Equilibrium Constants (Kp and Kc) and their Significance

Introduction

When a chemical process achieves equilibrium, the equilibrium constant (typically indicated by the symbol K) offers information about the relationships between the reacting species. It is worth noting that various distinct categories of kinetic parameters give links between balanced chemical reactions. 

Chemical equilibrium 

Chemical equilibrium is an important aspect of the biological and chemical processes. When a closed container is used to evaporate a liquid, the molecules with higher kinetic energy escape from the liquid surface and turn into the vapour phase. The number of liquid molecules from the vapour phase stick to the liquid surface and are retained in the liquid phase. It gives rise to the vapour pressure because of the equilibrium. The number of leaving molecules equals the number of returning liquid vapour. In this stage of the process, it can be said that the system has reached an equilibrium state. In equilibrium, the forward reaction rate equals the backward response rate:

i.e., rf = rb Or, kf × α × [A]a[B]b = kb × α × [C]c [D]d

The above-mentioned double arrow indicates that the process is simultaneously processed from both sides. The mixture of reactants and products in the state of equilibrium is known as an equilibrium mixture. 

Kequ = kf/kb = [C]c [D]d/[A]a [B]b = Kc

Here, A and B are the reactants, C and D are the product and Kc is the equilibrium constant.

Equilibrium constants (Kp and Kc) 

The equilibrium constant is the symbol that denotes the relationship between product and reactants when it reaches the stage of equilibrium in a chemical reaction. It usually presents K as the symbol of constants. Equilibrium constants of a concentration of any chemical reaction at the equilibrium stage defines the relationship between products and reactants of any reaction at the equilibrium stage. 

The equilibrium constant Kc for the reaction in liquids is measured in Moles per litre. Similarly for a gaseous reaction, the formula is, K equ = kf/kb = [[PC]c [PD]d]/[[PA]a [PB]b] = Kp. here Kp denotes the constant in terms of partial pressure. 

Different Kp and Kc values denote different conversions. For example, larger Kc and Kp values = higher conversion percentage and product formation. Lower Kc and Kp values = lower conversion percentage and product formation. Medium Kc and Kp value = optimum product formation.

Relationship between Kp and Kc

Kp and Kc are the two constants of equilibrium of the ideal gas mixture, which is considered under reversible reaction. Kp is denoted as the equilibrium constant which represents the atmospheric pressure, whereas Kc is denoted as the expression of concentration in morality. 

The relationship between Kp and Kc is presented in a simple derivation considering the reversible reaction.

‘a’ mole of ‘A’ reactant is reacted with ‘b’ mole of reactant ‘B’ to give ‘c’ moles of product ‘C’ and ‘d’ moles of the product ‘D’; “aA + bB⇌cC + dD”. Here a, b, c, d are the stoichiometric coefficients of products A, B and C, D. 

Similarly, the equilibrium constant Kc for the reaction in a reversible way is derived as, 

Kc= [C]c [D]d / [A]a [B]b

Here, C is the molar concentration of the ‘c’ product. D is the molar concentration of product ‘d’. Similarly, A is for ‘a’ product and B for product ‘b’. 

The derivation of the Kp equation of atmospheric pressure is given below.

Kp= PCcPDd/ PAaPBb, where, PC is the partial pressure of Product ‘c’ and similarly PD for ‘d’, PA for ‘a’ and PB for ‘b’.  

The derivation of sorting out  the relation between Kp and Kc is PV = nRT. Here P stands for the pressure of the ideal gas; V stands for volume of the ideal gas. n is the number of moles, R the universal gas constant and T for temperature. 

Therefore, it can be proven that Kc and Kp are the equilibrium constant of the gaseous mixture, and in a reversible reaction, they are proportional to each other.

Conclusion 

A chemical reaction’s calculated value is the quantity of its response differential at equilibrium constant, a condition reached by a moving chemical equation after some time has passed. It is worth mentioning that several different types of kinetic parameters provide linkages between chemical processes in balance.