Chemistry involves different kinds of matter and change of these matters from one form to the other. This occurs by different types of reactions. One of the essential categories among various categories of reactions includes Redox Reactions (Oxidation and Reduction). There are many different types of redox reactions. Let us go through the types of redox reaction (Oxidation and Reduction reactions) along with its example to further understand the classification of redox reactions.
Types of Redox Reactions (Oxidation and Reduction)
Redox reactions can be primarily classified into five different types:
- Combination Reactions
- Decomposition Reactions
- Displacement Reactions
- Disproportionate Reactions
Let us go through each type of redox reaction one-by-one.
Browse more Topics under Redox Reactions
- Balance Redox Reactions
- Classical Idea of Redox Reactions
- Oxidation Number
- Redox Reactions and Electrode Potential
- Redox Reactions as the Basis of Titrations
- Redox Reactions – Electron Transfer Reactions
1) Combination Reactions
In this type of oxidation and reduction reactions, that is, redox reactions, two species of any atoms or molecules combine to form a single species of the compound. The schematic representation of the reaction is
This type of reaction refers to redox reactions only when both the species (A & B) or either of the species are present in their elemental form. This type of redox reaction (oxidation and reduction) reaction is just the opposite of the decomposition reaction.
It is a subtype of combination reaction in which one of the species will always be elemental dioxygen. Refer to the examples below.
2) Decomposition Reactions
It is just the opposite of combination reactions. This type of reactions involves breaking down of a single compound into two or more different compounds. However, at least one product out of the two or more components must be in its elemental form. Examples of this category of oxidation and reduction reactions are
All the above reactions take place under heat as a necessary condition.
However, not all the decomposition reactions are redox process/reactions. For instance:
3) Displacement Reactions
In the above reaction, the X atom replaces atom Y from the compound YZ. There are mainly two types of displacement reactions:
- Metal-displacement reactions
- Non-metal displacement reactions
i) Metal-Displacement Reactions
In this type of displacement reaction, an elemental metal displaces a metallic compound present in the reaction. The metal which is a better reducing agent displaces the other metal. Refer to the examples below
ii) Non-Metal Displacement Reactions
In this type of reactions, either a metal or a non-metal will displace another non-metal of a compound present in the reaction. Usually, the non-metal undergoing displacement is hydrogen. However, in certain cases, halogens or oxygen undergoing displacement can occur.
First, we will discuss the reactions involving hydrogen displacement. According to the capability of reducing metal or non-metal, following non-metal displacement reaction cases occur:
- Good reducing agents displace hydrogen in cold water. All alkali metals and few alkaline earth metals such as Calcium, Strontium, and Barium are very good reducing agents.
- In another case, less active metals can react with steam to form hydrogen gas. Examples of less active metals are magnesium and iron.
- There are metals that cannot displace hydrogen when reacting in cold water. However, these metals can displace hydrogen from acids. Refer to the examples below
- Metals such as cadmium and tin do not react with steam. However, it reacts with acids and forms dihydrogen gas.
- There are metals such as silver and gold that are very less reactive. They generally occur in the native form and are not reactive even with dilute HCl.
Reactivity of Metals and Non-Metals
Reactivity of Metals
The determination of the order of the reactivity of metals is possible by observing the rate of evolution or rate of displacement of hydrogen from dilute/aqueous acids and water. The order of reactivity can also be determined by the tendency of metals to lose electrons which in turn signifies the metals reducing capacity.
For instance, the rate of reactivity of Na than Mg. The rate of reactivity of Mg is faster than Fe. Therefore, Fe reacts at the slowest rate. Additionally, metals like silver and gold do not react altogether. The order of reactivity of metal can be seen in metal activity series.
Reactivity of Non-Metals
Non-metals also have activity series similar to metals. As we know, non-metals have the electron accepting tendency whereas metals have the electron losing tendency. Thus, the reactivity of non-metals depends on the oxidizing power of a non-metal. If we consider the halogen group, the oxidizing power of group number 17 decreases from fluorine to iodine.
Hence, fluorine has the highest oxidizing capacity and it will be able to displace chlorine, bromine, and iodine from a chloride, bromide, or an iodide solution respectively. The reacting capacity of F2 is so high that it can even displace oxygen from water.
However, chlorine is only capable of displacing bromine from the solution of bromide ions and iodine from the solution of iodide ions. The ionic equations of chlorine are
Similarly, bromine can only displace iodine from a solution of iodide ion.
Displacement of halogen is also possible by oxidation of their respective halide ions by the use of appropriate chemical oxidizing agents.
There are many oxidizing agents present such as KMnO4, Cr2O7, MnO2, etc for the oxidation of Cl–, Br–and I –. However, oxidations of fluorine by these chemical agents are not possible because of its strongest oxidizing capacity. Hence, there is no way to change F– ions to F2 by chemical agents. Fluorine can only be converted into F2 from F– electrolytic method.
4) Disproportionation Reactions
This is a special type of oxidation and reduction reaction (redox reaction). In disproportionation reaction, the same species will undergo oxidation and reduction. This type of redox reaction can only occur if one of the elements in the reaction consists of three oxidation states.
Moreover, the element in the reacting compound will have to be present in the intermediate oxidation state whereas the lower and the higher oxidation states are present for oxidation and reduction, respectively. A common example of such kind of reaction is decomposition of hydrogen peroxide where oxygen species will be experiencing disproportionation. Refer to the examples below:
In the above example, peroxide oxygen is present in -1 state. It undergoes oxidation and reduction simultaneously and changes to oxidation state -1 to zero in O2 and also decreases to -1 to -2 in H2O.
Solved Question for You
Q. Why fluorine does not show disproportionation tendency in the given reaction?
Solution: In the above equation, fluorine does not show disproportionation reaction because fluorine is the most electronegative element. Hence, it cannot show any positive oxidation state. Therefore fluorine among all the halogens will not show a disproportionation tendency even though it will attack water and produce oxygen.