Tuesday, November 30, 2010


I've written a bit in the past on equilibrium expressions in chemistry, and have also noted how thermodynamics (i.e. equilibrium) is one of the two foundational concepts in all of chemistry, while kinetics is the other foundational concept. A reaction can be characterized as favorable through thermodynamic expressions, but because said reaction can take forever to take place, it can, for all intents and purposes, be considered as not taking place. This is all related to the energy diagram of a reaction:

The distance between the starting point on the left and the ending point on the right gives the change in energy of a system or species. This is the thermodynamic aspect of a reaction, and is directly related to equilibrium. The center portion, where the energy is changing in all kinds of ways, is related to the kinetics of a reaction.

Think about a trampoline. When you jump high into the air on a trampoline, it took a considerable amount of energy to get you up there (from the trampoline, from your legs). This is similar to the high points on the reaction diagram above. The molecule undergoing reaction needs to change its shape in order to become a new compound, but in order to get there it has to "jump up" into a shortly lived shape, much like jumping on a trampoline will only propel you into the air for a short amount of time. The amount of energy it takes to change a molecules shape is what slows a reaction down.

In the above diagram, we have two high points. The first high point, because of how much energy the molecule starts with, takes more energy to be put into the molecule in order to get the molecule to that energetically packed shaped. This would be termed the "slow step" of a reaction. The molecule briefly takes a more comfortable shape before having to undergo one more energetically unfavorable conformation change, and then it drops back down to the final product. However, because the molecule already has a lot of energy gained from the previous conformation, this step is much faster since it only requires a little shove to get going over the final hill.

This is the theoretical picture of a reaction, but kinetic relationships are usually represented with equations. Deriving these equations will be the topic of my next blog post.

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