The picture of a central positively charged nucleus of an
atom surrounded by negatively charged swarming electrons explains so much
detail in the nature of our world. This
is known as the atomic model. The
behavior of gas, liquids and solids are almost completely explained with just
that model (after including quantum effects).
Basic things like weather, water behavior, rock mechanics and fire to
name but a small few.
The behavior of gas follows another simple rule using the
atomic model when kept in a constant volume chamber. The gas pressure is approximately equal to
the number of gas molecules multiplied by the temperature (although this
requires the correct units and proportionality constants to be used). This means that if you double the temperature
of the gas, you double the pressure in the vessel. Similarly, if you double the number of gas
molecules present, you again double the pressure in the vessel.
When burning gasoline in your car or burning other fossil
fuels at the local power plant, the reaction chamber design has to incorporate
the changes in pressure and temperature associated with the combustion
reaction. At the moment of combustion,
if the reaction chamber is a fixed volume, the pressure is directly
proportional to both the number of gas molecules and the temperature in the
chamber.
The temperature of the chamber is largely driven by the
energy released in the combustion reaction.
When the organic molecules being burned are mixed with sufficient oxygen
and heat, they combine into soot, smoke, water (H2O) and carbon dioxide (CO2). A high temperature burn with optimal oxygen
will almost completely oxidize the fuel converting it more fully over to pure H2O
and CO2. The chemical energy released in
the burning fuel in this oxidation reaction is what we typically refer to as
fire.
The chemical potential energy in the fuel is actually in the
form of electrons being further away on average from the nucleus of their atoms
than is available in CO2 and H2O. By
converting the fuel completely over to CO2 and H2O, all the electrons on
average are closer to the nucleus of the atoms to which they were bound than
they were when they were part of the fuel.
Like dropping a weight from an altitude can turn a wheel and do work,
allowing the negatively charged electrons to get closer to the positively
charge protons in the nucleus can do work by releasing heat. That heat is what causes energy transfer to
the molecules sufficient to raise their orbital electrons to higher energy
levels such that when they spontaneously drop to lower energy levels, they give
off the light we associate with fire.
The number of gas molecules is also proportional to the
pressure. The pressure is an average of
how many gas molecules are bouncing off the walls of the vessel in any given
time interval. The gas molecules are
like little bullets bouncing off the wall continually resulting in an average
force per area we call pressure. If you
double the number of molecules in a fixed volume, you basically double the
number of these atomic bullets being fired at the walls and so you double the
pressure.
This is the case for a typical combustion reaction where the
fuel is initially liquid or solid and the combustion products being mostly CO2
and heated H2O are gaseous. The initial
volume taken up by the fuel is largely negligible because the phase change from
a liquid or a solid over to a gas generally creates gas taking up around a
thousand times more volume than the initial volume of the fuel. In other words, a cubic inch of fuel will
result in something in the range of 1000 cubic inches of gas. It is this large expansion of volume that
causes the pressure increase sufficient to move a piston or turn a turbine
although we will have to wait until next week to consider changing volumes in
the gaseous equations of state.
