The Anatomy of a Rate Law
This material is replicated on a number of sites as part of the SERC Pedagogic Service Project
In this activity, students try to derive the mathematical forms of rate laws involving elementary reactions. It begins with a guided classroom discussion meant to bring out the main factors that affect chemical reaction rates. It ends with a short writing assignment that is meant to help them cement the concepts and learn some basic paragraph organization skills.
- explain what kinds of factors control chemical reaction rates,
- explain how the variables in a rate law relate to the kinds of factors that control chemical reaction rates,
- be able to recognize and implement proper paragraph organization.
Context for Use
Description and Teaching Materials
There is also a student reading regarding paragraph organization (Microsoft Word 43kB Aug12 09).
Teaching Notes and Tips
- The rate of an elementary reaction will be proportional to the number and proximity of reactant molecules (where there is only one reactant molecule,) or the likelihood that the reactant molecules will all collide (where there is more than one reactant molecule). Therefore, we can make rate laws where rate is proportional to the concentrations of the reactants taken to their stoichiometric powers.
- Rates will also be proportional to the speed at which multiple reactant molecules travel, or for single reactant molecules, how vigorously dissociating bonds vibrate.
- The rate constant folds in factors having to do with kinetic energy and reaction-specific factors like molecular shape, electronic configuration, and so on. The Arrhenius equation includes RT, which has to do with kinetic energy, as well as the activation energy and pre-exponential factor, which are reaction-specific.
- The rate orders for compound reactions can't always be surmised from the stoichiometry.
References and Resources
- The Organizing Scientific Writing activity may aid students in developing their scientific writing skills.