Essential Concepts of Chemical Kinetics

Sep 6, 2024

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Lecture on Chemical Kinetics

Key Concepts

  • Chemical Kinetics: Study of reaction rates, measuring how fast reactions occur.
  • Thermodynamics vs. Kinetics:
    • Thermodynamics: Determines if a reaction is spontaneous (negative ΔG).
    • Kinetics: Describes the speed of a reaction.
  • Examples:
    • Conversion of diamond to graphite: Spontaneous but occurs very slowly.
    • Combustion of glucose: Spontaneous and energetically favorable but slow at room temperature due to high activation energy.

Reaction Rates

  • Reaction rate: Change in concentration of products/reactants over time.
    • Units: Molarity (M) per second, minute, etc.
    • Rate for reactants uses a negative sign (concentration decreases).
    • Rate for products uses a positive sign (concentration increases).

Rate of Reaction

  • Average Rate of Appearance/Disappearance:
    • For appearance of product B: Δ[B]/Δt (change in concentration over change in time).
    • For disappearance of reactant A: -Δ[A]/Δt.
  • Stoichiometry in Rate Expression:
    • For reaction: aA + bB → cC + dD
    • Rate = -(1/a)(Δ[A]/Δt) = -(1/b)(Δ[B]/Δt) = (1/c)(Δ[C]/Δt) = (1/d)(Δ[D]/Δt).

Rate Law Expression

  • Differential Rate Law: Shows rate dependence on concentration.
  • Order of Reaction:
    • First order with respect to a reactant: Rate ∝ [A]^1.
    • Second order: Rate ∝ [B]^2, etc.
    • Overall order: Sum of individual orders.

Example Calculations

  • Average Rate:
    • Example: Disappearance of NO over 20 seconds.
    • Δ[NO]/Δt = (Final concentration - Initial concentration) / Time interval.
  • Rate Law Expression:
    • Example: [Rate] = k[A]^m[B]^n using initial rate method.
    • Determine order by observing change in rate with change in concentration.

Practice Problem Steps

  1. Given Data: Concentrations and initial rates.
  2. Determine Order:
    • Hold one reactant constant, change the other.
    • Observe rate change to determine order.
  3. Calculate Rate Constant (k):
    • Use data to solve: Rate = k[A]^x[B]^y.
  4. Units of k:
    • Derived from rate expression to match units of rate and concentration.
  5. Calculate Initial Rate:
    • With given concentrations, use Rate = k[A]^x[B]^y.

Important Concepts

  • The rate is proportional to concentration, affected by order of reaction.
  • Zero order means change in concentration does not affect rate.
  • Understanding stoichiometry is crucial for calculating the rate.