Collision theory and rates of reaction

Nature of science:

The principle of Occam’s razor is used as a guide to developing a theory—although we cannot directly see reactions taking place at the molecular level, we can theorize based on the current atomic models. Collision theory is a good example of this principle. (2.7)

• Species react as a result of collisions of sufficient energy and proper orientation.
• The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.
• Concentration changes in a reaction can be followed indirectly by monitoring changes in mass, volume and colour.
• Activation energy (E_a) is the minimum energy that colliding molecules need in order to have successful collisions leading to a reaction.
• By decreasing E_a, a catalyst increases the rate of a chemical reaction, without itself being permanently chemically changed.

Applications and skills:

• Description of the kinetic theory in terms of the movement of particles whose average kinetic energy is proportional to temperature in Kelvin.
• Analysis of graphical and numerical data from rate experiments.
• Explanation of the effects of temperature, pressure/concentration and particle size on rate of reaction.
• Construction of Maxwell–Boltzmann energy distribution curves to account for the probability of successful collisions and factors affecting these, including the effect of a catalyst.
• Investigation of rates of reaction experimentally and evaluation of the results.
• Sketching and explanation of energy profiles with and without catalysts.

• Depletion of stratospheric ozone has been caused largely by the catalytic action of CFCs and is a particular concern in the polar regions. These chemicals are released from a variety of regions and sources, so international action and cooperation have been needed to ameliorate the ozone depletion problem.

Theory of knowledge:

• The Kelvin scale of temperature gives a natural measure of the kinetic energy of gas whereas the artificial Celsius scale is based on the properties of water. Are physical properties such as temperature invented or discovered?

Utilization:

• Syllabus and cross-curricular links:
• Topic 5.3—what might be meant by thermodynamically stable vs kinetically stable?
• Topic 13.1—fireworks and ions
• Option A.3—everyday uses of catalysts Option
• B.2—enzymes
• Biology topic 8.1—metabolism

• Calculation of reaction rates from tangents of graphs of concentration, volume or mass vs time should be covered.
• Students should be familiar with the interpretation of graphs of changes in concentration, volume or mass against time.

• Aims 1and 8: What are some of the controversies over rate of climate change? Why do these exist?
• Aim 6: Investigate the rate of a reaction with and without a catalyst.
• Aim 6: Experiments could include investigating rates by changing concentration of a reactant or temperature.
• Aim 7: Use simulations to show how molecular collisions are affected by change of macroscopic properties such as temperature, pressure and concentration.
• Aim 8: The role that catalysts play in the field of green chemistry.