Write the equilibrium-constant expression for the reaction a (g) + b (g) = 2c (g) + d (g)

Chemical reaction: A (g) + B (g) ⇄ 2C (g) + D (g).

The equilibrium constant (Keq) is a ratio of the concentration of the products (in this reaction C and D) to the concentration of the reactants (in this reaction A and B):

Keq = [C]² · [D] / [A] · [B].

[A], [B] are equilibrium concentrations of reactants.

[C], [D] are equilibrium concentrations of products.