Section 4 Interactions of MatterChapter 21

Energy• Energy is the ability to do work.

• It is not a form of matter, because it does not have mass or volume. However, energy is involved in every interaction of matter.

• The law of conservation of energy states that energy cannot be destroyed nor created.

Section 4 Interactions of MatterChapter 21

Role of Energy in Physical Change

• Potential energy is the energy an object has as a result of its position, shape, or condition.

• Kinetic energy is the energy of motion.

• Adding the kinetic energy of all atoms or molecules in a substance gives you that substance’s thermal energy.

Section 4 Interactions of MatterChapter 21

• Phase Change and Energy An object’s thermal energy determines its physical state. When a substance changes from one physical state to another, the amount of thermal energy in that object changes.

Role of Energy in Physical Change, continued

Section 4 Interactions of MatterChapter 21

• In a chemical change, or reaction, energy is involved in the rearrangement of atoms.

• Exothermic Chemical Reaction A chemical reaction that releases energy is an exothermic reaction.

• Endothermic Chemical Reaction A chemical reaction that absorbs energy is called an endothermic reaction.

Role of Energy in Chemical Change

Section 4 Interactions of MatterChapter 21

• Change, Energy, and Disorder Even though energy cannot be destroyed, energy can sometimes be transferred into a form that cannot be used to do work.

• Most processes result in an increase in randomness or disorder.

• Entropy is a measure of disorder or randomness.

Role of Energy in Chemical Change, continued