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. (2.7)

• 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.

• The first catalyst used in industry was for the production of sulfuric acid. Sulfuric acid production closely mirrored a country’s economic health for a long time. What are some current indicators of a country’s economic health?

Theory of knowledge:

• Reaction mechanism can be supported by indirect evidence. What is the role of empirical evidence in scientific theories? Can we ever be certain in science?

Utilization:

• Cancer research is all about identifying mechanisms; for carcinogens as well as cancer-killing agents and inhibitors.

Syllabus and cross-curricular links:
Topic 20.1—organic mechanisms especially S_N1 and S_N2
Option A.3—catalysts
Biology topic 8.1—enzymes acting as catalysts
Aims:

• Aim 7: Databases, data loggers and other ICT applications can be used to research proposed mechanisms for lab work performed and to carry out virtual experiments to investigate factors which influence rate equations.

• Calculations will be limited to orders with whole number values.
• Consider concentration–time and rate–concentration graphs.
• Use potential energy level profiles to illustrate multi-step reactions; showing the higher E_ain the rate-determining step in the profile.
• Catalysts are involved in the rate-determining step.
• Reactions where the rate-determining step is not the first step should be considered.
• Any experiment which allows students to vary concentrations to see the effect upon the rate and hence determine a rate equation is appropriate.