Which is an oxidation half reaction

Historically, the term "oxidation" was used because the redox reactions that were first systematically investigated took place in oxygen, with oxygen being reduced and the other species being oxidized, hence the term oxidation reaction.

However, it was later realized that this case oxidation reactions involving oxygen was just one possible scenario. For example consider the redox reaction shown below. So oxidation reactions need not involve oxygen. This redox reaction is actually the sum of two separate half-reactions a reduction half-reaction and an oxidation half-reaction. Example 1. This is represented in the following reduction half-reaction note that the electrons are on the reactants side :.

Consider the example burning of magnesium ribbon Mg. When magnesium burns, it combines with oxygen O2 from the air to form magnesium oxide MgO according to the following equation:. Since there are 2 Mg on left side, a total of 4 electrons are lost according to the following oxidation half reaction:.

On the other hand, O 2 was reduced: its oxidation state goes from 0 to Thus, a reduction half-reaction can be written for the O 2 as it gains 4 electrons:. When chemical reaction, especially, redox reaction takes place, we do not see the electrons as they appear and disappear during the course of the reaction.

What we see is the reactants starting material and end products. Due to this, electrons appearing on both sides of the equation are canceled. After canceling, the equation is re-written as. In any given oxidation-reduction reaction, there are two half-reactions — oxidation half- reaction and reduction half-reaction. The atom that is oxidized is the reducing agent, and the atom that is reduced is the oxidizing agent. Note: the oxidizing and reducing agents can be the same element or compound.

Redox reactions are comprised of two parts, a reduced half and an oxidized half, that always occur together. The reduced half gains electrons and the oxidation number decreases, while the oxidized half loses electrons and the oxidation number increases. Simple ways to remember this include the mnemonic devices OIL RIG, meaning " oxidation is loss " and " reduction is gain. Those given off in the oxidation half reaction are taken up by another species in the reduction half reaction.

Hence, what is oxidized is the reducing agent and what is reduced is the oxidizing agent. Note: the oxidizing and reducing agents can be the same element or compound, as in disproportionation reactions discussed below.

Combination reactions are among the simplest redox reactions and, as the name suggests, involves "combining" elements to form a chemical compound. As usual, oxidation and reduction occur together. The general equation for a combination reaction is given below:.

In this reaction both H 2 and O 2 are free elements; following Rule 1 , their oxidation states are 0. The product is H 2 O, which has a total oxidation state of 0. According to Rule 6 , the oxidation state of oxygen is usually A decomposition reaction is the reverse of a combination reaction, the breakdown of a chemical compound into individual elements:. This follows the definition of the decomposition reaction, where water is "decomposed" into hydrogen and oxygen.

Note that the autoionization reaction of water is not a redox nor decomposition reaction since the oxidation states do not change for any element:. A single replacement reaction involves the "replacing" of an element in the reactants with another element in the products:.

A double replacement reaction is similar to a single replacement reaction, but involves "replacing" two elements in the reactants, with two in the products:.