Concept of chemical equilibrium

Chemical equilibrium is defined as a state where both reactants and products are present in the  concentration and have no tendency to change further with time. There will be no observable changes in the properties of any particular system in the present.  The equilibrium rate of forward and backward reactions is equal.  

For example, the mixture of reactants and products in the equilibrium state 

In equilibrium, two opposite reactions occur in equal rate, and therefore there are no changes in the substances involved. At this particular point, the reaction must be considered complete. The quantitative formulation of equilibrium is a reversible reaction A ⇋ B + C, the velocity of the reaction to its right, r1, which is given by the mathematical expression which is based on the mass law action. “r1 = k1[A]”, here the k1 is the ratio rate. [A] is represented as the concentration of the reactant A. 

The reaction’s forward and backward rates have to be necessarily conditioned for any chemical equilibrium.

It is a temperature-dependent formula observed by the Van’t Hoff equation. A catalyst addition can affect both the backward and forward reaction in the same way and do not affect the equilibrium constant. Both the reactions will speed up with the help of a catalyst and increase the speed from which the equilibrium rate can be reached.  

The law of chemical equilibrium helps to properly understand the rate of forward and backward reaction and rate of change in a chemical reaction. 

Law of chemical equilibrium

The rate of forwarding reaction  =  rate of backward reaction.

The formula mentioned above is known as the Law of chemical equilibrium. Here K is the equilibrium constant, and it defines the molar concentration of the product. The ratio defines the amount of reactant and product used to understand and analyse the chemical behaviour. 

 This constant can be presented as the partial pressures of the reactants and the products for gas-phase reactions. If this is expressed by partial pressure, then the K is represented as Kp. 

Kp = PCc x PDd / PAa x PBb

Here, PA, PB, PC, PD are the partial pressures of A,B,C,D in the mixture of reactions. 

Examples of chemical equilibrium

As an example, let us analyse an experiment on hydrogen and iodine. The vapour of these two products is kept on a closed vessel at a constant temperature. The reaction starts to proceed and then stops after some time by converting some portion of iodine and hydrogen to hydrogen iodide. The reason for stopping the process was that the two constants reached the equilibrium level after reaching a particular point of dynamism. 

Similarly, let us take some amount of hydrogen iodide in the closed vessel and keep it at the same constant temperature after some time. It will be converted into hydrogen molecules and iodine molecules. Therefore, the rest of the product will remain unchanged. However, two constants will remain the same and be fixed in both the experiment and reaction. This reaching of the dynamic stage is called the equilibrium. 

Types of chemical equilibrium

The law of chemical equilibrium is of two types: homogeneous equilibrium and heterogeneous equilibrium.

Homogeneous equilibrium

In this type of equilibrium, all the reacting components are stated in one phase of matter, such as solid, gas, or liquid. These types of reactions are classified in three different ways.

1. The reaction when no mole number changes the net of the system (Δn = 0).
2. Mole number will increase due to reaction time (Δn = +ve).
3. Mole numbers will decrease due to reaction time. (Δn = -ve).

Example

H2 (g) + I2 (g) ⇌ 2HI (g)  , (Δn = 0)

PCl5  ⇌ PCl3 + Cl2  , (Δn = +ve)

N2 + 3H2 ⇌ 2NH3  , (Δn = -ve)

Heterogeneous equilibrium

In this type of equilibrium, the reacting components do not stay in the same matter phase. For  example, calcium carbonates decompose to calcium oxide and dioxide.

 CaCO3 (s) ⇌ CaO (s) + CO2

The equation includes the three different phases of chemical equilibrium. 

Thermodynamic derivation of the law of chemical equilibrium

The thermodynamic derivation of the law of chemical equilibrium states that any kind of direct or reverse reaction is possible in this system. If this process occurs in any ratio or system, the reaction can proceed in two different and opposite directions. Hence, the macroscopic parameters of any system do not change the relationship between the concentration of the reacting substance and the constant at a particular temperature. The equilibrium of any chemical reaction can be presented as ∑νiμi = 0, where the μi represents the chemical potential of each component. Moreover, νi is the stoichiometric coefficient of each component of the reaction. 

Conclusion

The  equilibrium state is one in which there is no net change in the centralizations of reactants and items. Regardless of the way that there is no evident change at harmony, this doesn’t imply that all compound responses have stopped.