Monday, December 7, 2009

The Two Pillars of Chemistry

It's that time of the semester where time spent outside of finishing projects and preparing for finals is scarce, but I wanted to get a quick post in. I've been wanting to do this post for awhile, so it should come fairly easily:

I remember well my days in Chem II where I learned that there chemical reactions were reducible to two major concepts: Kinetics and Thermodynamics. The first deals with how fast a reaction proceeds, and the second deals with what is energetically favorable. And the great thing is that this can all be displayed using only one graph:

This is a reaction energy diagram that I nabbed from here. The energy being referred to is the Potential Energy of the main molecules under study in a chemical reaction: What you start with and what you finish with. In this particular diagram, the starting molecule (labeled Reactants) only undergoes one quick change before becoming a new molecule (labeled Products). You'll notice that in this reaction the products have a lower Potential energy than the Reactants. Because of the Law of Conservation of Energy, this energy doesn't just disappear, but is released into the environment (Which, as far as chemists are concerned, is "Not the Molecule). An everyday example of this happens if you have a gas-burning stove, or inside your car engine. The energy is released and heats up your food, or drives the piston down. The comparison between the Potential energy of your starting products to your ending products is what chemists use to gauge whether or not a reaction is "Thermodynamically favorable", and entails Pillar One of chemistry: Thermodynamics. In this particular reaction diagram, the reaction is thermodynamically favored because the products have a lower potential energy than the reactants -- this works, in a lot of ways, like gravity. Objects "like to" get closer the center of the earth, and molecules like to have a lower potential energy.

I actually often give chemicals personalities and say "Likes to do..." more often than may be healthy, but personification helps in simplifying the theory.


(OK, I fess up: these aren't official pillars, I'm just raising a big hullabaloo)

Deals with the section of the graph where the Potential Energy raises temporarily. The maximum of this curve is termed the "Transition State", because... well, to sound redundant, it's very transitory, and it's in transition from one molecule to another. The higher this maxima is in comparison to the starting Potential Energy of the Reactants, the slower the rate is going to be.

So, really, the two pillars of chemistry tie back to the most fundamental concept of dynamics: Energy. But one quick look at Diamond tells you that it's important to separate Energy into Kinetics and Thermodynamics -- it is thermodynamically favorable for Diamond to spontaneously decompose into Graphite. However, the transition state is so high, this is very... very... slooooowwwww.....