Chemical Kinetics

(a) Principles Underlying the Measurement of Reaction Rate by Sampling and Quenching

Sampling: Small portions of the reaction mixture are removed at different time intervals to analyze concentration changes.


Quenching: A process that rapidly stops the reaction in the sampled portion to prevent further reaction. This can be achieved by:

  • Cooling the sample rapidly

  • Adding a reagent to neutralize the reactant or catalyst

  • Dilution to slow the reaction significantly


Analysis: The quenched samples are analyzed using methods such as titration, spectrophotometry, or chromatography to determine reactant/product concentrations over time.

(d) Concept of a Rate-Determining Step

Definition: The slowest step in a reaction mechanism that controls the overall rate of reaction.


  • Characteristics:

    • Governs the overall reaction rate.

    • Higher activation energy than other steps.

    • Rate equation reflects species involved in the rate-determining step.


  • Example: In a two-step reaction, if the first step is slow and the second is fast, the rate is determined by the first step.

(e) Link Between Reaction Kinetics and Mechanism

  • Reaction mechanism: A series of elementary steps that describe how a reaction proceeds at the molecular level.

  • Kinetics provides experimental support for mechanisms:

    • The rate equation indicates which reactants are involved in the slowest step.

    • Intermediate species may appear in mechanisms but not in rate equations.

    • Orders of reaction suggest how molecules collide and react.


Example: If a reaction is experimentally found to be first-order in A and second-order in B, the mechanism must account for this dependency.

(f) The Arrhenius Equation and Its Applications

Equation: k = Ae - Ea / RT

k = rate constant

Ae = frequency factor (pre-exponential factor)

Ea = activation energy

R = gas constant (8.314 J mol K)

T = temperature in Kelvin


Effects

  • Effect of Temperature:

    • Increasing increases , leading to a higher reaction rate.

    • A small increase in can significantly increase reaction speed due to the exponential relationship.

  • Effect of a Catalyst:

    • Lowers , increasing without affecting .

    • Provides an alternative reaction pathway with lower activation energy.

  • Determination of Activation Energy:

    • Plot vs. (Arrhenius plot) gives a straight line with slope .

    • Enables calculation of and from experimental data.