Energy
Energy exists in a variety of forms. Energy associated with chemical reactions is evolved as heat. Other forms of energy are:
- light energy
- sound energy
- electrical energy
- mechanical energy
- chemical energy
Contents
Examples of energy
All physical and chemical changes are accompanied by changes in energy. Here are a few common processes and their associated energy changes:
- eating an orange results in a release of energy
- burning charcoal in a summer BBQ results in a release of energy
- ice melting in your thirst quenching drink uses energy
- boiling water in a kettle uses energy
The chemical energy stored in the battery
is converted to electrical energy
to useful light energy emitted from a flashlight
and to less useful heat energy.
Types of energy
Two general classifications of energy are potential energy and kinetic energy.
Potential energy
 Because of the boulder's altitude, it has stored energy capable of making a splash in the water if it were rolled down. This stored energy is the boulder's potential energy. |
Potential energy is stored energy. It is the energy that an object possesses because of its position.
The classic example is a boulder sitting on a cliff above a river (see image to the left). Some substances like charcoal and propane have a lot of stored energy within the relative positions and arrangements of atoms of the substances. This stored energy is also potential energy, and it is referred as chemical energy. Different forms of potential energy include:
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Kinetic energy
 When the boulder rolls down the cliff, its potential energy is converted to kinetic energy. How much kinetic energy the boulder possesses depends on the mass and the velocity of the boulder. |
Kinetic energy is the energy of motion.
When we light the charcoal or burn propane, the potential energy of these substances are converted to heat energy by giving off heat. Heat is a form of kinetic energy. Kinetic energy depends on the object's mass and velocity. The mathematical equation that relates kinetic energy to mass and velocity is K.E. = ½mv2. |
Endothermic and exothermic reactions
When physical or chemical changes occur, energy is either produced or absorbed. When the process produces energy, it is an exothermic process. When the process absorbs energy, it is an endothermic process.
The melting of snow is an endothermic process. A source of heat causes the phase transition from solid to liquid.
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The heat energy from the sun causes snow to melt. In the solid form, snow has regular hexagonal shapes. The hexagonal units touch, and are tightly packed in a regular arrangement. The snow absorbs the heat energy of the sun. In the melting process, the sun's heat energy is converted to the liquid water's kinetic energy. As the snow melts, the water molecules increase in kinetic energy resulting in losing their regular hexagonal shapes, and becoming mobile so that the water molecules in the liquid state can glide over each other. In the end, when snow melts to become a puddle of water, an endothermic process has occurred. |
Law of Conservation of Energy
Now that we understand that energy can be converted from one form to another, it is as important to know the scientific law that governs energy. It is known as the Law of Conservation of Energy.
- In any physical and chemical changes, energy is neither created nor destroyed.
Scientists have reached the conclusion that although energy has many different forms that are interconvertable, when one form of energy disappears, some other form of energy of equal magnitude must appear, and vice versa. In other words, the total quantity of energy in the universe is constant.
Practice usages of heat of reactions
Hand warmers
Commercially available hand warmers contain iron particles, carbon, salt water and other chemicals such as cellulose and vermiculite, which act as insulators. The hand warmer is activated when the protective packaging is opened and exposed to air. The average temperature of handwarmers is ~ 57°C. Maximum temperature is ~69°C.
Air will penetrate the hand warmers and the oxygen reacts chemically with the iron, causing the iron to rust or oxidize. The result of the oxidation process is the production of heat resulted from an exothermic reaction. The process occurs rapidly with the help of the salt solution. The carbon particles help spread the heat throughout the entire package. The other chemicals added help to provide long-lasting heat. Once all of the iron have been converted to Fe2O3, rust, the hand warmer will not be able to provide heat.
Some hand warmers use a supersaturated solution in a sealed bag to generate heat. The supersaturated solution could be sodium acetate or calcium nitrate. A piece of metal is usually inserted into the sealed bag. Upon flexing the metal, salt crystals will begin to form as the supersaturated solution crystallized into a solid. As the crystallization process involves generating heat, usable heat will usually last about 30 minutes. Unlike the previous type of hand warmer involving the oxidation of iron, this type of hand warmers can be reheated and reused.
Cold packs
For cold packs that are used by athletes, the packet usually contains water and a packet of ammonium chloride. The cold pack is activated by breaking the barrier which separates the ammonium chlorid and water. Once the barrier is broken, ammonium chloride begins to dissolve in water, and almost instantly, the pack becomes cold. This happens because the chemical reaction absorbs heat from the environment.
Content suitability
BCIT courses: CHEM 0011
