|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