Redox reactions — reactions in which there’s a simultaneous transfer of electrons from one chemical species to another — are really composed of two different reactions: oxidation (a loss of electrons) and reduction (a gain of electrons).
The electrons that are lost in the oxidation reaction are the same electrons that are gained in the reduction reaction. These two reactions are commonly called half-reactions; the overall reaction is called a redox (reduction/oxidation) reaction.
Oxidation
There are three definitions you can use for oxidation:
The loss of electrons
The gain of oxygen
The loss of hydrogen
Loss of electrons
One way to define oxidation is with the reaction in which a chemical substance loses electrons in going from reactant to product. For example, when sodium metal reacts with chlorine gas to form sodium chloride (NaCl), the sodium metal loses an electron, which is then gained by chlorine.
The following equation shows sodium losing the electron:
When it loses the electron, chemists say that the sodium metal has been oxidized to the sodium cation. (A cation is an ion with a positive charge due to the loss of electrons.)
Reactions of this type are quite common in electrochemical reactions, reactions that produce or use electricity.
Gain of oxygen
Sometimes, in certain oxidation reactions, it’s obvious that oxygen has been gained in going from reactant to product. Reactions where the gain of oxygen is more obvious than the gain of electrons include combustion reactions (burning) and the rusting of iron. Here are two examples.
Burning of coal:
Rusting of iron:
In these cases, chemists say that the carbon and the iron metal have been oxidized to carbon dioxide and rust, respectively.
Loss of hydrogen
In other reactions, oxidation can best be seen as the loss of hydrogen. Methyl alcohol (wood alcohol) can be oxidized to formaldehyde:
In going from methanol to formaldehyde, the compound went from having four hydrogen atoms to having two hydrogen atoms.
Reduction
Like oxidation, there are three definitions you can use to describe reduction:
The gain of electrons
The loss of oxygen
The gain of hydrogen
Gain of electrons
Reduction is often seen as the gain of electrons. In the process of electroplating silver onto a teapot, for example, the silver cation is reduced to silver metal by the gain of an electron. The following equation shows the silver cation gaining the electron:
When it gains the electron, chemists say that the silver cation has been reduced to silver metal.
Loss of oxygen
In other reactions, it’s easier to see reduction as the loss of oxygen in going from reactant to product. For example, iron ore (primarily rust) is reduced to iron metal in a blast furnace by a reaction with carbon monoxide:
The iron has lost oxygen, so chemists say that the iron ion has been reduced to iron metal.
Gain of hydrogen
In certain cases, a reduction can also be described as the gain of hydrogen atoms in going from reactant to product. For example, carbon monoxide and hydrogen gas can be reduced to methyl alcohol:
In this reduction process, the CO has gained the hydrogen atoms.
One’s loss is the other’s gain
Neither oxidation nor reduction can take place without the other. When those electrons are lost, something has to gain them.
Consider, for example, the net-ionic equation (the equation showing just the chemical substances that are changed during a reaction) for a reaction with zinc metal and an aqueous copper(II) sulfate solution:
This overall reaction is really composed of two half-reactions, shown below.
Oxidation half-reaction — the loss of electrons:
Reduction half-reaction — the gain of electrons:
Zinc loses two electrons; the copper(II) cation gains those same two electrons. Zn is being oxidized. But without that copper cation (the oxidizing agent) present, nothing will happen. It’s a necessary agent for the oxidation process to proceed. The oxidizing agent accepts the electrons from the chemical species that is being oxidized.
The copper(II) cation is reduced as it gains electrons. The species that furnishes the electrons is called the reducing agent. In this case, the reducing agent is zinc metal.
The oxidizing agent is the species that’s being reduced, and the reducing agent is the species that’s being oxidized. Both the oxidizing and reducing agents are on the left (reactant) side of the redox equation.