NTRODUCTION TO REACTION KINETICS

Chemical Kinetics - Reaction Rates

Kinetics, the study of the rates of chemical reactions, has a profound impact on our daily lives. Even though some reactions are thermodynamically favorable, such as the conversion of diamonds into graphite, they do not occur at a measurable rate at room temperature. Other reactions, like the explosive reaction between vinegar and baking soda, occur almost instantaneously. Imagine a world where all thermodynamically favored processes occurred at the same rate--life could not exist under such circumstances because biological processes rely on the kinetic stability of many unstable compounds. Kinetics answers questions about rate, how fast reactions go, and mechanisms, the paths molecules take in going from reactants to products.

To describe the rate of a reaction, we will derive the rate law for a chemical reaction and discuss the factors affecting rate. Additionally, we will describe the experimental techniques, such as the method of initial rates and fitting data to plots based on the integrated rate law, used to determine the rate law for an unknown reaction.

In our discussion on mechanisms, we will discuss how to determine the path a reaction takes by analyzing and predicting the series of elementary steps that comprise a mechanism. By comparing the rate law for a proposed mechanism and other mechanistic predictions to experimental data, we can test the validity of a mechanism. Mechanisms can never be proven exactly, but we can rule out mechanisms that disagree with experimental observations. We will use reaction coordinate diagrams to understand and to visualize reaction mechanisms, thermodynamics, and activation energies. Catalysts and intermediates can be important factors in reaction mechanisms, and they provide interesting examples of mechanism problems.

Chemical kinetics is the branch of chemistry which addresses the question: "how fast do reactions go?" Chemistry can be thought of, at the simplest level, as the science that concerns itself with making new substances from other substances. Or, one could say, chemistry is taking molecules apart and putting the atoms and fragments back together to form new molecules. (OK, so once in a while one uses atoms or gets atoms, but that doesn't change the argument.) All of this is to say that chemical reactions are the core of chemistry.

If Chemistry is making new substances out of old substances (i.e., chemical reactions), then there are two basic questions that must be answered:

1. Does the reaction want to go? This is the subject of chemical thermodynamics.

2. If the reaction wants to go, how fast will it go? This is the subject of chemical kinetics.

Here are some examples. Consider the reaction,

2 H₂(g) + O₂(g) → 2 H₂O(l).

We can calculate Δ_rG^o for this reaction from tables of free energies of formation (actually this one is just twice the free energy of formation of liquid water). We find that Δ_rG^o for this reaction is very large and negative, which means that the reaction wants to go very strongly. A more scientific way to say this would be to say that the equilibrium constant for this reaction is very very large.

However, we can mix hydrogen gas and oxygen gas together in a bulb or other container, even in their correct stoichiometric proportions, and they will stay there for centuries, perhaps even forever, without reacting. (If we drop in a catalyst - say a tiny piece of platinum - or introduce a spark, or even illuminate the mixture with sufficiently high frequency uv light, or compress and heat the mixture, the mixture will explode.) The problem is not that the reactants do not want to form the products, they do, but they cannot find a "pathway" to get from reactants to products.

Another example: consider the reaction,

C(diamond) → C(graphite).

If you calculate Δ_rG^o for this reaction from data in the tables of thermodynamic properties you will find once again that it is negative (not very large, but still negative). This result tells us that diamonds are thermodynamically unstable. Yet diamonds are highly regarded as gem stones ("diamonds are forever") and are considered by some financial advisors as a good long-term investment hedge against inflation. On the other hand, if you were to vaporize a diamond in a furnace, under an inert atmosphere, and then condense the vapor, the carbon would come back as graphite and not as diamond.

How can all these things be?

The answer is that thermodynamics is not the whole story in chemistry. Not only do we have to know whether a reaction is thermodynamically favored, we also have to know whether the reaction can or will proceed at a finite rate. The study of the rate of reactions is called chemical kinetics.

The study of chemical kinetics requires new definitions, new types of experimental data, and new theories and equations to organize the data. We begin with the definition of reaction rate in next post.