Chemical Equilibrium: Equilibrium Constant?

📌 CONCEPT: The equilibrium constant (Kc) is a numerical value that represents the ratio of the concentrations of the products to the concentrations of the reactants at equilibrium, indicating the extent of a chemical reaction towards the forward or reverse direction.

📐 RULE / FORMULA: The equilibrium constant (Kc) is calculated using the formula: Kc = [C]ⁿ[D]ᵐ / [A]ᵐ[B]ᵐ, where [C] and [D] are the concentrations of the products, [A] and [B] are the concentrations of the reactants, and n and m are their stoichiometric coefficients.

💡 WORKED EXAMPLE: In the reaction NH3 + HCl ⇌ NH4Cl, the equilibrium constant (Kc) is given as 5.5 x 10^-4 at 25°C. If the concentration of NH3 is 0.1 M and the concentration of HCl is 0.2 M, we can calculate the concentration of NH4Cl using the formula: Kc = [NH4Cl] / ([NH3] * [HCl]). Rearranging the formula to solve for [NH4Cl], we get [NH4Cl] = Kc * [NH3] * [HCl]. Substituting the given values, we get [NH4Cl] = 5.5 x 10^-4 * 0.1 * 0.2 = 1.1 x 10^-5 M.

⚠️ COMMON MISTAKE: Students often confuse the equilibrium constant (Kc) with the equilibrium expression, which is a mathematical representation of the equilibrium constant. Another common mistake is assuming that the equilibrium constant is a fixed value, when in reality, it can change with temperature and pressure.