Reversible Reactions

Reversible Reactions A reversible reaction is simply a reaction that can run backwards. But, there’s a catch. ALL reactions are reversible. We can see this when we look at an energy diagram for a reaction. 

We can think about this graph in a very different way. Think of the energy graph as a hill between your house and your friend’s house. Imagine that you want to ride a bike to your friends house. You’ll have to work to get up the hill, but you have a longer ride down the other side, so overall, it’s a “good” ride. In chemistry terms, we put in less heat to break the bonds than we get out when the bonds in the products are created, so the overall reaction is exothermic. 

All of that is stuff that’s been discussed before. What we need to recognize here is that you can still go home. A hill is just a hill. Going home will require more effort -- there is more uphill and less downhill, but you can still go home. In chemistry terms, we will be breaking the bonds that we had made and making the bonds we had originally broken. So, in this case the reaction will be endothermic, but it can still occur.

What this means is, again, that all reactions can (and do) run in both directions. This may seem a little odd, so let’s look at an extreme example. Surely, forest fires don’t go backwards, right?

Let’s take a look at the reaction. Forest fires are combustion reactions and trees are almost entirely C, H and O. So we can write the reaction this way 

It seems crazy to argue that forest fires can happen backwards, but lets take a look at this reaction.  The reaction combines "tree" and oxygen to release carbon dioxide, water and energy. Backwards would be water, carbon dioxide and energy combining to form a tree and releasing oxygen. Chances are you've heard of that reaction. Biologist call it photosynthesis. 

Chemists, always looking for the "lazy" way, don't write this as two reactions. Instead we replace the arrow in the middle with a double arrow that shows the reaction can run in both directions, like this:

The take-away- The essential idea here is that ALL reactions are reversible if the conditions permit. The consequences of this idea are much bigger than you might expect and comprise the rest of this chapter.