What NEET Asks
- Conceptual understanding: Questions on the definition of rate law, its experimental nature, and distinction between order and molecularity are common.
- Unit calculations: Deriving the units of the rate constant (k) for different reaction orders is a frequently tested skill.
- Experimental data analysis: Determining the order of reaction and rate constant from given initial rate data is a recurring problem type.
Key Points
- Rate Law (Rate Expression): Mathematically expresses the rate of reaction in terms of molar concentrations of reactants, each raised to some power.
- Experimental Determination: The rate law cannot be predicted from the stoichiometry of a balanced chemical equation; it must be determined experimentally.
- Order of Reaction (x, y): The sum of the powers (exponents) of the concentration terms in the rate law. These exponents may or may not be equal to the stoichiometric coefficients.
- Rate Constant (k): A proportionality constant in the rate law. Its value is characteristic for a given reaction at a particular temperature and is independent of reactant concentrations.
- Units of Rate Constant: Depend on the overall order of the reaction. For an n-th order reaction, units are typically
(mol L^-1)^(1-n) s^-1. - Molecularity vs. Order: Molecularity is theoretical (from mechanism), while order is experimental. They are only the same for elementary reactions.
Must-Know Formula / Reaction
Rate Law: Rate = k[A]^x[B]^y
Rate: Reaction rate (e.g., mol L^-1 s^-1)k: Rate constant[A], [B]: Molar concentrations of reactants A and Bx, y: Orders with respect to reactants A and B (experimentally determined)Overall Order (n) = x + y
Units of Rate Constant (k): k = (mol L^-1)^(1-n) s^-1
n: Overall order of the reaction
Common Mistakes
- Students often confuse order of reaction with molecularity. Remember, molecularity applies only to elementary steps, while order is experimental for overall reactions.
- Don't assume order always equals stoichiometric coefficients. This is true only for elementary reactions or reactions where one reactant is in large excess.
- Incorrectly calculating the units of the rate constant (k). Always use the formula
(mol L^-1)^(1-n) s^-1or derive it from the rate law.
Rapid Revision
The rate law, Rate = k[A]^x[B]^y, is always experimentally determined. 'x' and 'y' are reaction orders, not necessarily stoichiometric coefficients. The overall order n = x+y dictates the units of the rate constant k, which are (mol L^-1)^(1-n) s^-1. k is temperature-dependent.