Energy is the ability to do work. There are many forms of energy and all forms of energy
can be converted from one form to another. Energy is always involved in a chemical
reaction. Sometimes the energy is released, or given off, as the reaction takes place.
Sometimes the energy is absorbed. Based on the type of energy change involved,
chemical reactions are classified as either exothermic reactions or endothermic reactions.
In either type of reaction, energy is neither created nor destroyed. It merely changes
position or form. The energy released or absorbed usually takes the form of heat or
visible light.
An exothermic reaction is a chemical reaction where energy is released. Heat comes
out of, or is released from, a reacting substance during an exothermic reaction. A
reaction that involves burning or combustion is a good example of an exothermic
Electrical Energy – toaster, batteries
Gravitational Energy - water falling from a waterfall
Light Energy - Light from the sun (Plants use light energy from photosynthesis!)
Elastic Energy - bungee cord or an elastic band moving across the room
Mechanical Energy – a pencil sharpener taking wood off of a pencil
Sound Energy - A student yelling and screaming in the halls or a telephone line
An endothermic reaction is a chemical reaction where energy is absorbed from their
surrounding. The energy absorbed during an endothermic reaction is usually in the form
of heat or light. The most familiar endothermic reactions are those that occur during
cooking. As you fry an egg, for example, the white begins to become solid. If you were
to remove the egg from the heat, the reaction would stop. The egg would remain half
cooked until you heated it again. This is a perfect example of energy being absorbed in
the form of heat.
The speed of a reaction is called a reaction rate. There are different factors, which can
affect the reaction rate. Reactions can be sped up or slowed down if you control one or
more of the following factors:
Temperature: Heat increases the reaction rate, while cold will slow it down. Why?
Particles are constantly in motion. Temperature is a measure of the energy of their
motion. Particles at a high temperature have more energy of motion than do particles at a
low temperature because particles at a high temperature collide more frequently.
Ex: For example, at the perfect temperature, you can cook a hamburger or a
steak to perfection. But, if you try to hurry it up with too much heat, other reactions
occur as well. You could end up with a black lump of overcooked and inedible
hamburger or steak!
Concentration: The concentration of a substance is the measure of how much of that
substance you have in a given area. If you dilute a solution, it means there is a small
amount of solute in solvent. If a solution is concentrated, it means there is a large amount
of solute in the solvent. The more concentrated the substance, the faster the reaction.
Ex: Let’s use juice mix as an example. Consider two pitchers of water. In the
first pitcher, we will add 1 tablespoon of orange juice crystals. In the second pitcher, we
will add 6 tablespoons of orange juice crystals. What is the difference between the two
pitchers of orange juice?
Surface area: The amount of exposed area that there is to react. The more surface area,
the faster the reaction. (The less surface area, the slower the reaction.) The rate of a
reaction can be increased by breaking the solid into smaller pieces because this increases
the surface area of the reactant.
Ex: Wood burns faster as sawdust than it does as logs. Why? Sawdust has a
much greater surface area exposed to air than do the logs. So oxygen particles from the
air can collide with more wood particles per second. Therefore, the reaction rate is
Catalyst: A catalyst is something that speeds up a reaction. Some chemical reactions
take place very slowly. The reactions involved with digesting a cookie are examples. In
fact, if these reactions proceeded at their normal rate, it could take weeks to digest one
cookie! Fortunately, there are certain substances that speed up the rate of a chemical
reaction. These substances are catalysts. It increases the rate of a reaction but is not
changed or involved in the reaction.
Ex: Your body contains some of the most important catalysts. In your body,
catalysts are called enzymes. Each enzyme increases the rate of a specific reaction
involved in the body’s metabolism.