Equilibrium constants Kc and Kp. Equilibrium Constants

Consider the generic reversible reaction below, where the lower case letters correspond to the balanced reaction coefficients and the upper case letters are the reactants and the products are all gaseous:

Considering each of the reaction directions separately, their development rates (Td) are given by:

*Direct reaction: aA + bB → cC + dD

All_direct = K_direct. [THE]^The. [B]^B

*Reverse reaction: cC + dD → aA + bB 

All_inverse = K_reverse. [Ç]^ç. [D]^d

The chemical equilibrium constant in terms of concentration in quantity of matter (K_ç) and in terms of partial pressure (K_P) will be given by dividing K_direct by K_reverse.

So we have:

K_direct. [THE]^The. [B]^B  = 1 → K_{direct__} = __[Ç]^ç. [D]^d___
K_reverse. [Ç]^ç. [D]^d K_reverse [THE]^The. [B]^B

 Being, K_ç =_K_direct_
K_reverse

So we have:

Kç =__[Ç]^ç. [D]^d___ or KP =__(Praça)^ç. (pD)^d___ THE]^The. [B]^B(Pan)^The. (pB)^B

Where p is the partial pressure of each substance at equilibrium.

In this way, each concentration is raised to an exponent corresponding to the coefficient of the respective substance in the reaction, and K_ç does not have unit*.

In addition, a very important aspect to be highlighted is that in this expressionneither solid components nor pure liquids should be represented., as only matters that can suffer variation participate in this expression. The concentration in amount of matter of a substance in the solid state is constant and thus is already included in the value of K itself._ç. The same goes for pure liquids like water. In short, only substances in the gaseous state and in aqueous solution participate in the expression.

Note the examples below:

K values_ç can show us if the concentration of reactants and products are the same or if one is greater than the other:

• if K_ç or K_P is equal to one (K_ç = 1), this means that the concentration of reactants and products is equal;
• if the value of K_ç or K_P be tall, this means that the products are in greater concentration, because in the expression of K_ç the products are in the numerator;
• if the value of K_ç or K_P is low, this means that the reagents are in higher concentration, because in the expression of K_ç the reagents are in the denominator.

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*K_ç and K_P they are dimensionless numbers, that is, pure numbers, with no unit related to any magnitude or relation between magnitudes.

By Jennifer Fogaça
Graduated in Chemistry

Source: Brazil School - https://brasilescola.uol.com.br/quimica/constantes-equilibrio-kc-kp.htm