1. Chemical Kinetics
Relationship between reaction rate and the variables that exert influence on them.
Mechanism of chemical reaction

2. Conceptual levels Primary concepts Secondary concepts
Chemical reaction: substances react, reagents, products
Reaction rate: chemical reactions proceed with different velocities
Secondary concepts
Energetics: influence of energy on the reaction rate
Catalysis: specific substances can change the rate of a reaction
Reaction path: reaction description at a molecular level

3. Phenomenological (empirical) level:
Kinetic law
The introduction of the notion of step reaction leads to the development of
Reaction mechanism
Molecularity
Reaction order
Rate determining step
Rate equation

4. Models of chemical reactions
Affinity model (17th-18th centuries): tendency of substances to combine with each other. In the affinity corpuscolar model, the rate of a reaction is related to the different degree of affinity between the particles.
Thermodynamic model (Collision theory): chemical reaction involve reacting molecules colliding with sufficient energy.
Kinetic model (Arrhenius law): activation energy, number of molecules that aquire more than a ‘critical energy’.
Transition state theory: link between thermodynamics and kinetic variables

5. Chemical reaction dynamics: a chemical reaction is viewed as the motion of a point in phase space, the co-ordinates of which are the distances between the molecules and their momentum. Notion of trajectory from the reactants to the products region through the col point of a potential energy hypersurface.
The collisional model is usually applied at the level of secondary school.
More rarely, concepts like ‘reaction mechanism’ and, as a consequence, ‘rate determining step’, ‘rate equation’ and ‘order of reaction’ are introduced.
However, the use of catalysts is often introduced in secondary textbooks.

6. Kinetics in the curricula
Kinetic laws are introduced in terms of concentrations
- a macro approach
Mechanisms are generally discussed in molecular terms
a micro approach
Kinetic law mathematically are differential equations, inhibiting a quantitative treatment at the level of secondary school.

7. Context 1: Pre-university level
Chemical kinetics is always simplified to a model of how reactions occur at the moelcular level.
However it is difficult to link such approach to the observables (change of colour, gas development, solid dissolution,…):
Macro/micro unbalance
2. Influence of reactant concentration, temperature and catalysis on the rate of chemical reactions.
Justi and Ruas (1997): ‘The majority of students did use appropriately the collision particle model’.
(this topic can help in understanding the discrete and dynamic character of the corspuscolar model)

8. It should be considered, however, the very abstract nature of collisions between microscopic, abstract entities in explaining the real and visible changes in reaction rates.
Everyday language: ‘collision’, ‘speed’, ‘crash’.
In many textbooks, the collision model is presented without introducing the concept of activation energy and of the orientation and spatial dependence of the collision outcome.

9. Common misconceptions
The forward reaction rate increases as the reaction ‘gets going’
The forward reaction rate equals the reverse reaction rate at all times.
The forward reaction is completed before the reverse reaction commence
A catalyst affects differently the rates of the forward and reverse reaction
Whn an equilibrium is re-established after a disturbance the rates of the forward and reverse reactions will be equal to those at the initial equilibrium.
Most of these misconceptions arose from the limitations of the collisional model.

10. Teachers’s ideas
Teacher’s knowledge about chemical kinetics often did not surpass that in textbooks that they used in their teaching.
‘mechanical’ application of collision theory
- Difference between reaction rate and mechanism
Quantitative assessment of Arrhenius equation and activation energy
Catalyst role and mechanism
Teachers (as textbooks) tend to apply ‘hybrid models’, i.e. a personal mixing of different theoretical models (collision theory + activated complex + transition state theory), without any discussion about the different contexts and limitations, nor reference to the historical context.

11. Teachers and textbooks should make the backgrounds of their expressed models clear, stating the contexts in which they are valid.
Distinction between the concept of chemical reaction at the level of macroscopic observations and at the level of ‘microscopic’ steps in a mechanism.

12. Context 2: University level
Textbooks do not generally discuss the applicability of rate equations.
There are a lot of technical papers, suggesting new experiments or methods, but poor methodological work.
Rarely, molecular dynamics of chemical reactions is introduced.
poor definition of ‘reaction coordinate’
Rarely, computer-assisted simulations were carried out.
‘The way computers are used in science teaching depends on teachers’ models of learning’.

13. Common misconceptions
Students may think that catalyst only lowers the energy of the transition state, and that both catalysed and uncatalysed reactions proceeds via the same mechanism.
Kinetic vs. thermodynamic quantities in TST (quasi-equilibrium hypothesis between the reactant and the transition state).
Students may not be able to establish a relationship between experimental data and the rate of a reaction.

14. Basic questions Why different reactions occur with different rates?
Why some factors influence the rate of a given reaction in a specific manner?
How do kinetic and thermodynamic factors act in a chemical reaction?
How does a given reaction takes place?

15. Core questions
How do students views about the meaning of fundamental chemical ideas influence their understanding of qualitative ideas of chemical kinetics?
How does students’ thinking about chemical kinetics change their views about fundamental chemical concepts?
How the students understanding of chemical kinetics influence their learning of other related ideas, e.g. thermodynamics and chemical equilibrium?
Are they able to interpret diagrams or establish relationships between experimental data and rate equations?

16. Suggestions
Beyond the mathematics, diagrams (concentration vs. time, energy vs. path of reaction) and concepts (activation energy, activated complex, catalysis) should be always introduced (also qualitatively).
Students ability to interpret diagrams
Students ability to treat experimental data
Unfortunately, very few chemical reaction systems are available that allow introductory students to easily determine the concentration of all reaction species at any istant of time.
In this connection, computer-aided simulations could be of great help (concentration analysis, diagrams, data treatment).

17. Issue Conceptual change
Dynamic nature of chemical processes

18. Selected readings