Introduction to Thermochemistry
What is Energy?
Work, as understood by scientists, means more than towing barges and lifting bales. The concept goes beyond physical labour to embrace a number of actions, but they all come down to the pushing or pulling of something against an opposing force. For example, when hot gases expand in the cylinder of a gasoline engine, they push back a piston and ultimately move the car. This is of course mechanical work.
 
Everyone agrees that a battery has energy. It also has the ability to do work, and it delivers this energy by pushing electrons through a wire. This pushing of electrons is referred to as electrical work. The current may run a small motor (and so be changed into mechanical work); or it can be passed through a bulb and converted into both heat and light, two other forms of energy.
 
There is kinetic energy, the energy of motion which is derived from the formula: KE = ½mv2 where m is the mass of the object in kilograms and velocity is in meters/sec. The units derived from this equation is kg m2/s2. The other kind is potential energy and it can be derived one way using PE = mgh
 
where m is again the mass in kilograms, g is the gravitational constant of 9.8 m/s2 and h is the height that the object is to fall. Again the units derived are kg m2/s2.
 
Origin of Chemical Energy
The term chemical energy is the special name often given to the form of potential energy that arises from the forces of attraction that bind atoms together in compounds. These forces of attraction are called chemical bonds. What is important now is the idea that when chemicals react to form new substances, atoms are exchanged as old bonds break and new bonds form. This process changes the potential energies of the atoms. Sometimes a reaction's products have more potential energy than it's reactants; in other reactions the products have less potential energy potential energy than the reactants.
 
In general all chemical reactions either liberate (exothermic reactions) or absorb (endothermic reactions) heat. The origin of chemical energy lies in the position and motion of atoms, molecules and subatomic particles. The total energy possessed by a molecule is the sum of all the forms of potential and kinetic energy associated with it. Kinetic energy actually contributes very little to the energy of a substance. The bonding energy is what is really important.