16. Chemical kinetics
16. Chemical kinetics
16.1 Rate expression and reaction mechanism
Nature of science:
Principle of Occam’s razor - newer theories need to remain as simple as possible while maximizing explanatory power. The low probability of three molecule collisions means stepwise reaction mechanisms are more likely.
Understandings:
Reactions may occur by more than one step and the slowest step determines the rate of reaction (rate determining step/RDS).
The molecularity of an elementary step is the number of reactant particles taking part in that step.
The order of a reaction can be either integer or fractional in nature. The order of a reaction can describe, with respect to a reactant, the number of particles taking part in the rate-determining step.
Rate equations can only be determined experimentally.
The value of the rate constant (k) is affected by temperature and its units are determined from the overall order of the reaction.
Catalysts alter a reaction mechanism, introducing a step with lower activation energy.
Applications and skills:
Deduction of the rate expression for an equation from experimental data and solving problems involving the rate expression.
Sketching, identifying, and analysing graphical representations for zero, first and second order reactions.
Evaluation of proposed reaction mechanisms to be consistent with kinetic and stoichiometric data.
16.2 Activation energy
Nature of science:
Theories can be supported or falsified and replaced by new theories - changing the temperature of a reaction has a much greater effect on the rate of reaction than can be explained by its effect on collision rates. This resulted in the development of the Arrhenius equation which proposes a quantitative model to explain the effect of temperature change on reaction rate.
Understandings:
The Arrhenius equation uses the temperature dependence of the rate constant to determine the activation energy.
A graph of 1/T against ln k is a linear plot with gradient -Eₐ/R and intercept, lnA.
The frequency factor (or pre-exponential factor) (A) takes into account the frequency of collisions with proper orientations.
Applications and skills:
Analysing graphical representation of the Arrhenius equation in its linear form lnk=-Eₐ/RT+lnA.
Using the Arrhenius equation k=Ae(-Eₐ/RT).
Describing the relationships between temperature and rate constant; frequency factor and complexity of molecules colliding.
Determining and evaluating values of activation energy and frequency factors from data.
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