Order of Reaction NEET Cheat Sheet: Everything You Need

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

• [A]: Concentration at time t
• [A]₀: Initial concentration
• n: Order of reaction
• k: Rate constant
• t: Time

Specific Formulas:

• Zero-Order: [A] = [A]₀ - kt ; Half-life t₁/₂ = [A]₀ / 2k
• First-Order: ln[A] = ln[A]₀ - kt or kt = 2.303 log([A]₀ / [A]) ; Half-life t₁/₂ = 0.693 / k
• Second-Order (for A → P): 1/[A] = 1/[A]₀ + kt ; Half-life t₁/₂ = 1 / (k[A]₀)

Common Mistakes

• 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.