CHEMICAL REACTIONS - CATALYSTS #8

Catalysts Speed It Up

A catalyst is like adding a bit of magic to a reaction. Reactions need a certain amount of energy in order 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 more easily. A catalyst is about energy. It doesn't have to be another molecule. If you fill a room with hydrogen gas (H₂) and oxygen gas (O₂), very little will happen. If you light a match in that room (or just produce a spark), most of the hydrogen and oxygen will combine to create water molecules (H₂O). 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 that a reaction needs is usually in the form of heat. When a catalyst is added, something special happens. Maybe a molecule shifts its 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 something called a chain reaction. In our earlier example, the spark decreased the required activation energy. You could also think of a catalyst like a bridge in some instances. Instead of letting reactions happen in the same (but faster) way, it can offer a new direction or chemical pathway in order to skip steps that require energy. 

Catalysts are also used in the human body. They don't cause explosions, but they can make very difficult reactions happen. They help very large molecules to combine. There is another interesting fact about catalysts. You know that 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 changes both rates and usually changes the equilibrium point. 

Inhibitors Slow It Down

There is also something called an inhibitor that works in exactly the opposite way as 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 inhibitors, 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. When you are watching television, you have no reason to keep breaking down sugars at the same rates you would if you were working out. 

CHEMICAL REACTIONS - EQUILIBRIUM II #7

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 "Ys" 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 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 that can change the equilibrium point: 

1. New molecules or substances are added that are not a part of the main reaction. 
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 reactant or product. 
5. There is a change in the total volume of the system. 

Equilibrium doesn't always mean that there are equal numbers of reactant andproduct molecules. Our equilibrium point may look like it is in the middle of the two concentrations, but it can be anywhere. It's all about balance and finding a happy point. There are times when everything becomes a product, and other reactions where nothing happens. It all depends on the molecules and conditions of the system. 

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." 

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CHEMICAL REACTIONS - EQUILIBRIUM I #6

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? Some of those chemicals want to go through a reverse chemical reaction and become the original molecules again. 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. 

There is a point where those two reactions happen and you can't tell that any reactions are happening. That's the point when the reaction looks like it is finished. In reality, some of the molecules are turning into products and some are turning back into reactants. You need to imagine that you're as small as a molecule and you're watching all of these parts bouncing around and changing back and forth. Just staring at a test tube, you won't generally notice a change in their numbers. That's what equilibrium really is. The overall reaction is happy. There is no pressure greater in one direction over 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 recombining to become A. Those reactions happen until they reach equilibrium. They reach equilibrium at the same point whether you start with all A, all B/C, or half A and half B/C. It doesn't matter. There is one special point where the forward and reverse 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 can combine and react by themselves. Eventually, they will reach equilibrium. Scientists say that 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. 

CHEMICAL REACTIONS - THERMODYNAMCIS #5

Heat and Cold

What are heat and cold? It's a pretty simple idea. When you think of heat, you probably think of fire. When you think of cold, you might think of an ice cube. It all has to do with kinetic energy in atoms. Heat has a lot of kinetic energy and gives it away. The cold doesn't have much energy and absorbs it from the surrounding area. Chemists measure heat in units called Joules. You may also hear about sinks and sources. If the temperature of an object is higher than the surrounding area, it is considered a heat source. If the temperature of an object is lower than the surrounding area, it is considered a heat sink. 

Thermochemistry

There are two kinds of heat in chemistry. The first is caused by physical activity. As you get more kinetic energy, there is more activity in the system. This extra activity makes more molecular collisions occur. The collisions create the heat. This happens when you increase the pressure in a system. Chemical processes cause the second type of heat. Instead of exciting a system and feeling the heat, chemical bonds are made and broken, and the energy is then released. A release of energy charges up the system and the molecules bounce around faster, resulting in that physical activity we just explained. The opposite can also happen. Sometimes bonds are made and broken and energy is absorbed. The system then gets colder as the temperature goes down. Those emergency icepacks you see when people hurt their ankles are good examples of chemical reactions that absorb energy. 

