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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.

Energy diagrams depict the reaction progress versus energy. For exothermic reactions, the reactants are drawn above the products because their energy is greater. However, for endothermic reactions, the reactants are drawn below the products. Catalysts lower activation energy, so they decrease the size of the hump within the diagram itself.

Alright so there's a lot of changes in energy that takes place within a reaction, and sometimes Chemists talk about it via number and words but sometimes it's actually easier to illustrate it using graph and pictures. So we're going to talk about energy diagrams and this is actually what an energy diagram looks like so on the y axis we have energy and on the x axis we're going to talk about reaction progress or the time going from reactants to products. So in this particular reaction, this reaction is unique. In this reaction the reactants are pretty high in energy they're, it's seems like they're pretty unstable they have high energy content. So as reactants are going to proceed to the products they're going to have to increase in energy to get to the activated complex, don't forget the activated complex is the transition state of the change between the reactants to products, is that in between stage or the transition stage. So they're actually very unstable and high in energy, so we're going to have to get some energy to get there we're going to call that the activation energy or e sub a. So we're going to get that energy and we're going to go down to our more stable less energetic products.

Okay so we're actually losing energy in this reaction, we're the energy. We're going to call this an exothermic reaction because energy is actually being exiting from the reaction. So our delta H which is our, a symbol of telling us the difference between the energy of the reactants and the energy of the product is going to be negative. Our energy is dropping so anytime you see the fact that this energy is negative or delta h is negative or energy is being released, then we know that it's an exothermic reaction, energy is exiting. Okay, so there's one thing in here too that we have to indicate and that is catalyst. Catalysts actually change the reaction mechanism or the pathway of the reaction going from reactants to products. So let's illustrate that on this.

Okay, so there's got change in the energy of the reactants or products is just changing the activity complex so if you have a catalyst the activated complex is going to be different. So it's not going to be as high in energy, so this green line will indicate the new pathway that the catalyst would use. So I'm going to write that down, in Biology you might know catalyst as an enzyme, it's basically the same thing. And so our ea is going to be lowered and this actually increases the rate or how fast a reaction is going to take place. So this is going to be our new ea so it doesn't require as much energy. Alright but there's also other ways to illustrate or other different types of reactions that we're going to talk about using energy diagrams.

Let's go over here, here's a different energy diagram it looks quite different our reactants in this case are very low in energy, they're very stable and our products are higher in energy a little bit less stable. So we're going to call this an endothermic reaction meaning energy is going to have to enter the reaction requiring a lot of energy. Our delta h is going to be positive because our reactants are lower in energy than our products. So we're actually going to increase our energy delta h is positive. Our activated complex is till going to be very high in energy as always and our activation energy is actually going to be very great in this case.

If we were to add a catalyst here to increase the reaction rate, the pathway would be lowered still higher than the products but lower and our activation energy would be from here. That would be our new activation energy ea, so catalysts would increase the rate of this reaction also. But not the activated complex would not be lower in energy than the products it'll always be higher because it's less stable, so these energy diagrams are the 2 main ones that you're going to see in class - endothermic and exothermic - and hopefully this will help you understand what they're actually saying, what they're telling you with energy within a reaction.