Law of Mass Action - NEET Notes, Formula & Common Mistakes
Law of Mass ActionEquilibriumNEET ChemistryNEET 2025Kc expression derivationChemical EquilibriumEquilibrium Constant
Law of Mass Action - NEET Notes, Formula & Common Mistakes
Equilibrium·2 min read·NEET 2026
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, so r_f = k_f [A]^a [B]^b.
Rate of reverse reaction (r_r) is proportional to [C]^c [D]^d, so r_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_c expression 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.
Frequently Asked Questions
What is the Law of Mass Action in simple terms?▾
The Law of Mass Action states that the rate of a chemical reaction is directly proportional to the concentration of the reactants, each raised to the power of its stoichiometric coefficient. This law helps us understand how reaction rates change with reactant amounts.
How is the Kc expression derived from the Law of Mass Action?▾
For a reversible reaction, the Law of Mass Action is applied to both forward and reverse reactions. At equilibrium, the rates of forward and reverse reactions are equal. By equating their rate expressions, the ratio of the forward and reverse rate constants (`k_f / k_r`) defines the equilibrium constant Kc, which is expressed as the ratio of product concentrations to reactant concentrations, each raised to their stoichiometric powers.
Why are pure solids and liquids excluded from the Kc expression?▾
Pure solids and liquids have constant concentrations at a given temperature, meaning their effective concentrations do not change during the reaction. Since these constant values are absorbed into the equilibrium constant (Kc), they are omitted from the explicit expression to simplify it.
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