0.16 Reaction rates  (Page 11/9)

Foundation

We will assume an understanding of the postulates of the Kinetic Molecular Theory and of the energetics of chemical reactions. We will also assume an understanding of phaseequilibrium and reaction equilibrium, including the temperature dependence of equilibrium constants.

Goals

We have carefully examined the observation that chemical reactions come to equilibrium. Depending on thereaction, the equilibrium conditions can be such that there is a mixture of reactants and products, or virtually all products, orvirtually all reactants. We have not considered the time scale for the reaction to achieve these conditions, however. In many cases,the speed of the reaction might be of more interest than the final equilibrium conditions of the reaction. Some reactions proceed soslowly towards equilibrium as to appear not to occur at all. For example, metallic iron will eventually oxidize in the presence ofaqueous salt solutions, but the time is sufficiently long for this process that we can reasonably expect to build a boat out of iron.On the other hand, some reactions may be so rapid as to pose a hazard. For example, hydrogen gas will react with oxygen gas sorapidly as to cause an explosion. In addition, the time scale for a reaction can depend very strongly on the amounts of reactants andtheir temperature.

In this concept development study, we seek an understanding of the rates of chemical reactions. We will defineand measure reaction rates and develop a quantitative analysis of the dependence of the reaction rates on the conditions of thereaction, including concentration of reactants and temperature. This quantitative analysis will provide us insight into the processof a chemical reaction and thus lead us to develop a model to provide an understanding of the significance of reactantconcentration and temperature.

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

Observation 1: reaction rates

We begin by considering a fairly simple reaction on a rather elegant molecule. One oxidized form ofbuckminsterfullerene C ₆₀ is C ₆₀ O ₃ , with a three oxygen bridge as shown in [link] .

Oxidized buckminsterfullerene

C ₆₀ O ₃ is prepared from C ₆₀ dissolved in toluene solution at temperatures of 0 °Cor below. When the solution is warmed, C ₆₀ O ₃ decomposes, releasing O ₂ and creating C ₆₀ O in a reaction which goes essentially to completion. We can actuallywatch this process happen in time by measuring the amount of light of a specific frequency absorbed by theC ₆₀ O ₃ molecules, called the absorbance . The absorbance is proportional to the concentration of theC ₆₀ O ₃ in the toluene solution, so observing the absorbance as a function of time is essentially the same as observing the concentration as afunction of time. One such set of data is given in [link] , and is shown in the graph in [link] .