are transferred to each sulfur atom.

Zinc donates two of its valence electrons to sulfur to form zinc sulfide

Figure 22.2

Reaction between zinc and sulfur.

Since the zinc is losing electrons in the reaction, it is being oxidized. The sulfur is gaining electrons and is thus being reduced. An oxidation-reduction reaction is a reaction that involves the full or partial transfer of electrons from one reactant to another. Oxidation is the full or partial loss of electrons or the gain of oxygen. Reduction is the full or partial gain of electrons or the loss of oxygen. A redox reaction is another term for an oxidation-reduction reaction.

Each of these processes can be shown in a separate equation called a half-reaction. A half-reaction is an equation that shows either the oxidation or the reduction reaction that occurs during a redox reaction.

$&text{Oxidation:} text{Zn} rightarrow text{Zn}^{2+}+2e^- \& text{Reduction:} text{S}+2e^- rightarrow text{S}^{2-}$

It is important to remember that the two half-reactions occur simultaneously. The resulting ions that are formed are then attracted to one another in an ionic bond.

Another example of an oxidation-reduction reaction involving electron transfer is the well-known combination of metallic sodium and chlorine gas to form sodium chloride:

$2text{Na} + text{Cl}_2 rightarrow 2 text{NaCl}$

The half-reactions are as follows:

$& text{Oxidation:} text{Na} rightarrow text{Na}^+ + e^- \& text{Reduction:} text{Cl} + e^- rightarrow text{Cl}^-$

We will concern ourselves with the balancing of these equations at another time.