EQUILIBRIUM BASICS

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EQUILIBRIUM BASICS

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 reverse 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 point is when the overall reaction is happy. There is no pressure to do more of one thing or another.

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.

IT HAPPENS ON ITS OWN

Another idea is that equilibrium is reached by itself with no outside forces acting on the system. If you put two substances in a mixture, they will combine and react by themselves. Eventually, they will reach equilibrium. Scientists say equilibrium happens through spontaneous processes. They happen on their own.

There is one last idea. Do you remember that some atoms and molecules have charges? A system "at equilibrium" appears to have no charge (neutral). 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 actions and conditions in a reaction. That "K" is the equilibrium constant.

MORE ABOUT EQUILIBRIUM

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".

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

THE POSITION OF EQUILIBRIUM

When a bunch of molecules is 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 that 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 for systems in equilibrium. The principle says that 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.

His exact words were, "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."

CATALYSTS SPEED IT UP

A catalyst is like adding a bit of magic to a reaction. Reactions need a certain amount of energy to happen. If they don't have it, oh well, the reaction probably can't happen. A catalyst lowers the amount of energy needed so that a reaction can happen easier. A catalyst is about energy; it doesn't have to be another molecule. If you fill a room with hydrogen gas and oxygen gas, very little will happen. If you light a match in that room (or just a spark), all of the hydrogen and oxygen will combine to create water molecules. It is an explosive reaction.

The energy needed to make a reaction happen is called the activation energy. As everything moves around, energy is needed. The energy a reaction needs is usually in the form of heat. When a catalyst is added, something special happens. Maybe a molecule shifts it's structure. Maybe that catalyst makes two molecules combine and they release a ton of energy. That extra energy might help another reaction to occur. In our earlier example, the spark added the activation energy.

Catalysts are also used in the human body, not to cause explosions but to make very difficult reactions happen. They help very large molecules combine. There is another interesting fact about catalysts. Catalysts lower the activation energy required for a reaction to occur. With the activation energy lower, the products can also combine more easily. Therefore, the forward and reverse reactions are both accelerated. It helps both reactions.

INHIBITORS SLOW IT DOWN

There is also something called an inhibitor that works exactly the opposite of catalysts. Inhibitors slow the rate of reaction. Sometimes they even stop the reaction completely. You might be asking, "Why would anyone need those?" You could use an inhibitor to make the reaction slower and more controllable. Without them, some reactions could keep going and going and going. If they did, all of the molecules would be used up. That would be bad, especially in your body.

Community Contributions - Articles by goIITians

EQUILIBRIUM BASICS

IT HAPPENS ON ITS OWN

MORE ABOUT EQUILIBRIUM

THE POSITION OF EQUILIBRIUM

LE CHATELIER, WHAT DID HE SAY?

CATALYSTS SPEED IT UP

INHIBITORS SLOW IT DOWN