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Chemical Kinetics

Rate laws, order of reaction, Arrhenius equation

Key Concepts
  • Rate of reaction: change in concentration per unit time
  • Rate law: rate = k[A]^m[B]^n (order determined experimentally, not from equation)
  • First-order kinetics: radioactive decay, many reactions in solution
  • Arrhenius equation: k increases with temperature
  • Activation energy (E_a): energy barrier that reactants must overcome
Important Formulae
First-order half-life t₁/₂ = 0.693/k
First-order integrated rate law ln[A] = ln[A]₀ − kt
Second-order half-life t₁/₂ = 1/(k[A]₀)
Arrhenius equation k = Ae^(−Ea/RT)
Temperature effect on k ln(k₂/k₁) = (Ea/R)(1/T₁ − 1/T₂)
Quick Tips
  • Molecularity: number of species in elementary step (integer, max 3); order: determined from experiment (can be fraction).
  • Rate-determining step is the slowest step of a mechanism.
  • For a 10°C rise in temperature, reaction rate approximately doubles (thumb rule).
Sample Practice Questions

  1. Molecularity of a reaction is:

    • Always equal to order
    • Number of molecules participating in elementary step
    • Rate of reaction
    • Activation energy divided by RT
    Show answer

    Answer: Number of molecules participating in elementary step

  2. Arrhenius equation: log(k₂/k₁) = (Ea/2.303R)(1/T₁ − 1/T₂). Increasing T generally:

    • Decreases k
    • Increases k
    • Has no effect on k
    • Makes k = 0
    Show answer

    Answer: Increases k

  3. Photochemical reactions are initiated by:

    • Heat
    • Light (photons)
    • Catalysts
    • Pressure
    Show answer

    Answer: Light (photons)

  4. For a first-order reaction, after 2 half-lives, fraction remaining:

    • 1/2
    • 1/4
    • 1/8
    • 1/16
    Show answer

    Answer: 1/4

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Practice Questions

Practice 20 randomly selected NEET questions on Chemical Kinetics. Answers are revealed after each question.

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