What is a redox reaction?
A redox reaction is any chemical reaction in which the oxidation numbers of molecules are changed. Redox reactions are a type of double replacement reaction where one reactant is being oxidized (losing electrons) while the other is being reduced (gaining electrons).
The reactants in a redox reaction
In a redox reaction,the reactants are either elements or compounds in which the atoms have different oxidation numbers. One reactant is oxidized (loses electrons) while the other is reduced (gains electrons).
The products in a redox reaction
In a redox reaction, there are always two reactants – one that is being oxidized (losing electrons) and one that is being reduced (gaining electrons). The substance that is being oxidized is called the “oxidizing agent” and the substance that is being reduced is called the “reducing agent”.
In any reaction, the oxidizing agent is always reduced and the reducing agent is always oxidized. This can be helpful in figuring out which reactant is which.
The products in a redox reaction are always the same as the reactants, just in different oxidation states. So, if we have a reaction where iron(III) oxide reacts with carbon to form iron and carbon dioxide, we can say that:
-The iron(III) oxide is being oxidized to iron(II) oxide.
-The carbon is being reduced to carbon dioxide.
The role of electrons in a redox reaction
In a redox reaction, the transfer of electrons between two species (reactants) results in a change in their oxidation state. This change can be used to determine the amount of each reactant that is converted to product.
The magnitude of the change in oxidation state for a particular species is known as its oxidizing power. The higher the oxidizing power, the greater the amount of reactant that is converted to product.
In general, electrons are transferred from the more electronegative species to the less electronegative species. However, this is not always the case. For example, in the reaction between iron and copper(II) sulfate, electrons are transferred from iron to copper(II), even though iron is less electronegative than copper(II). This is because the overall reaction is thermodynamically favored (that is, it has a negative free energy change).
The following table lists some common redox couples and their respective oxidizing powers:
| Species | Oxidizing power |
|---|---|
| Fluorine | 2.87 V |
| Oxygen | 1.23 V |
| Chlorine | 1.36 V |
| Bromine | 0.99 V |
| Iodine | 0.54 V |
The importance of redox reactions
Redox reactions are extremely important in many areas of chemistry, including biochemistry, since they allow electrons to be transferred between molecules. This type of reaction is key in the process of photosynthesis, as well as in the production of ATP, the energy molecule used by cells. Redox reactions are also responsible for the corrosion of metals and the rusting of iron.