0.19 Reaction kinetics  (Page 16/7)

Introduction

In the previous study, we found that the rates of chemical reactions depend on the concentrations of the reactants, which is not a surprising outcome. However, when analyzed in detail using the concept of a “rate law,” the results are surprising. In particular, it is not at all obvious why the exponents on the concentrations in the rate laws differ as they do and do not always correspond to their stoichiometric coefficients in the balanced chemical equation. There is no obvious pattern from one reaction to the next, or even between reactants in a single reaction. We need to develop a model to understand the variations from one reaction to the next, and this requires us to understand why reaction rates are related to reactant concentrations.

We will find that many reactions proceed quite simply, with reactant molecules colliding and exchanging atoms. In other cases, we will find that the process of reaction can be quite complicated, involving many molecular collisions and rearrangements leading from reactant molecules to product molecules. The rate of the chemical reaction is determined by these steps.

It is a common observation that reactions tend to proceed more rapidly with increasing temperature. Similarly, cooling reactants can have the effect of slowing a reaction to a near halt. From our study of the Kinetic Molecular Theory of Gases, we recall that temperature is related to the kinetic energy of the particles, so it seems to make sense that increasing the energy by increasing the temperature would increase reaction rates. However, we also can remember that not all reactions are endothermic. Reactions can be exothermic or endothermic, so many reactions do not require a net input of energy. Why then would an exothermic reaction speed up when the temperature is higher? We need to understand the role of energy in the reaction rate.

Foundation

In this study, we will rely on our understanding of the postulates and conclusions of the Kinetic Molecular Theory, including the relationship of temperature to kinetic energy and the model of gas molecules moving independently but occasionally colliding. We will of course assume we have measured and observed reaction rate laws in the previous study. We will also rely on our understanding of bonding and bond energies, specifically as they relate to the energies of chemical processes.

Observation 1: collision model for simple reactions

In the previous study, we observed the dependence of reaction rates on the concentration of reactants, and we have fit these data to equations called rate laws. Although this is very convenient, it does not provide us insight into why a particular reaction has a specific rate law.

To begin building our model to understand the concentration dependence of a reaction rate law, we consider a very simple reaction between two molecules in which a single atom is transferred between the molecules during the reaction. Our example is a reaction important in the decomposition of atmospheric ozone O ₃ by aerosols: