Kinetics I: Introduction to kinetics

At this point, I hope you’ve looked at a lot of the other tutorials about chemical reactions.  If you haven’t, head on over to the links at the right side of your page and read them all.  Aren’t they marvelous?

portrait-of-friendly-family-of-four-in-the-studio-100209431

Those tutorials saved our family. Thanks, Mr. Guch!

Unfortunately, I haven’t yet had much to say about the kinetics of chemical reactions.  If you’re not sure what that means, read on.


What is kinetics?

Chemical kinetics is the study of how fast chemical reactions take place.  If we say that “a reaction went really fast”, we’re making a statement about the kinetics of that chemical reaction.  Likewise, if we say that we’re “really annoyed that the reaction is going so slow”, that’s a statement about kinetics, too.

rate

As you get older, the rate at which this man goes from looking handsome to looking stupid increases exponentially.

As you might guess, there are also numerical ways of describing the rates of chemical reactions (called, straightforwardly enough, rate laws), but they’re nothing more than numerical ways of saying that “the reaction is fast” or “the reaction is slow.”


A common misunderstanding:  Kinetics and thermodynamics are the same thing

If you know anything about thermodynamics, you know that some reactions are exothermic (they give off energy) and some are endothermic (they absorb energy).  If something blows up and sets the surroundings on fire, it’s exothermic.  If something sits and gets cold, it’s endothermic.  You get the idea.

Kinetics doesn’t deal with this sort of thing at all.  Instead of worrying about how much energy will be given off in a reaction, it deals with how fast the reaction moves.  Kinetics doesn’t care whether a reaction is exothermic or endothermic – it just deals with the rate at which it goes from reagents to products.

Let’s use the example of the combustion of gasoline.  As is the case with all combustion reactions, when gasoline combines with oxygen to form carbon dioxide and water, energy is given off.  That’s why your car’s engine gets hot when it’s been running for a while.  If you didn’t already know that burning gasoline is hot, please step calmly away from the computer and find a grownup to assist you.

fire

A spontaneous reaction at my last Labor Day barbeque.

However, the combustion of gasoline isn’t necessarily a fast process.  Think about this:  In any automobile tank, there is a combination of gasoline and air.  Given that the reagents of gasoline and oxygen are both present, shouldn’t this combustion reaction take place?  Let’s see what kinetics and thermodynamics tell us about this:

  • Thermodynamics tells us that the reaction of gasoline and oxygen is both exothermic and spontaneous.  As a result, when you put these two things together, they should undergo an exothermic reaction with one another.
  • Kinetics tells us that the reaction of gasoline and oxygen is very, very slow.  After all, you don’t see cars exploding all over the freeway when you go for a drive.  From what you can see, you can conclude that gasoline undergoes combustion very slowly when it sits in a tank by itself, but very quickly when you stick it into the appropriate engine parts of a car.

So, we’ve got one reaction, and two ways of interpreting them:

  • Thermodynamics:  Gasoline and oxygen spontaneously ignite when in each others’ presence.
  • Kinetics:  Gasoline and oxygen react unbelievably slowly inside a gas tank, but the reaction rate is greatly accelerated in the engine of a car.

My point is this:  Kinetics and thermodynamics complement each other because they both tell us different things about chemical reactions.  Thermodynamics tells us if a reaction will be spontaneous, while kinetics tells us how fast it will undergo that spontaneous process.  Both pieces of information are good to know, because it’s not much fun to perform a spontaneous reaction that takes eight billion years to finish.


“Spontaneous vs. fast”

You probably have the idea that something that is “spontaneous” will happen all by itself if all of the needed reagents are present.  You probably also have the idea that if something is spontaneous, it happens pretty quickly.  For example, if you put a walrus into a tree, it’s not hard to tell that it will spontaneously fall out.

walrus

The view from my front porch.

However, what we don’t know about this process is whether or not the walrus will fall out of the tree quickly.  If I were to glue the walrus in the tree, it would eventually fall out of the tree after the glue weakened, or the tree broke, or something else unlikely occurred.  Because it does eventually fall out of the tree, this is considered a spontaneous process.  However, even though this process is spontaneous, the glue ensures that it’s a slow process.

Likewise, a fast process might not be a spontaneous process.  To initially get the walrus in the tree, you might use a catapult to launch it into the branches.  Once you fire off the catapult, this process is a quick one.  However, it can hardly be said that the process of getting a walrus into a tree is spontaneous.


Keep reading, because there’s a lot more to the kinetics saga!


 

Photo credits:

  • Weird-looking guy:  Image courtesy of photostock at FreeDigitalPhotos.net
  • Happy looking family:  Image courtesy of stockimages at FreeDigitalPhotos.net
  • My barbeque:  Image courtesy of Witthaya Phonsawat at FreeDigitalPhotos.net
  • Walruses in my front yard:  Image courtesy of Hal Brindley at FreeDigitalPhotos.net
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