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What is the importance of equilibrium constant?

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Answer. Knowing the numerical value of equilibrium constant of a chemical reaction, direction as well as extent of the reaction can be predicted.

(i) Predicting Direction of a Reaction
Direction of a reaction at a particular moment can be predicted by inserting the concentration of the reactants and products at that particular moment in the equilibrium expression. Consider the gaseous reaction of hydrogen with iodine.
                  H2(g) + l2(g) <====> 2HI(g) Kc = 57.0 at 700 K

We withdraw the samples from the reaction mixture and determine the concentrations of H2(g), I2(g) and HI(g). Suppose concentrations of the components of the mixture are:

[H2] t = 0.10 mol dm-3 [I2] t = 0.20 mol dm-3
and [HI]t = 0,40 mol dm-3
The subscript t with the concentration symbols means that the concentrations are measured at some time t, not necessarily at equilibrium. When we put these concentrations into the equilibrium constant expression, we obtain a value called the reaction quotient Qc. The reaction quotient for this reaction is calculated as:

       Q = [Hl] 2        =   (0.40) 2      =8.0
           [H2] t [l2] t      (0.10) (0.20)

As the numerical value of Qc (8.0) is less than Kc (57.0), the reaction is not at equilibrium. It requires more concentration of product. Therefore, reaction will move in the forward direction.
The reaction quotient Qc .is useful because it predicts the direction of the reaction by comparing the value of Qc with Kc.
Thus, we can make the following generalization about the direction of the reaction,
If Qc < Kc; the reaction goes from left to right, i.e., in forward direction.
                  
If Qc > Kc; the reaction goes from right to left, i.e., in reverse direction.
                  
If Qc = Kc; no net reaction occurs, i.e., the reaction has attained the equilibrium
                            

(ii) Predicting Extent of a Reaction
Numerical value of the equilibrium constant predicts the extent of a reaction. It indicates to which extent reactants are converted to products. In fact, it measures how far a reaction proceeds before establishing equilibrium state.
In general there are three possibilities of predicting extent of reactions as explained below
(a)      Large numerical value of Kc: The large value of Kc indicates that at equilibrium position the reaction mixture consists of almost all products and reactants are negligible. The reaction has almost gone to completion. For example, oxidation of carbon monoxide goes to completion at 1000 K.
2CO(g) + O2(g) <====> 2CQ2(g)            Kc = 2.2x1022

(b)      Small numerical value of Kc: When the Kc value of reaction is small, it indicates that the equilibrium has established with a very small conversion of reactants to products. At equilibrium position, almost all reactants are present but amount of products is negligible. Such type of reaction never goes to completion. For example;
         2NH3(g)   N 2(g) + 3H2(g) Kc= 3.0 x 10-9         
(c)      Numerical value of Kc is neither small nor large. Such reactions have comparable, amounts of reactants and products at equilibrium position. For example:
                  N2O4 (g) <====> 2 NO 2(g)         Kc = 0.211
It indicates that the rates of decomposition of N2O4 and combination of NO2 to form
N2O4 are almost comparable to each other.
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