Reaction quotients

If you let a chemical reaction run for long enough, it will reach equilibrium.  That’s just a fact of life.  Reaction + time = equilibrium.

However, how can you tell if a reaction has reached equilibrium yet?  The answer: Calculate the reaction quotient, Q.

q

Or you could ask this guy.

Let’s learn how.


What is Q?

Let’s consider the equation we’ve been looking at for the last few tutorials and take a look at the equilibrium expression:

ex2

Now, let’s take a look at the expression for the reaction quotient:

qexpression

See anything familiar?  If not, go take a look again.  Yep, you got it:  K and Q are the same thing.  This leads to an obvious question:  Why do we have the same  expression for two different values?

Not exactly.  Check it out:

  • K describes a system at equilibrium.  For the reaction above, K describes the concentrations of every chemical in your beaker when everything has reached equilibrium.
  • Q describes a system which may or may not be at equilibrium.  The very instant you throw your reagents into a flask, you can start calculating Q.  Not surprisingly, Q will change over the course of the reaction as the concentrations of the reagents and products shift.  However, as the equation suggests, once the reaction finally reaches equilibrium, K and Q are the same.

Essentially, Q is just a way of monitoring the progress of a chemical reaction that hasn’t yet achieved equilibrium.


If I look at Q, what does it tell me?

It tells you whether a chemical reaction has reached equilibrium.

  • If Q > K, this means that there are more products in your particular mixture than there will be at equilibrium.  As a result, as time progresses, you can expect to see some of the products revert back to reagents.
  • If Q < K, this means that there are more reagents in your mixture than there will be at equilibrium.  As time passes, you’ll see more products being formed.
  • If Q = K, you’ve reached equilibrium.  You’ve reached your final concentrations of products and reagents.

In a practical sense, Q allows you to figure out if your reaction is done yet.  When you throw a bunch of chemicals into a beaker, you can monitor the concentrations of each chemical to figure out what Q is.  When the Q you calculate is equal to the K value you know for the reaction, then you’re done!


Photo credits:

John de Lancie, the man who played “Q” on several Star Trek TV series:  By “Pinguino” (“Pinguino’s” flickr account) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)%5D, via Wikimedia Commons.

Advertisements
This entry was posted in Uncategorized. Bookmark the permalink.