Final answer:
The correct equilibrium expression for the reaction aA + bB → cC + dD is Keq = [C]^c[D]^d / [A]^a[B]^b, following the law of mass action. In this case, the expression given as 'E. [C][D]^2/[A]^3[B]^4' represents the concentrations of the chemical species at equilibrium, with the products in the numerator and reactants in the denominator.
Step-by-step explanation:
The correct equilibrium expression for a general reaction aA + bB → cC + dD is Keq = [C]c[D]d / [A]a[B]b, where the square brackets indicate the molar concentrations of the reactants and products at equilibrium, and the superscripts indicate the coefficients from the balanced chemical equation. This expression is derived from the law of mass action, which states that at equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant.
To find the equilibrium constant expression, you simply raise the concentration of each reactant and each product to the power of their respective coefficients in the balanced equation and place the products in the numerator and the reactants in the denominator. In the context of the given question, the equilibrium expression is represented as 'E. [C][D]^2/[A]^3[B]^4', where [A], [B], [C], and [D] are the concentrations of the substances A, B, C, and D, respectively.