The Anatomy of a Rate Law

This material is replicated on a number of sites as part of the SERC Pedagogic Service Project

Summary

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.

Used this activity? Share your experiences and modifications

Learning Goals

Students should be able to:
1. explain what kinds of factors control chemical reaction rates,
2. explain how the variables in a rate law relate to the kinds of factors that control chemical reaction rates,
3. be able to recognize and implement proper paragraph organization.

Context for Use

This exercise was made for an upper-division introductory to geochemistry class. The assignment takes approximately two hours of class time plus students will also spend a few hours out of class working on it.

Description and Teaching Materials

Activity sheets and instructions (Microsoft Word 63kB Aug12 09) for rate law activity, and associated writing assignment.
There is also a student reading regarding paragraph organization (Microsoft Word 43kB Aug12 09).

Teaching Notes and Tips

Here are a few points I make sure the discussion hits.
1. 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.
2. Rates will also be proportional to the speed at which multiple reactant molecules travel, or for single reactant molecules, how vigorously dissociating bonds vibrate.
3. 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.
4. The rate orders for compound reactions can't always be surmised from the stoichiometry.

Assessment

A grading rubric for the writing assignment is included with the activity sheets and instructions (Microsoft Word 63kB Aug12 09).