Chemical Reactions
Let's start with the idea of a chemical reaction. Reactions occur when two or more molecules interact and the molecules change. Bonds between atoms are broken and created to form new molecules. That's it. What molecules are they? How do they interact? What happens? The possibilities are infinite.
When you are trying to understand chemical reactions, imagine that you are working with the atoms. Imagine the building blocks are right in front of you on the table. Sometimes we use our chemistry toys to help us visualize the movement of the atoms. We plug and unplug the little connectors that represent chemical bonds. There are a few key points you should know about chemical reactions:
1. A chemical change must occur. You start with one molecule and turn it into another. Chemical bonds are made or broken in order to create a new molecule. One example of a chemical reaction is the rusting of a steel garbage can. That rusting happens because the iron (Fe) in the metal combines with oxygen (O2) in the atmosphere. Chemical bonds are created and destroyed to finally make iron oxide (Fe2O3).
When a refrigerator or air conditioner cools the air, there is no reaction in the air molecules. The change in temperature is a physical change. When you melt an ice cube, it is a physical change. When you put bleach in the washing machine to clean your clothes, a chemical change breaks up the molecules in your stains. 2. A reaction could include atoms, ions, compounds, or molecules of a single element. You need to remember that a chemical reaction can happen with anything, just as long as a chemical change occurs. If you put pure hydrogen gas (H2) and pure oxygen gas in a room, they might be involved in a reaction to form water (H2O). However, it will be in very very small amounts. If you were to add a spark, those gases would be involved in a violent chemical reaction that would result in a huge explosion (exothermic). Another chemical reaction might include silver ions (Ag+). If you mix a solution with silver ions with a solution that has chloride (Cl-) ions, silver chloride (AgCl) precipitate will form and drop out of solution.
3. Single reactions often happen as part of a larger series of reactions. When a plant makes sugars, there might be as many as a dozen chemical reactions to get through the Calvin cycle and eventually create (synthesize) glucose (C6H12O6) molecules. The rusting example we used earlier only showed you the original reactants and final products of the chemical reaction. There were several intermediate reactions where chemical bonds were created and destroyed. The silver chloride example only focused on the ions. In reality, the two solutions were created when two salts dissociated (split into ions) in water. Rate of Reaction
The rate of a reaction is the speed at which a chemical 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 others can happen in less than one second. If you want to think of a very slow reaction, think about how long it takes plants and ancient fish to become fossils (carbonization). The rate of reaction also depends on the type of molecules that are combining. If there are low concentrations of an essential element or compound, the reaction will be slower.
There is another big idea for rates of reaction called collision theory. The collision theory says that as more collisions in a system occur, there will be more combinations of molecules bouncing into each other. If you have more possible combinations there is a higher chance that the molecules will complete the reaction. The reaction will happen faster which means the rate of that reaction will increase.
Think about how slowly molecules move in honey when compared to your soda even though they are both liquids. There are a lower number of collisions in the honey because of stronger intermolecular forces (forces between molecules). The greater forces mean that honey has a higher viscosity than the soda water.
Factors That Affect Rate
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. Temperature: When you raise the temperature of a system, the molecules bounce around a lot more. 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. To the chemistry lab! Sometimes you will mix solutions in ice so that the temperature of the system stays cold and the rate of reaction is slower.
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. Sometimes, when you are in a chemistry lab, you will add one solution to another. When you want the rate of reaction to be slower, you will add only a few drops at a time instead of the entire beaker. Pressure: 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 density of molecules increases the number of collisions. When you decrease the pressure, molecules don't hit each other as often and the rate of reaction decreases.
Pressure is also related to concentration and volume. By decreasing the volume available to the molecules of gas, you are increasing the concentration of molecules in a specific space. You should also remember that changing the pressure of a system only works well for gases. Generally, reaction rates for solids and liquids remain unaffected by increases in pressure.
Measuring Reaction Rates
Scientists like to know the rates of reactions. They like to measure different kinds of rates too. Each rate that can be measured tells scientists something different about the reaction. We're going to take a little time to cover a few different measures of reaction rates. Forward Rate: The rate of the forward reaction when reactants combine to become products.
Reverse Rate: The rate of the reverse reaction when products break apart to become reactants.
Net Rate: The forward rate minus the reverse rate.
Average Rate: The speed of the entire reaction from start to finish.
Instantaneous Rate: The speed of the reaction at one moment in time. Some reactions can happen quickly at the start and then slow down. You have one average rate, but the instantaneous rates can tell you the whole story.
Scientists measure all of these rates by finding out the concentrations of the molecules in the mixture. If you find out the concentration of molecules at two different times, you can find out what direction the reaction is moving toward and how fast it is going. Even if the concentrations are equal at the two points of measurement, scientists still learn something. If the concentrations are stable during two measurements, the reaction is at an equilibrium point. One Step at a Time
There is still more to know about measuring the rates of reactions. Since many reactions happen in several steps, the rate for each step needs to be measured. There will always be one step that happens at the slowest speed. That slowest step is called the rate-limiting step. That rate-limiting step is the one reaction that really determines how fast the overall reaction can happen. If you have six steps in your series of reactions and the third step goes incredibly slow, that is the rate-limiting step. As far as the overall reaction is concerned, none of the other rates really matter. If you want to speed up the overall reaction, you would focus on that slowest step. Don't forget that if you only speed up one step, another step may become the new rate-limiting step. You should always understand how all of the steps are involved in the overall reaction. 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.
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).
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.
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 and productmolecules. 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."
