Which concentrations are included in the equilibrium constant (K) expression for a chemical reaction?

• A. All reactant and product concentrations, including solids and liquids
• B. Only the concentrations of reactants
• C. Only the concentrations of solids and liquids
• D. Only the concentrations of gases and aqueous species

Answer: (D)

Verified step by step guidance

1. Understand that the equilibrium constant expression, \(K\), is written in terms of the concentrations of species involved in the reaction at equilibrium.
2. Recall that pure solids and pure liquids have constant concentrations because their densities do not change during the reaction, so they are not included in the \(K\) expression.
3. Identify that only the concentrations of gases and aqueous (dissolved) species are variable and thus included in the equilibrium constant expression.
4. Write the general form of the equilibrium constant expression as \(K = \frac{[\text{products}]^{\text{coefficients}}}{[\text{reactants}]^{\text{coefficients}}}\), where the brackets denote concentrations of gases and aqueous species only.
5. Conclude that the correct answer is: only the concentrations of gases and aqueous species are included in the equilibrium constant expression.