WHAT THE HECK IS EQUILIBRIUM?
Equilibrium is a pretty easy topic. Big name, but easy idea. FIRST. When you have a system made up of a bunch of molecules, those molecules sometimes combine. That's the idea of a chemical reaction. SECOND. A chemical reaction sometimes starts at one point and moves to another.

Now imagine the reaction finished and you have a pile of new chemicals. Guess what? Those chemicals want to go through a chemical reaction and become the original molecules. We don't know why, sometimes they just do.

Put those two ideas together and you have equilibrium.
(1) Two reactants combine to make a product.
(2) Products like to break apart and turn back into the reactants.
(3) There is a point where those two reactions happen and you can't tell that any reactions are occurring. That is when the overall reaction is happy. There is no pressure to do more of one thing, or another.

THAT'S EQUILIBRIUM! (TA-DA)

But wait, there's more...There are some other traits of equilibrium. Equilibrium always happens at the same point in the reaction no matter where you start. So if you start with all of substance A it will break up and become B and C. Eventually, B and C will start combining to become A. Those reactions happen until they reach equilibrium. They reach equilibrium at the same point if you start with all B and C, or half A and half B,C. It doesn't matter, There is one special point where the two reactions cancel each other out.

Another idea is that equilibrium is reached by itself, with no outside forces acting on it. If you put two substances in a mixture, they combine and react by themselves. Eventually they will reach equilibrium. Scientists say equilibrium happens through SPONTANEOUS PROCESSES.

One last thing...You know how we learned that some atoms and molecules have charges, well a system at equilibrium appears not to have a charge. All the pluses and minuses cancel each other out and give a total charge of "0".

Scientists use the letter "K" to add up all of the conditions in a reaction. That "k" is the EQUILIBRIUM CONSTANT.

LOOK AT IT THIS WAY...
Let's look at this equilibrium thing in a different way. Start with a table. There is a glass on the table. We'll pour a whole bunch of "X" into that glass. Eventually, some of that "X" breaks down into two pieces of "Y". That's one chemical reaction taking place.

If you have another glass and you pour a bunch of "Y" into it, those "Y"s will eventually combine to make an "X".

Using scientific terms, the "X" DISSOCIATES into two pieces of "Y" and the pieces of "Y" are going through a RECOMBINATION to become "X". If you put the two equations together you get this equation.

Now we have one glass, with both reactions happening at the same time. If we look inside the concentration of the molecules move in one direction, and then the other. Eventually you won't see the concentrations change anymore. It's like nothing is happening in the glass. That's equilibrium. The two reactions are still going on, but at a speed where they cancel each other out and you can see no change. The reactions are at a "happy" position.

THINGS EFFECT THE POSITION OF EQUILIBRIUM
When a bunch of molecules are left alone, they reach a state of equilibrium. But that position of equilibrium can change if something happens to the molecules. Here's a list of things which can change the equilibrium point...

(1) New molecules or substances are added.
(2) The temperature of the system is changed.
(3) The pressure of the system is changed.
(4) The concentrations are changed, like adding more water to a solution or adding more of one substance.

LE CHATELIER, WHAT DID HE SAY?
There was a French guy named Henri Le Chatelier and he came up with a principle which talked about systems in equilibrium. Basically the principle says if you have a system in equilibrium and you do anything to it that messes up the equilibrium, the system will try to move back to the original state of equilibrium. Or, if you have a happy system, and you make it unhappy, it will try to make itself happy again.

He used a few more words...

"A system in equilibrium, when subjected to a stress resulting from a change in temperature, pressure, or concentration, and causing the equilibrium to be upset, will adjust its position of equilibrium to relieve the stress and reestablish equilibrium."

Hmmmm. The happy thing is a whole lot easier.