Catalysts Speed It Up
A catalyst is like adding a bit of magic to a chemical 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 all about energy. 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. You can also add a catalyst to that room and get one little reaction started. In that situation, you could add a little palladium (Pd) to act as the catalyst.
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. 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.Acids and Bases Are Everywhere
Every liquid you see will probably have either acidic or basic traits. Water (H2O) can be both an acid and a base, depending on how you look at it. It can be considered an acid in some reactions and a base in others. Water can even react with itself to form acids and bases. It happens in really small amounts, so it won't change your experiments at all. It goes like this:
2H2O --> H2O + H+ + OH- --> H3O+ + OH-
See how the hydrogen ion was transferred?
Most of the time, the positive and negative ions in distilled water are in equal amounts and cancel each other out. Most water you drink from the tap has other ions in it. Those special ions in solution make something acidic or basic. In your body there are small compounds called amino acids. The name tells you those are acids. In fruits there is something called citric acid. That's an acid too. But what about baking soda? When you put that in water, it creates a basic solution. Vinegar? Acid.
So what makes an acid or a base? A chemist named Svante Arrhenius came up with a way to define acids and bases in 1887. He saw that when you put molecules into water, sometimes they break down and release an H+ (hydrogen) ion. At other times, you find the release of an OH- (hydroxide) ion. When a hydrogen ion is released, the solution becomes acidic. When a hydroxide ion is released, the solution becomes basic. Those two special ions determine whether you are looking at an acid or a base. For example, vinegar is also called acetic acid. (Okay, that gives away the answer.) If you look at its atoms when it's in water, you will see the molecule CH3COOH split into CH3COO- and H+. That hydrogen ion is the reason it is called an acid. Chemists use the word "dissociated" to describe the breakup of a compound.
Scientists use something called the pH scale to measure how acidic or basic a liquid is. Although there may be many types of ions in a solution, pH focuses on concentrations of hydrogen ions (H+) and hydroxide ions (OH-). The scale measures values from 0 all the way up to 14. Distilled water is 7 (right in the middle). Acids are found between 0 and 7. Bases are from 7 to 14. Most of the liquids you find every day have a pH near 7. They are either a little below or a little above that mark. When you start looking at the pH of chemicals, the numbers can go to the extremes. If you ever go into a chemistry lab, you could find solutions with a pH of 1 and others with a pH of 14. There are also very strong acids with pH values below 1, such as battery acid. Bases with pH values near 14 include drain cleaner and sodium hydroxide (NaOH). Those chemicals are very dangerous. Names to Know
Let's look at the whole picture now. There is a scale for acids and bases just like everything else. Here are a couple of definitions you should know:Acid: A solution that has an excess of H+ ions. It comes from the Latin word acidus, which means "sharp" or "sour".
Base: A solution that has an excess of OH- ions. Another word for base is alkali.
Aqueous: A solution that is mainly water. Think about the word aquarium. AQUA means water.
Strong Acid: An acid that has a very low pH (0-4).
Strong Base: A base that has a very high pH (10-14).
Weak Acid: An acid that only partially ionizes in an aqueous solution. This means that not every molecule breaks apart. Weak acids usually have a pH close to 7 (3-6).
Weak Base: A base that only partially ionizes in an aqueous solution. This means that not every molecule breaks apart. Weak bases usually have a pH close to 7 (8-10).
Neutral: A solution that has a pH of 7. It is neither acidic nor basic.
More Ideas About Acids and Bases
We told you about that guy Arrhenius and his ideas about concentrations of hydrogen and hydroxide ions. You're also going to learn about Brønsted-Lowry ideas. These two chemists from Denmark and England looked at acids as donors and bases as acceptors. What were they donating and accepting? Hydrogen ions. It's a lot like the first definition we gave, where an acid breaks up and releases/donates a hydrogen ion. This newer definition is a little bit more detailed. Scientists used the new definition to describe more bases, such as ammonia (NH3). Since bases are proton acceptors, when ammonia was seen accepting an H+ and creating an ammonium ion (NH4+), it could be labeled as a base. You didn't have to worry about hydroxide ions anymore. If it got the H+ from a water molecule, then the water (H2O) was the proton donor. Does that mean the water was the acid in this situation? Yes.
A chemist named Lewis offered a third way to look at acids and bases. Instead of looking at hydrogen ions, he looked at pairs of electrons (remember our pictures with dot structures in Atoms and Elements?). In Lewis' view, acids accept pairs of electrons and bases donate pairs of electrons. We know that both of these descriptions of acids and bases use completely opposite terms, but the idea is the same. Hydrogen ions still want to accept two electrons to form a bond. Bases want to give them up. Overall, Lewis' definition was able to classify even more compounds as acids or bases.
What Really Happens?
What really happens in those solutions? It gets a little tricky here. Let's look at the breakup of molecules in aqueous (water-based) solutions one more time for good measure. Acids are compounds that dissociate (break) into hydrogen (H+) ions and another compound when placed in an aqueous solution. Remember that acetic acid example? Bases are compounds that break up into hydroxide (OH-) ions and another compound when placed in an aqueous solution. We'll talk about baking soda in a few paragraphs.Let's change the wording a bit. If you have an ionic/electrovalent compound and you put it in water, it will break apart into two ions. If one of those ions is H+, the solution is acidic. The strong acid hydrogen chloride (HCl) is one example. If one of the ions is OH-, the solution is basic. An example of a strong base is sodium hydroxide (NaOH). There are other ions that make acidic and basic solutions, but we won't be talking about them here.
That pH scale we talked about is actually a measure of the number of H+ ions in a solution. If there are a lot of H+ ions, the pH is very low. If there are a lot of OH- ions compared to the number of H+ ions, the pH is high.
