For a reaction aA + bB ⇌ cC + dD, write the expression for Kc.

For a reaction aA + bB ⇌ cC + dD, write the expression for Kc.

• A. Kc = [C]^c [D]^d / ([A]^a [B]^b)
• B. Kc = [A]^a [B]^b / ([C]^c [D]^d)
• C. Kc = ([C][D])/([A][B])
• D. Kc = [A]^a [B]^b [C]^c [D]^d

Answer: (A)

The expression for Kc comes from the law of mass action: at equilibrium, the ratio of product concentrations to reactant concentrations, each raised to its stoichiometric power, stays constant. For aA + bB ⇌ cC + dD, the products C and D appear with exponents c and d, while the reactants A and B appear with exponents a and b. So the equilibrium constant in terms of concentrations is Kc = [C]^c [D]^d / ([A]^a [B]^b). This reflects that increasing product concentrations or decreasing reactant concentrations shifts the system toward the products, and the same form defines the constant at a given temperature. If you consider the reaction quotient Q at any moment, it uses the same arrangement, and at equilibrium Q equals Kc. In gases, a related constant Kp uses pressures; Kp and Kc are linked by Kp = Kc(RT)^(Δn) where Δn = (c + d) − (a + b).

The expression for Kc comes from the law of mass action: at equilibrium, the ratio of product concentrations to reactant concentrations, each raised to its stoichiometric power, stays constant. For aA + bB ⇌ cC + dD, the products C and D appear with exponents c and d, while the reactants A and B appear with exponents a and b. So the equilibrium constant in terms of concentrations is Kc = [C]^c [D]^d / ([A]^a [B]^b). This reflects that increasing product concentrations or decreasing reactant concentrations shifts the system toward the products, and the same form defines the constant at a given temperature. If you consider the reaction quotient Q at any moment, it uses the same arrangement, and at equilibrium Q equals Kc. In gases, a related constant Kp uses pressures; Kp and Kc are linked by Kp = Kc(RT)^(Δn) where Δn = (c + d) − (a + b).