Order of Reaction NEET Cheat Sheet: Everything You Need
Order of ReactionChemical KineticsNEET ChemistryNEET 2025Reaction OrderZero Order ReactionFirst Order Reaction
Order of Reaction NEET Cheat Sheet: Everything You Need
Chemical Kinetics·2 min read·NEET 2026
What NEET Asks
Direct numerical problems on integrated rate laws and half-life for zero and first-order reactions are frequent.
Conceptual questions distinguishing order from molecularity and identifying reaction order from experimental data or units of rate constant.
Graphical representation of concentration vs. time or rate vs. concentration for different orders.
Key Points
Order of Reaction: Sum of the powers of the concentration terms of reactants in the experimentally determined rate law. It can be an integer, zero, or a fraction.
Zero-Order Reactions: Rate is independent of reactant concentration. Rate = k.
First-Order Reactions: Rate is directly proportional to the first power of reactant concentration. Rate = k[A].
Second-Order Reactions: Rate is proportional to the square of one reactant concentration or the product of two reactant concentrations, each raised to the first power. Rate = k[A]² or k[A][B].
Molecularity vs. Order: Molecularity is always a whole number (theoretical for elementary steps), while order is experimental and can be zero or fractional.
Units of Rate Constant (k): Vary with the order of reaction. General formula: (mol L⁻¹)^(1-n) s⁻¹, where n is the order.
Must-Know Formula / Reaction
General Integrated Rate Law (n ≠ 1):[A]^(1-n) - [A]₀^(1-n) = (n-1)kt
Students often confuse the order of reaction with molecularity; remember, order is experimental.
Don't forget that the units of the rate constant k depend on the overall order of the reaction.
Misinterpreting the graphical representation of integrated rate laws (e.g., straight line plots).
Rapid Revision
Order of reaction is derived from the rate law experimentally. Zero order has constant rate, first order's half-life is constant, second order's half-life depends on initial concentration. Memorize integrated rate laws and half-life formulas for quick application.
Frequently Asked Questions
How do I determine the order of a reaction?▾
The order of a reaction is determined experimentally, not from the stoichiometry of the balanced chemical equation. It's found by analyzing how the reaction rate changes with varying reactant concentrations or by fitting experimental data to integrated rate laws.
What is the significance of the half-life for different reaction orders?▾
For a zero-order reaction, half-life decreases with decreasing initial concentration. For a first-order reaction, half-life is constant and independent of the initial concentration. For a second-order reaction, half-life increases with decreasing initial concentration.
Can the order of a reaction be fractional or zero?▾
Yes, the order of a reaction can be zero, an integer, or even a fraction. Zero order implies the rate is independent of that reactant's concentration. Fractional orders often indicate complex reaction mechanisms involving intermediates.
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