Slow reactions
Chemical reactions 
that occur very slowly and can take a long time for completion are called slow reactions.
Usually covalent compounds are involved in slow reactions.
Some reactions can take days, weeks and months to complete; they are called very slow reactions.
For example, Milk may take several hours or a day to convert to curd, while it may take even longer for iron to corrode.
Rusting of Iron
4Fe + 3O2 -> 2Fe2O3
(Iron) (Oxygen) (Rust – Iron Oxide)
Formation of crude oil by a geochemical reaction and disintegration of radium are other examples of slow reactions.
Fast reactions
Chemical reactions that complete in a very short time, such as less than 10 -6 seconds, they are called fast reactions.
Examples: Magnesium ribbon is burnt in the flame of Bunsen burner; it quickly gets combusted with a noticeable spark.
2Mg(s) + O2 (g) heat > 2MgO (s)
Similarly, a neutralization reaction between acids and bases is a fast reaction.
Example: When hydrochloric acid reacts with the base, ammonium hydroxide, it forms salt and water.
HCl (aq) + NaOH (aq) -> NaCl (aq) + H2O (l)
(Acid) (Base) (Common Salt) (Water)
Another example of fast reaction is formation of silver chloride precipitate when sodium chloride solution is mixed with silver nitrate solution.
Ag+NO3- + Na+Cl– -> AgCl + NaNO3
Since, these fast reactions occur between ions, they are also known as ionic reactions.
Besides slow and fast reactions, there is another category called moderate reactions.
Irreversible Reactions
A fundamental concept of chemistry is that chemical reactions occurred when reactants reacted with each other to form products. These unidirectional reactions are known as irreversible reactions, reactions in which the reactants convert to products and where the products cannot convert back to the reactants. These reactions are essentially like baking. The ingredients, acting as the reactants, are mixed and baked together to form a cake, which acts as the product. This cake cannot be converted back to the reactants (the eggs, flour, etc.), just as the products in an irreversible reaction cannot convert back into the reactants.
An example of an irreversible reaction is combustion. Combustion involves burning an organic compound—such as a hydrocarbon—and oxygen to produce carbon dioxide and water. Because water and carbon dioxide are stable, they do not react with each other to form the reactants. Combustion reactions take the following form:
CxHy+O2→CO2+H2O
Reversible Reactions
In reversible reactions, the reactants and products are never fully consumed; they are each constantly reacting and being produced. A reversible reaction can take the following summarized form:
A+B⇌k1k−1C+DA+B⇌k1k−1C+D
This reversible reaction can be broken into two reactions.
Reaction 1:
A+B−→k1C+DA+B→k1C+D
Reaction 2:
C+D−→−k−1A+BC+D→k−1A+B
These two reactions are occurring simultaneously, which means that the reactants are reacting to yield the products, as the products are reacting to produce the reactants. Collisions of the reacting molecules cause chemical reactions in a closed system. After products are formed, the bonds between these products are broken when the molecules collide with each other, producing sufficient energy needed to break the bonds of the product and reactant molecules.
Imagine a ballroom. Let reactant A be 10 girls and reactant B be 10 boys. As each girl and boy goes to the dance floor, they pair up to become a product. Once five girls and five boys are on the dance floor, one of the five pairs breaks up and moves to the sidelines, becoming reactants again. As this pair leaves the dance floor, another boy and girl on the sidelines pair up to form a product once more. This process continues over and over again, representing a reversible reaction.
Unlike irreversible reactions, reversible reactions lead to equilibrium: in reversible reactions, the reaction proceeds in both directions whereas in irreversible reactions the reaction proceeds in only one direction. If the reactants are formed at the same rate as the products, a dynamic equilibrium exists. For example, if a water tank is being filled with water at the same rate as water is leaving the tank (through a hypothetical hole), the amount of water remaining in the tank remains consistent.
Reversible reaction and dynamic nature of equilibrium.
A reversible reaction attains equilibrium of dynamic nature and not of static nature. The fact that the properties of a system become constant at the equilibrium stage may give the impression that both the forward and backward processes stop altogether. This however is not true. At equilibrium, forward and backward reactions go at equal speeds, but do not stop. The rate of formation of the products exactly equals to the rate of formation of reactants again. As a result the concentration of the reactants and products and other properties of the system remain unchanged. Thus the equilibrium is dynamic in nature.
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