Chemistry 30

1.1 The Concept of Energy and Forms of Energy

What is energy? If you're like most students, you probably have an idea of what energy is but might find it hard to actually define it.

Energy is defined as the ability to do work or produce heat. You likely know that energy can come in many forms - light energy, nuclear energy, and electrical energy to give just a few examples. In chemistry, heat energy is often what we are interested in, and the second part of the definition for energy will have more meaning for us. We will often discuss energy in terms of heat.

So, what exactly is heat, or thermal energy? This is something else that we "know it when we see it" but find it difficult to define. In previous science classes you've most likely learned that the individual atoms and molecules that make up all substances are in constant motion. This energy of motion is called kinetic energy. The energy associated with this motion is heat, or thermal energy.	What happens to molecular motion as temperature is increased? Follow this link to see how temperature and molecular motion are related. The molecular motion within sample of metal is shown. The initial temperature is set at 50°C. Use the slider control to increase the temperature - Source: Ted Greenbowe, Chemical Education Research Group Iowa State University (http://www.chem.iastate.edu/)
How does the temperature of two objects change when brought into contact? Select two metals and adjust the starting temperatures of one or both. What happens to the temperature of the two samples as they are brought together? How do the temperatures of the two objects compare at the end? Source: Source: Ted Greenbowe, Chemical Education Research Group Iowa State University (http://www.chem.iastate.edu/)
	Heat can also be defined as the flow of energy. This energy transfer is always from the hotter substance to the colder object. For example, If you hold an ice cube in your hand, your hand feels colder. Your hand feels colder because it is giving heat (or losing heat) to the ice cube; the ice cube does not transfer "coldness" to your hand. This is an important point, so keep it in mind.
In addition to kinetic energy there is another basic form of energy - potential energy. Potential energy is energy of position; we often refer to it as stored energy.
Kinetic Energy Energy of motion. A rock rolling down a hill has kinetic energy. Atoms and molecules have kinetic energy as they are always in motion (click here for more information). Potential Energy Stored energy. A rock sitting on top of the hill has stored energy.

Many of the different forms of energy that you may think of, such as light energy, electrical energy, and thermal energy are specific types of kinetic energy.

What about potential energy - can you think of an example? We will be concerned about one type of potential energy in this unit - chemical bond energy. Chemical bond energy arises from the attractive forces between molecules, and between atoms. This form of energy plays an important role in chemical reactions.

Nuclear energy is another form of potential energy, and is the energy stored within an atom as the forces holding the parts of an atom together. Chemical reactions do not involve nuclear changes or energy, and we won't be examining it further in this unit.