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A half reaction
is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
s of individual substances involved in the redox reaction.
Consider the reaction below:
- Cl2 + 2Fe2+ → 2Cl− + 2Fe3+
The two elements involved, iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
, each change oxidation state; iron from 2+ to 3+, chlorine from 0 to 1−. There are then effectively two half
-reactions occurring. These changes can be represented in formulas by inserting appropriate electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s into each half-reaction:
- Fe2+ → Fe3+ + e−
- Cl2 + 2e− → 2Cl−
In the same way given two half-reactions it is possible, with knowledge of appropriate electrode potential
Electrode potential, E, in electrochemistry, according to an IUPAC definition, is the electromotive force of a cell built of two electrodes:* on the left-hand side is the standard hydrogen electrode, and...
s, to arrive at the full (original) reaction.