How can I interpret all of the symbols in an energy diagram?

Question:  How can I interpret all of the symbols in an energy diagram?  I get confused between all the terms and what they mean.

Answer:

In all diagrams, you have a flat looking area in the front and one at the back.  The one in the front represents the reagents and the one in back represents the products.  Remember that it's the energy of the reagents and products that are represented, not the identities of the reagents and products.  For exothermic reactions, the products are shown lower on the diagram than the reagents and for endothermic reactions, the opposite is true.

Toward the middle of the energy diagram there should be a peak, as on a mountaintop.  This represents the transition state (often abbreviated Ts).  The transition state is the high energy state where the reagents have halfway turned into the products.  It's a high energy state because at the transition state the chemicals are neither products nor reagents - they're in the middle somewhere.  There should be an arrow shown that represents the difference in height between the reagents and transition state.  This is the activation energy,
abbreviated Ea.  The activation energy is the amount of energy that's required for a reaction to take place.  Think of it as trying to throw a tennis ball over a building - if you throw it with a lot of energy, but not enough energy to get it over the top of the house, the tennis ball will just bounce right back to where it started.  It's the same thing with chemical reactions - if the reaction doesn't have enough energy to reach the transition state, it just goes back the way it was.  The activation energy is always positive, because reactions require energy to get started.  Reactions that move very quickly typically have lower transition state energies than those that move slowly.

There's also a term that shows the difference in energy between the reagents and products, shown as DH.  If this arrow is pointing up, this means that the products have more energy than the reagents (it's an endothermic reaction) and if it points down, this means that energy is given off (exothermic reaction).

Although you didn't ask this, chemical reactions can be sped up by catalysis, which is when something is added to the reaction mixture that helps the reagents combine in a way that turns them into products.  Because the catalyst isn't actually  onsumed in the reaction, you can usually get it back at the end.  On an energy diagram, catalysts work by creating a new transition state that's lower in energy than the original transition state. For reactions that can go both forwards and backwards, there's an idea called the "Principle of Microscopic Reversibility", which says that reactions that go backwards do the same things as those that go forwards, except in the opposite order.  As a result, if you have products that can turn back into reagents, the transition state for the reverse reaction will be the same as for the forward reaction.


Do you have a question for Mr. Guch?  Email him at misterguch@chemfiesta.com.