Definition of Dynamic Equilibrium

Dynamic equilibrium is the state of a reversible reaction where the forward reaction rate is equal to the reverse reaction rate, resulting in no observable net change in the system. Reactions are continuing to proceed in the forward and reverse direction dynamically; however, there is no net change in the amount of product or starting material. Dynamic equilibrium is also called steady state. In the early stage of a reversible reaction, the forward reaction proceeds more quickly than the reverse until the equilibrium state is reached when the forward and reverse rates are equal. An example of an important dynamic equilibrium is the equilibrium between ice and water at 0 ºC, where ice is melting at the same rate as the water is freezing.

Definition of Dynamic Equilibrium

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Reversible and Irreversible Reactions

Introduction

 The world believed that all chemical reactions were irreversible until 1803 when French chemist Claude Louis Berthollet introduced the concept of reversible reactions.

Irreversible Reactions

 As children, we learned that chemical reactions occurred when reactants reacted with each other to form products. These unidirectional reactions are known as irreversible reactions. In other words, irreversible reactions are reactions where the reactants convert to products and where the products cannot convert back to the reactants. These reactions are essentially like cooking eggs. Cooking the eggs is an  example of irreversible reaction,It does not matter how we cook the eggs,when we have cooked eggs we can not bring it back to uncooked postion

As stated above, a egg cannot be "uncooked" and so the reaction is irreversible.

A real-life example of an irreversible reaction is combustion. Combustion usually involves the burning of an organic compound, like a hydrocarbon, and oxygen to produce carbon dioxide and water. Since water is stable in its polyatomic state, like below, it will not react with the other product, CO₂, to form the reactants. Combustion can take the following form:

C_xH_y + O₂ → CO₂ + H₂O

Reversible Reactions

 In reversible reactions, the reactants and products are never used up. In fact, they are both constantly reacting and being produced. A reversible reaction can take the following summarized form:

 his reversible reaction can be broken into two reactions.

Reaction 1: 

A+B−→k1C+D

Reaction 2: 

C+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 because the molecules collide with each other, producing sufficient energy needed to break the bonds of the product and reactant molecules. 

Below is an example of the summarized form of a reversible reaction and a breakdown of the reversible reaction N₂O₄ ↔ 2NO₂

Reaction 1 and Reaction 2 happen at the same time because they are in a closed system.

Blue: Nitrogen    Red: Oxygen 

                                             Reaction 1                                      Reaction 2

 

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 because reversible reactions have the reaction proceeding in both directions while irreversible reactions only have the reaction proceeding in one direction.

 If the reactants are being made at the same rate as the products are being made, 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 and Irreversible Reactions

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Law of Mass Action

Law stating that the rate of any chemical reaction is proportional to the product of the masses of the

reacting substances, with each mass raised to a power equal to the coefficient that occurs in the chemical equation. This law was formulated over the period 1864–79 by the Norwegian scientists Cato M. Guldberg and Peter Waage but is now of only historical interest. This law was useful for obtaining the correct equilibrium equation for a reaction, but the rate expressions it provides are now known to apply only to elementary reactions. By the term active mass we mean the molar concentration,or number of moles per dm3 in a dilute solution.


a A + b B --> c C + d Dwhere a, b, c, d are the coefficients for a balanced chemical equation.

The mass action law states that if the system is at equilibrium at a given temperature, then the following ratio is a constant.

[C]c [D]d ------------- = Keq [A]a [B]b

The square brackets "[ ]" around the chemical species represent their concentrations. This is the ideal law of chemical equilibrium or law of mass action.The units for K depend upon the units used for concentrations. If M is used for all concentrations, K has units

M^c+d-(a+b)

Accordin to law of Mass Action

the rate of forward reaction is proportional to [A] [B],while the rate of reserve reaction is equal to kr [C] [D] where kf and kr are the rate of constant for the forward and reverse reactions,respectively

Law of Mass Action

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