Equilibrium Review

 

Consider this reaction:

 

 

2A(g)  +  3B(g)  +  Energy  ß---à  C(g)  +  3D(g)

 

 

1. Is this reaction Endo or Exo thermic?   Endo.  Energy is on the reactant side

2. Which side is products, which side is reactants?  Left is always reactants

3. Write the Keq expression for this reaction.  [C][D]3/[A]2[B]3

4. Write the Keq expression for the reverse of this reaction?  (reciprocal of what you see above)

5. If the Keq of the forward reaction is 3.75 x 10-9, does the reaction favor products or reactants?  (reactants.  Keq’s less than one favor reactants)

6. What would the numeric value for the Keq be if the reaction were written in the reverse order?  (1/3.75 x 10-9)

 

 

 

If you did                                            Favor Products                                     Favor Reactants

 

Increase Pressure                                    x

 

Increase Volume                                                                                                  x

 

Increase Temp.                                      x

 

Decrease Temp.                                                                                                 x

 

Add more A(g)                                                           x

 

Remove A(g)                                                                                                      x

 

Remove C(g)                                          x

 

Why?  The following should answer your questions to why I checked the spots that I did above:  They are also helpful for studying for the quiz!

 

1.  For gasses ONLY (not liquids and solids):  Avogadro’s principle states that the same number of molecules of gas can fit into a given volume and pressure at a given temperature.  Think about the analogy of the parking lot with cars that can fit into any space.  Gas molecules are the cars and no matter their size (big or small) only one car can fit into one  parking space.

      So, if you put the gas under pressure (or reduce the volume – eg. Reduce the number of parking spaces) then, the reaction will favor the side with the least number of particles.   on it.  If you look at the reaction above, you’ll see that the product side has FOUR particles while the reactant side has FIVE particles.

 

2.  Notice that energy is on the left side (reactant side).  That means it requires energy (and is thus endothermic).  That means that if you add heat, it will drive it to the products side.  Likewise, if you remove heat, it will drive it to the reactant’s side.

 

3.  Think of the Earth and its water cycle.  Think that the Earth is a self contained unit which includes the ground and the atmosphere above it.  Think of water (in its liquid state) as being the reactant.  Think of clouds (in its vapor state) as being the product.

      Imagine if you will, for just the sake of argument that 90% of all of the Earth’s water is on the ground and 10% is in the air.  This is the equilibrium point.  Yeah, it might rain, but the sun also evaporates water from the ocean to make more clouds.  This keeps the ratio of water:clouds pretty much 90:10.  (Think about that:  For as much as it rains, it evaporates the same amount.  Not necessarily at the same place, however.  It might rain in Hawaii from water we wasted here in San Clemente).

     What if some alien from space stole all of the water vapor (clouds)?  Don’t you think that some of the water in the oceans would be evaporated to replace what was stolen and return the Earth’s balance to 90:10?  Sure it would!  Of course, the 90:10 split would represent a lesser amount of water, but it would still be split 90% water on the ground and 10% water in the air.

       Therefore, if you remove products (clouds) then the reaction will favor making more products (clouds). 

       If that same alien came and poured water on our planet and filled up the oceans more, then our sun would soon distribute that new water into the atmosphere so that it would continue to represent 90:10.  So, if you add reactant (adding more water to the ocean) then you will favor making more product (clouds) so that the equilibrium can be reached.

      Of course, the same would be true if you removed water (reactant).  You would favor more reactants.  And, if you added product (clouds) you would favor making more reactant.

 

      Bottom line:  There is a certain equilibrium reached which must be maintained.  If you tweek the equilibrium by adding more reactant or product or by heating it or cooling it, the system will adjust itself until the equilibrium is reached again.

 

      Think about this:  In the examples above, I said that 90% of the water on Earth is on the ground and 10% is in the air.  This equilibrium ratio itself can be altered however.  How?  By global warming for one thing.  If the world get hotter, then we might wind up with an 80% and 20% water:cloud ratio.

     So, as it stands now, there is a fixed equilibrium point of the Earth’s water.  BUT, that could change if the world’s temperatures are changed.