Oxidation and reduction are two types of chemical reactions that go hand-in-hand to form redox reactions. Historically, oxidation was defined according to the behavior of oxygen in a reaction, but the modern definition is somewhat different. Here is the definition of oxidation, along with examples of oxidation reactions.
Oxidation Definition
Oxidation is the loss of electrons or increase in oxidation state of a molecule, atom, or ion in a chemical reaction. The opposite process is called reduction, which is a gain of electrons or the decrease in the oxidation state of a molecule, atom, or ion. In a redox reaction, one chemical species is reduced, while another is oxidized. Redox stands for reduction-oxidation.
Originally oxidation meant adding oxygen to a compound. This is because oxygen gas (O2) was the first known oxidizing agent. Adding oxygen to a compound typically meets the criteria of electron loss and an increase in the oxidation state, but the definition of oxidation was expanded to include other types of chemical reactions.
In organic chemistry, you might encounter a definition for oxidation involving hydrogen. This definition deals with protons (hydrogen ions) rather than electrons, so it may cause confusion. Still, it’s good to be aware. According to this definition, oxidation is the loss of hydrogen, while reduction is the gain of hydrogen.
An examples is oxidation of ethanol into ethanal:
CH3CH2OH → CH3CHO
Ethanol is considered oxidized because it loses hydrogen. Reversing the equation, ethanal can be reduced by adding hydrogen to it to form ethanol.
Oxidation Examples
A classic example of oxidation occurs between iron and oxygen in moist air, forming iron oxide or rust. The iron is said to have oxidized into rust. The chemical reaction is:
4 Fe + 3 O2 + 6 H2O → 4 Fe(OH)3 or 2Fe2O3·6H2O
The iron metal is oxidized to form the iron oxide known as rust.
Fe → Fe2+ + 2 e−
Meanwhile, oxygen is reduced.
O2 + 4 e− + 2 H2O → 4 OH−
Another example of oxidation where an element combines with oxygen is the reaction between magnesium metal and oxygen to form magnesium oxide.
2 Mg (s) + O2 (g) → 2 MgO (s)
Many metals oxidize. Tarnish is the name given to the oxidation of silver. The green or blue patina that forms on copper in damp air is another example of oxidation.
But, oxidation does not require oxgen! An example of a reaction is that between hydrogen and fluorine gas to form hydrofluoric acid:
H2 + F2 → 2 HF
In this reaction, hydrogen is oxidized and fluorine is reduced. The reaction may be better understood if it is written in terms of two half-reactions.
H2 → 2 H+ + 2 e–
F2 + 2 e– → 2 F–
Electrochemical reactions are great examples of oxidation reactions. When a copper wire is placed into a solution that contains silver ions, electrons are transferred from the copper metal to the silver ions. The copper metal is oxidized. Silver metal whiskers grow onto the copper wire, while copper ions are released into the solution.
Cu(s) + 2 Ag+(aq) → Cu2+(aq) + 2 Ag(s)
Using OIL RIG to Remember Oxidation and Reduction
Remember, the modern definition of oxidation and reduction involves electrons (not oxygen or hydrogen). One way to remember which species is oxidized and which is reduced is to use OIL RIG. OIL RIG stands for Oxidation Is Loss, Reduction Is Gain.
Oxidizing and Reducing Agents
An oxidizing agent is a substance that oxidizes another material. In other words, an oxidizing agent donates oxygen (or donates hydrogen) to another species. A reducing agent is a substance that reduces another material. So, a reducing agent removes oxygen from another species (or gives it hydrogen).
References
- Haustein, Catherine Hinga (2014). K. Lee Lerner and Brenda Wilmoth Lerner (eds.). Oxidation–Reduction Reaction. The Gale Encyclopedia of Science (5th ed.). Farmington Hills, MI: Gale Group.
- Hudlický, Miloš (1990). Oxidations in Organic Chemistry. Washington, D.C.: American Chemical Society. ISBN 978-0-8412-1780-5.
- IUPAC (1997). “Oxidation”. Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”). ISBN 0-9678550-9-8. doi:10.1351/goldbook