A bonfire. Combustion consists of redox reactions involving free radicals.

Combustion is an example of an oxidation-reduction reaction.

When a chemical bond forms, the bonding electrons enter a molecular orbital within the internuclear space pulling the bonded atoms together. This represents an electrostatic potential energy decrease. When there is an electronegativity difference between two atoms, the more electronegative atom pulls the electron or electrons the other atom has brought to the new bond inward towards its more powerful nucleus. This drawing in of negative charge by the more electronegative atom increases the magnitude of internal energy decrease above and beyond what is normally seen. This extra energy makes polar bonds stronger. As a general rule, they release more energy when they form. This phenomenon is the basis of oxidation-reduction. Oxidation-reduction is simply an accounting system to keep track of the extra energy released when a very electronegative element (high reduction potential) gains electron control (is reduced). Oxidation-reduction tells the story in terms of a simple narrative of electron control.

As a framework which lends a great deal of coherence to biochemistry, oxidation-reduction is one of the most important topics in physical science for future doctors. The central narrative of bioenergetics, for example, is the transduction of redox potential into phosphoryl transfer potential.

WikiPremed Resources

Redox and Electrochemitry Practice
Problem set for Oxidation-Reduction & Electrochemistry in PDF format

Answer Key
Answers and explanations

Oxidation-Reduction Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for Oxidation-Reduction
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals


Understand how to distinguish oxidation-reduction reactions from metathesis reactions, and be able to identify the oxidizing agents and reducing agents within redox reactions.

Be well practiced in assigning oxidation numbers.

Gain an intuitive ability to predict the redox behavior of various substances from their chemical structure, whether they will tend to act as oxidizing or reducing agents.

Comprehend the application of chemical bonding and chemical thermodynamic principles to judging the spontaneity of redox reactions.

Be able to narrate the events that occur when a copper wire is suspended in a solution containing silver ions.

Understand what occurs within the various processes known as corrosion.

Have a basic familiarity with the methods for balancing redox reactions, the oxidation number method and the half-reaction method.

Be prepared to identify common oxidizing and reducing agents.

Be able to describe how a potentiometer or a redox indicator may be used to determine the endpoint of a redox titration.

Suggested Assignments

Review terminology for oxidation reduction using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Perform the practice items for oxidation reduction. Here is the answer key for the practice items.

Read pp. 191-195 in ExamKrackers Chemistry. Perform practice items 129-136 on pg 196.

Review the web resources on oxidation reduction.

Conceptual Vocabulary for Oxidation-Reduction

Redox, shorthand for reduction/oxidation reaction, is a term used to describe chemical reactions in which atoms have their oxidation state changed.
Oxidizing agent
An oxidizing agent is a substance that gains electrons in a redox chemical reaction.
Reducing agent
A reducing agent is the element or a compound that reduces another species in a redox reaction. It is the electron donor in the redox.
Oxidation number
Within a molecule or complex, the oxidation number of an element is the charge that it would have if the compound were composed of ions, with assignment of shared electrons based on electronegativity.
Oxidation state
The oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. It is usually numerically equal to the oxidation number.
A half reaction is either the oxidation or reduction reaction component of a redox reaction.
Reduction potential
Standard reduction potential is the tendency of a chemical species to acquire electrons and thereby be reduced.
Electron transfer
Electron transfer is the process by which an electron moves from one atom or molecule to another atom or molecule.
Electron acceptor
A terminal electron acceptor is a compound that receives or accepts an electron during cellular respiration or photosynthesis.
Electron donor
In general terms, an electron donor gives up an electron during cellular respiration.
Superoxide is the free radical product of the one-electron reduction of dioxygen.
A peroxide is a compound containing an oxygen-oxygen single bond.
Metathesis reaction
Metathesis is a bimolecular process involving the exchange of bonds between the two reacting chemical species, which results in the creation of products with similar or identical bonding affiliations.
Denitrification is the process of reducing nitrate and nitrite, which can be thought of as the opposite of nitrogen fixation
An oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule to another.
Inner sphere electron transfer
Inner sphere electron transfer proceeds via a covalent linkage between the two redox partners, the oxidant and the reductant.
Outersphere electron transfer
Outer sphere electron transfer refers to an electron transfer event that occurs between chemicals that remain separate species before, during, and after the electron transfer event.
Advanced terms that may appear in context in MCAT passages
Osmium tetroxide
Osmium tetroxide is the chemical compound with one oxmium atom bonded to four oxygens. It is an important oxidizing agent in the laboratory.
Perchlorates are the salts derived from perchloric acid, which are important oxidizing agents for their tendency not to react unless heated.
Redox signaling
Redox signaling is the concept that free radicals, reactive oxygen species, and other electronically-activated species act as messengers in biological systems.
Marcus theory
Marcus Theory was originally developed to explain outer sphere electron transfer, but was later extended to inner sphere electron transfer by Noel Hush.