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When wood or charcoal is burned, it appears as if energy is destroyed and wasted. In fact, the energy in these kinds of fuels is not destroyed when the fuels are burned. It is simply converted to other forms of energy such as heat and light. When you are seated on a desk in class, you are possessing potential energy. When you stand up and walk away from the classroom, you are transforming the potential (chemical) energy in your muscles to kinetic energy.

The Law of Conservation of Energy states that energy can neither be created nor destroyed but it can only be changed from one form to another. When we use energy, it does not disappear. We simply convert it from one form to another. Potential (chemical) energy in a dry cell is converted to electrical energy which is finally converted to sound energy in radio speakers. In a tape record player, the same chemical energy is ultimately converted to kinetic energy to drive the cassettes. When the potential energy is all used up, the batteries are dead.

In the case of rechargeable batteries, their potential energy is restored through recharging. The chemical energy in your mobile phone battery can be converted into sound, light, text, etc. The main energy changes that occur in a variety of simple situations are:

  • Battery chemical to electrical, sound or light;
  • Car engine chemical to mechanical and then kinetic;
  • Light bulb electrical to light and heat;
  • Solar heat to electrical and kinetic;
  • Running water kinetic to electrical;
  • Muscles chemical to kinetic, etc.

All energy changes that occur during chemical and physical changes must conform to the Law of Conservation of Energy, that is, energy can only be changed from one form into its equivalent of another form with no total loss or gain. The most common form of energy in chemistry is the heat change.

A chemical reaction must involve some change in energy. As the reaction occurs, chemical bonds of reactant molecules are broken while those of the product molecules are formed. Energy is given out when a chemical bond forms and it is consumed when a bond is broken.

For example of combustion (respiration) of glucose in living cells: During respiration process, the bonds of glucose and oxygen are broken down while those of carbon dioxide and water are formed. Heat is absorbed when chemical bonds are broken and it is released when the bonds are formed. The total amount of heat absorbed by the reactants is equal that released by the products. Heat absorbed is given a positive sign (+ve) while heat given out is assigned a negative sign (-ve). So the total energy change is equal to zero. This means that no energy has been created or destroyed

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