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.