What NEET Asks
- Direct application of the Law of Mass Action to write equilibrium constant expressions (Kc, Kp).
- Conceptual questions on the factors affecting reaction rates and equilibrium.
- Problems involving the relationship between K, reaction stoichiometry, and direction of equilibrium.
Key Points
- Law of Mass Action: At a given temperature, the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants, each raised to the power equal to its stoichiometric coefficient in the balanced chemical equation.
- For a general reversible reaction:
aA + bB <=> cC + dD - Rate of forward reaction (
r_f) is proportional to[A]^a [B]^b, sor_f = k_f [A]^a [B]^b. - Rate of reverse reaction (
r_r) is proportional to[C]^c [D]^d, sor_r = k_r [C]^c [D]^d. - At equilibrium,
r_f = r_r. - Therefore,
k_f [A]^a [B]^b = k_r [C]^c [D]^d. - Rearranging gives
k_f / k_r = ([C]^c [D]^d) / ([A]^a [B]^b) = K_c(Equilibrium constant in terms of concentration).
Must-Know Formula / Reaction
For aA + bB <=> cC + dD:
K_c = ([C]^c [D]^d) / ([A]^a [B]^b)
[X]= Molar concentration of species X at equilibrium.a, b, c, d= Stoichiometric coefficients of the balanced reaction.K_c= Equilibrium constant in terms of molar concentrations.
Common Mistakes
- Students often forget to raise the concentration terms to their respective stoichiometric coefficients.
- Don't confuse the rate constants (
k_f,k_r) with the equilibrium constant (K_c). - Pure solids and pure liquids are not included in the
K_cexpression as their concentrations remain constant.
Rapid Revision
The Law of Mass Action states reaction rates depend on reactant concentrations. At equilibrium, forward and reverse rates are equal, leading to K_c = k_f / k_r = (Products)^coeff / (Reactants)^coeff for the general reaction aA + bB <=> cC + dD.