Chemical Reactions

Let's start with the idea of a reaction. In chemistry, a reaction happens when two or more molecules interact and something happens. That's it. What molecules are they? How do they interact? What happens? Those are all the possibilities in reactions. The possibilities are infinite. There are a few key points you should know about chemical reactions.

Key Points

1. A chemical change must occur. You start with one compound and turn it into another. That's an example of a chemical change. A steel garbage can rusting is a chemical reaction. That rusting happens because the iron (Fe) in the metal combines with oxygen (O₂) in the atmosphere. When a refrigerator or air conditioner cools the air, there is no reaction. That change in temperature is a physical change. Nevertheless, a chemical reaction can happen inside of the air conditioner.

2. A reaction could include ions, molecules, or pure atoms. We said molecules in the previous paragraph, but a reaction can happen with anything, just as long as a chemical change occurs (not a physical one). If you put pure hydrogen gas (H₂) and pure oxygen gas in a room, they can be involved in a reaction. The slow rate of reaction will have the atoms bonding to form water very slowly. If you were to add a spark, those gases would create a reaction that would result in a huge explosion. Chemists would call that spark a catalyst.

3. Single reactions often happen as part of a larger series of reactions. Take something as simple as moving your arm. The contraction of that muscle requires sugars for energy. Those sugars need to be metabolized. You'll find that proteins need to move in a certain way to make the muscle contract. A whole series (hundreds actually) of different reactions are needed to make that simple movement happen.

RATES OF REACTIONS

The rate of a reaction is the speed at which a reaction happens. If a reaction has a low rate, that means the molecules combine at a slower speed than a reaction with a high rate. Some reactions take hundreds, maybe even thousands of years while other can happen in less than one second. The rate of reaction depends on the type of molecules that are combining.

There is another big idea for rates of reaction called collision theory. The collision theory says that the more collisions in a system, the more likely combinations of molecules will happen. If there are a higher number of collisions in a system, more combinations of molecules will occur. The reaction will go faster, and the rate of that reaction will be higher.

Reactions happen, no matter what. Chemicals are always combining or breaking down. The reactions happen over and over but not always at the same speed. A few things affect the overall speed of the reaction and the number of collisions that can occur.

Concentration: If there is more of a substance in a system, there is a greater chance that molecules will collide and speed up the rate of the reaction. If there is less of something, there will be fewer collisions and the reaction will probably happen at a slower speed.

Temperature: When you raise the temperature of a system, the molecules bounce around a lot more (because they have more energy). When they bounce around more, they are more likely to collide. That fact means they are also more likely to combine. When you lower the temperature, the molecules are slower and collide less. That temperature drop lowers the rate of the reaction.

Pressure: Pressure affects the rate of reaction, especially when you look at gases. When you increase the pressure, the molecules have less space in which they can move. That greater concentration of molecules increases the number of collisions. When you decrease the pressure, molecules don't hit each other as often. The lower pressure decreases the rate of reaction.

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.

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.