In previous Study Guides we worked with and defined matter. Matter is easier to understand because we can touch it and see it. Energy, on the other hand, is a more challenging concept. Energy is defined as the capacity to do work or transfer heat. Heat is the thermal energy transferred from one object to another due to a temperature difference between the two objects. Work is the energy required to move an object against a force.

Kinetic Energy and Potential Energy

Objects, including molecules, possess kinetic energy, the energy of motion. The kinetic energy of an object, E_k, depends on the mass, m, of the object and its speed, v.

\(\displaystyle E_k\;=\;\frac{1}{2}mv^2\)

The kinetic energy increases with increasing speed and mass. The faster the object is moving, the greater the kinetic energy of that object. The greater the mass of an object, the higher the magnitude of its kinetic energy. A motorcycle moving at a speed of 65 mi/hr has a greater kinetic energy than a motorcycle of the same mass moving at a speed of 35 mi/hr. A large semi truck traveling the same speed as a motorcycle has a greater kinetic energy because the mass of the truck is larger than that of the motorcycle. In chemistry our interest is in the energy of atoms, ions, and molecules–they all possess kinetic energy because they have mass and are in motion.

We can think of Potential Energy, E_p as stored energy by virtue of the position of the object relative to other objects. This could be energy that is stored in a spring that is stretched or a weight that is held above the floor. Both the spring and the weight have potential energy. Once the weight is released, the potential energy decreases while the kinetic energy increases. The same principle applies to the stretched spring.

Energy Units

In this section we will use the SI unit of the Joule, J when calculating energies.

\(\displaystyle 1\;J\;=\;1\;kg\times\frac{m^2}{s^2}\)
Please see the discussion on units of energy in Energy: A Derived Unit , study guide. Because the joule is such a small energy unit, kJ are commonly used. For example, 53526.2 J is equal to 53.5 kJ.

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