4. Stoichiometry

L et'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 amounts. 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 t he time to explain that reactions are dependent upon 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 compound (NaCl). When you look at the equation, you see it takes one atom of sodium (Na) to combine with one atom of chlorine (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 (Na) and only one atom of chlorine (Cl) you can only make one molecule of sodium chloride (NaCl). Nothing you can do will change that. 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). What does stoichiometry look at here? First, look at the equation. Four hydrogen (H) atoms and two oxygen (O) 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 you need will help you figure out how much of each substance you will need to make the reaction happen.

5. Thermodynamics heat and cold

What are heat and cold? It's a pretty simple idea. You think of heat, you think of fire. You think of cold, you think of an ice cube. It all has to do with kinetic energy. Heat has a lot of kinetic e nergy and gives it away. The cold doesn't have much and absorbs energy from the area. Scientists measure heat in units called Joules.

Thermochemistry

T here 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 (such as 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 area then gets colder. 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.