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Tips on Differentiating Between a Catalyst and an Intermediate - 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.

Tips on differentiating between a catalyst and an Intermediate. Before we can differentiate between them we have to understand what they are. Let's look at a catalyst. The definition of a catalyst is a substance that causes or accelerates a chemical reaction by providing an alternative pathway and lowering the activation energy without itself being affected.

If we look at an energy diagram that I drew here, without a catalyst the activation energy is very high, with the catalyst the activation energy is really low. Just so you know what a catalyst does, that's exactly what it does. However, basically, in a nut shell, it's, we put it in a reaction and we get it out unchanged and retrieve it unchanged.

An Intermediate is derivative of an initial material formed before the desired product in a chemical reaction. Essentially, we produce it then it's reacted. It's a produced as a product of one reaction and another. So it goes from product to reactant and in elementary, in mechanism a catalyst goes from reactant to product.

What the heck I'm I talking about? So let's look at an actual reaction mechanism. We have 2 elementary steps one here and one here. We want to decide which one of these compounds or elements or ions is an intermediate and which one is a catalyst. Let's actually add these together to get a full reaction and I have S2O8-2 plus 2Fe+2, plus 2Fe+3, plus 2I basically just add all the reactants all up. Now I'm going to add the products up; 2Fe+2 plus 2SO4-2 plus 2Fe+3 plus I2.

To cross out things that I have on both sides of the reaction. Here are two things I'm going to look at, I have 2Fe+2 and that is going from here to here. I went from a reactant to a product. I put it in and I got it out unchanged. Fe+2 is a catalyst. I went from left to right I put it in, I got it out.

Another one is 2Fe+3. This is here. I produced it in the first reaction and then I reacted with it in the second reaction. I found it in the products and I absorbed it in the reactants. So Fe+3 is an intermediate.

That's how you differentiate in our drawing we both cross them out, we don't put them in the reaction. But that is the difference between a catalyst and an intermediate. Catalyst gets from reactant to product, intermediate goes from product to reactant.

So if you want to write this equation including the catalyst, we know it that's it's there, best way to write it is like this; S2O8-2 plus 2I-2 that's my big arrow. 2SO4-2 plus I2 and on top of that arrow you're going to put your catalyst. In this case our catalyst is the Iron Fe+2.

So hopefully this helped you decipher between a catalyst and an intermediate and now you understand the difference between the two.