There is energy all around us. Just as matter is all around us, energy is always there. Usually, you will feel this energy as heat. Let's say it's really hot out today. Why is it hot? One big reason is that there is a lot of heat/energy coming from the Sun. The Sun is a big furnace, and that furnace heats the Earth. When a lot of the Sun's radiant energy makes it to Earth, it transmits energy to the atoms and molecules in the air and ground. Those molecules heat up. The Sun makes your molecules more excited because of the energy hitting you. You should remember that only a small percentage of the Sun's energy makes it to Earth. We're talking about millionths of a percent. The Sun gives off more energy than you can imagine, and it doesn't end there. There are also millions of stars that are bigger than our Sun. There's a lot of energy in the Universe. 

Energy in Chemical Bonds

We just talked about energy in a star. There is also energy stored in the bonds between atoms. How about when you burn a piece of wood? When you burn something, you release the energy from the chemical bonds in the wood. Where did the energy come from? The Sun. A plant needs the Sun to grow. Light hits the plant and is used by a process called photosynthesis. The plant captures the Sun's energy and stores it in the chemical bonds. You have probably heard of glucose (C₆H₁₂O₆), which is one of the smallest sugar building blocks made by plants. The plant uses glucose to power certain processes, to manufacture the cellulose, and as a building block in the cellulose itself. When you burn a piece of wood, you are releasing all of the energy stored up. You experience that energy as heat and light (fire). 

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CHEMICAL REACTIONS - STOICHIOMETRY #4

Stoichiometry

Let's start with how to say this word. Five syllables: 
STOY-KEE-AHM-EH-TREE. It's a big word that describes a simple idea. Stoichiometry is the part of chemistry that studies amounts of substances that are involved in reactions. You might be looking at the amounts of substances before the reaction. You might be looking at the amount of material that is produced by the reaction. Stoichiometry is all about the numbers. 

All reactions are dependent on how much stuff you have. Stoichiometry helps you figure out how much of a compound you will need, or maybe how much you started with. We want to take the time to explain that reactions depend on the compounds involved and how much of each compound is needed. 

What do you measure? It could be anything. When you're doing problems in stoichiometry, you might look at...
- Mass of Reactants (chemicals before the reaction)
- Mass of Products (chemicals after the reaction)
- Chemical Equations
- Molecular Weights of Reactants and Products
- Formulas of Various Compounds 

Now, an example. Let's start with something simple like sodium chloride (NaCl). You start with two ions and wind up with an ionic/electrovalent compound. When you look at the equation, you see that it takes one sodium ion (Na⁺) to combine with one chlorine ion (Cl^-) to make the salt. When you use stoichiometry, you can determine amounts of substances needed to fulfill the requirements of the reaction. Stoichiometry will tell you that, if you have ten million atoms of sodium and only one atom of chlorine, you can only make one molecule of sodium chloride. Nothing you can do will change that. It's like this: 

10,000,000 Na + 1 Cl --> NaCl + 9,999,999 Na

Let's bump it up a level. When you mix hydrogen gas (H₂) and oxygen gas (O₂), nothing much happens. When you add a spark to the mixture, all of the molecules combine and eventually form water (H₂O). You would write it like this: 

2H₂ + O₂ --> 2H₂O

What does stoichiometry look at here? First, look at the equation. Four hydrogen atoms and two oxygen atoms are on each side of the equation. It's an important idea to see that you need twice as many hydrogen atoms as you do oxygen atoms. The number of atoms in the equation will help you figure out how much of each substance you will need to make the reaction happen. If you make this an extreme example and fill a sealed container with one million hydrogen molecules and only one oxygen molecule, the spark won't make an explosion. There is no monster reaction to be created when there is only one oxygen molecule around. You will make two water molecules and be done.