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Understanding Energy Diagrams - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

So in front of us we have a energy diagram, and for the reaction x plus y yields Z. As you can see the reactants have the energy, potential energy is on the y axis. And so, the reactants have a particular energy, and the products have a particular energy. And just to explain like what this is just a reminder.

From here, so 1, we'll just say 1 is the activation energy. That’s the energy it takes for the reaction to occur. So it's from the reactants to the highest point on the curve in the transition state. So up here we have the transition state. And so the difference between the reactants and the transition state, as in 1, this difference, is what we call the activation energy. You might see that as E sub a.

The difference between the reactants and the products as in 2, we're going to call that the delta H. It's the energy difference between the products and the reactants, and mixes delta H of the reaction.

And three is, if we did the reverse reaction, going from Z as being the reactants to x plus y, it being the products, that the activation energy it would be 2. So it's Ea for the backwards reaction. So the question asks then, now that we understand what this energy diagram is looking at, is if we added a catalyst and a catalyst is this substance that is put into the reaction that changes the reaction mechanism. Therefore lowering the activation energy, it is not consumed within the reaction. What you put in is what you get out, and that's for another time to talk about.

But, the addition of a catalyst there was this reaction that caused a change to which it indicated energy differences. So let's just draw what a catalyst would do. So since it lowers the activation energy, it doesn’t do anything to the reactants, they would be the same in energy. And then it would lower the activation energy, let's say there. And then it would do anything to the energy of the products either.

So now my Ea will be, let's just draw a line, from here to here, it would be my Ea. So what would change? Well number 1 would change. It would decrease in a magnitude. And letter 2 would change; that activation energy for the backwards reaction would also change. It would decrease as well. And so the delta H would remain the same. Sorry, 3, which is also delta H, would remain the same. This is Ea for the forward reaction, this is Ea for the backward reaction.

So understanding how catalyst affect the energy diagrams is pretty important. You'll have questions on that. And understanding how that energy diagram works, but it's actually graphing is also pretty important as well. So hopefully this is helped you understand exactly what energy diagrams are used for and how they work.