Energy

Energy is usually defined as the ability to do work.

Energy may appear in a variety of forms. Most commonly, energy in reactions is evolved as heat. Some other forms of energy are light, sound, mechanical energy, electrical energy, and chemical energy. Energy can be converted from one for to another. An example of this is heat from burning fuel being used to vaporize water to steam. The energy of steam is used to turn the turbine wheels to produce mechanical energy. The turbines turns the generator armature to produce electricity. The electricity is the available in our homes.

Two general classifications of energy are potential energy and kinetic energy. Potential energy is due to position: Kinetic energy is energy of motion. The difference can be illustrated by a boulder sitting on the side of a mountain. It has a high potential energy due to its position above the valley floor. If it falls, its potential energy is converted to kinetic energy. This illustration is very similar to the situation of electrons cascading to a lower energy level in the atomic model.

When physical or chemical changes occurs, energy is involved. If the heat content of the products is higher than the heat content of the reactants, the reaction is endothermic. If the heat content of the products is lower than the heat content of the reactants, the reaction is exothermic. The change of heat content can be designated DH (read delta H). the heat content is also referred to as the enthalpy.

DH = H products - H reactants

If DH is positive, the reaction is endothermic, and if the DH is negative, the reaction is exothermic.

Conservation of Energy

Experiments have shown that energy is neither gained nor lost in physical or chemical changes. This principle is known as the Law of Conservation of Energy and is often stated as follows: Energy is neither created or destroyed in ordinary physical and chemical changes.