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. (2.5)

• 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
• Using the Arrhenius equation
• 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.

Guidance:

• Use energy level diagrams to illustrate multi-step reactions showing the RDS in the diagram.
• Consider various data sources in using the linear expression in The expression ln   given in the data booklet.

• The flashing light of fireflies is produced by a chemical process involving enzymes.
• The relationship between the “lock and key” hypothesis of enzymes and the Arrhenius equation.

Syllabus and cross-curricular links:
Topic 6.1—collision theory
Aims:

• Aims 4 and 7: Use of simulations and virtual experiments to study effect of temperature and steric factors on rates of reaction.
• Aim 6: Experiments could include those involving the collection of temperature readings to obtain sufficient data for a graph.
• Aim 7: Graphing calculators can be employed to easily input and analyse data for Ea and frequency factor values.