Download do some work or to transfer some heat

ENERGY
of any system is defined as the ability of that system to
do some work or to transfer some heat.
Different forms of energy are related to different
properties of matter: mechanical, electrical, chemical,
radiation, heat energy.
Mechanical energy is the energy of a body as a whole, no
matter what’s its inner structure. It may be in the form
of KINETIC energy, or energy of motion. A moving
body may do some work.
The energy of resting body depends of its position. This energy is
termed as POTENTIAL energy. A boulder on the top of a hill has
some potential energy. It may fall down, doing some work. This is
mechanical potential energy.
In chemistry we are interested in the inner potential energy of
substance, i.e. the energy due to the intrinsic composition &
structure of substance. Some substances, as water, carbon dioxide,
are very stable. They are not inclined to decompose or to react
with other substances, provided no other very active chemicals
available.
Their potential energy is low.
It also means that bonding between atoms in such substances is
strong.
Other substances are unstable. They can readily decompose,
sometimes with an explosion. It means that their intrinsic potential
energy is high.
We say that the chemical rxn is EXOTHERMIC if heat is released
in the course of that reaction. It means that the potential energy of
system goes down in the course of such a reaction.
The reaction is termed ENDOTHERMIC, if accompanied by the
heat absorption. The energy of the system is going up in the course
of an endothermic reaction.
In science we are always looking for something which is constant,
conservative. Therefore two most fundamental laws of Nature are
those of CONSERVATION OF ENERGY & CONSERVATION
OF MASS.
When heat is released in an exothermic reaction, the energy is
transferred to the environment: some bodies around are heated,
though the intrinsic energy of reacting system is going down.
However, the overall amount of energy in the world does not
change.
What is released by the system, is absorbed by the environment, &
vice versa. This is the law of conservation of energy.
Energy cannot be created or destroyed in a chemical reaction or in
a phase transition. It can only be converted from one form to
another one.
Since we already know that all substances consist of atoms, & each
atom has it own definite mass, & that atoms are not destroyed or
created in chemical reactions, it means that true is also the LAW
OF MASS CONSERVATION in chemical reactions:
There is no observable change in the quantity of matter (i.e. is in
mass) during a chemical reaction or a physical change.
Energy units: calory & Joule.
1 cal is the amount of heat necessary to raise the temperature
of 1g of water by 1oC.
1kcal=103cal.
Joule is a smaller unit:
Conversion: 1cal = 4.18 J
1 kJ = 103J
(Origin of this unit: 1J = 1 kg.m2/s2
Energy = force x distance
Force, by the 2nd Newton’s Law is F=ma
(mass per acceleration)
m[kg], a[m/s2],
therefore: work=FL [kg m2/s2]) force times distance
Conversion: 1cal = 4.18 J
this mechanical equivalent of heat was obtained
experimentally.
Heat is the Energy of Molecular Motion. Temperature reflects
the kinetic energy of motion of particle in matter.
Specific Heat is the energy required to raise the
temperature of 1 g of material by 1 centigrade (or 1 K).
o
Sp.Heat = Q/m∆t
Specific heat is independent of the amount of material & is the
property of the material.
Water has the highest known specific heat
transp
o
1 cal/g C = 4.18J/g
o
C
Metals have low specific heats. Water has high specific heat:
1cal/goC.