Let's start with the idea of a reaction. In chemistry, a reaction is 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.

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 garbage can rusting is an example of a chemical change. Rust occurs because the iron metal is combining with oxygen in the atmosphere to form a new substance, rust. A refrigerator or air conditioner cooling air, is that a chemical change? No, it is a physical change because nothing new is created. (2) Electrons must be rearranged. A reaction can happen with anything, as long as electrons are rearranged between the elements. When electrons are rearranged new substances are formed and a chemical change has occurred.

The substances present before the change and the substances formed by the change are the 2 kinds of substances involved in a chemical reaction. A substance that enters into a chemical reaction is called a reactant. A substance that is produced by a chemical reaction is called a product. So a general description of a chemical reaction is, reactants changing into products.

Chemist have a convenient way of representing chemical reactions. A chemical equation uses symbols and formulas to describe what occurs during a chemical reaction. An equation is another example of chemical shorthand. Instead of using words to create a sentence to describe what happens in a reaction, an equation can be used. A chemical equation is much like the work of an accountant who has to show where every penny has come from and where it has gone to. The French chemist Antoine Lavoisier established The Law of Conservation of Mass that states mass is neither created nor destroyed in any chemical reaction. Therefore chemical equations must be balanced. Balancing of equations requires the same number of atoms on both sides of a chemical reaction. The mass of all the reactants (the substances going into a reaction) must equal the mass of the products (the substances produced by the reaction). To balance an equation, follow these steps:

1. Write a chemical equation with correct symbols and formulas.

H2 + O2 yields H20

2. Count the number of atoms of each element on each side of the yield sign.

3. Balance atoms by using coefficients.

2H2 + O2 yeilds 2H2O

Chemist have identified four general types of reactions: synthesis, decomposition, single replacement, and double replacement.

In a synthesis reaction two simple substances combine to form a new, more complex substance. Look at the image below:

In a decomposition reaction, a complex substance breaks down into two simpler substances. Look at the image below:

In a single replacement reaction, an uncombined element replaces an element that is part of a compound. These reactions can be written as follows:

In a double replacement reaction, different atoms in two different compounds replace each other. The reactions can be written as follows:

A catalyst is like adding a bit of magic to a reaction. Reactions need a certain amount of energy to happen. A catalyst lowers the amount of energy needed so that a reaction can happen easier. 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. If heat energy is released we describe the the chemical reaction as being an exothermic reaction. The word exo means "out of" and the word thermic refers to "heat." So an exothermic reaction releases heat. What about a reaction in which heat is absorbed? We describe this type of reaction as an endothermic reaction. Endo is a prefix that means "into." There is another type of catalyst called an inhibitor that slows down the speed of a reaction, instead of speeding it up.

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 which are combining. There is another big idea with rates of reaction... collision theory. The study of reaction rates and the collision theory is known as kinetics. The collision theory says that the more collisions in a system, the more likely combinations of molecules will happen. So 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. Over and over again but not always at the same speed. There are a few things which really affect the speed of the reaction and the number of collisions that can occur.

(1) CONCENTRATION: If there is more of a substance in a system there is a higher 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.

(2) 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 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.

(3) SURFACE AREA: Surface area refers to how much of a material is exposed. An increase in surface area increases the collisions between reacting particles. For example, a given quantity of wood burns faster as sawdust than it does as a log.

Additional assignments- print and turn in your work, or email your answers to me at jburks1@cullmancats.net. You might need to install ShockWave:
1. Online Practice Test
2. Interactive Tutor