RE: nitrous in your car?

["Lewis, Gary M" <Gary.Lewis@West.Boeing.com>]

I laugh because most folks don't seem to know that ole' Bob Myers is I
believe Robert Myers, PhD, Chemistry.  I affectionately refer to him as the
"Benevolent God of Chemistry."

As soon as he opens his mouth (or hits send) on a chemistry related subject,
I quickly close mine.  Talk about wealth of knowledge...

Thank God...
Gary


> Caution y'all - pedant alert!  Delete now unless you want to become
> terminally bored.  :-)
> 
> At 04:44 AM 09/30/1999 -0500, you wrote:
> >>Once all the liquid has been evaporated then the PV=nRT equation
> applies.
> >
> >
> >I agree with everything you wrote except the above statement.  How can
> the
> >Ideal gas law NOT apply?  Regardless of the state or amount of nitrous in
> >the tank, if you increase the temperature of the tank (without releasing
> >gas) the pressure is going to increase.  
> 
> True - the pressure will increase.  This pressure increase is due,
> however,
> primarily to the increasing equilibrium vapor pressure of the liquid N2O
> rather than some application of the ideal gas law (which is only a first
> approximation at best).  Remember, we are dealing with a change of state
> not just a gas in a close system.  This makes the system far more complex
> than just a single phase gaseous system and the ideal gas law as typically
> encountered in a freshman chemistry class is not capable of handling the
> situation.  What happens when a sample of gas/liquid at equilibrium is
> compressed (i.e, the volume available is reduced) at constant temperature?
> Some more gas liquifies.  The pressure stays essentially constant and does
> not increase as the ideal gas law would predict.  Similarly, what happens
> when the volume of the container is increased?  More liquid vaporizes and
> the pressure remains essentially constant.
> 
> One could argue that the "n" of the equation (PV = nRT) is changing since
> the number of moles of gas is changing as liquid is either condensed or
> evaporated but this is not the way the ideal gas law is commonly used.  In
> this case then N(total) = n(liquid) + n(gas).  The n(gas) is the "n" you
> would need to use in the ideal gas equation if you were trying to use it
> in
> this situation.  It is usually regarded as a constant depending upon which
> particular sample of a gas you were describing.  It will not be constant
> in
> this case.
> 
> Remember the definition of an ideal gas?  Among other small details, the
> individual molecules possess zero volume and they do not interact with
> each
> other except by colliding with each other in perfectly elastic collisions.
> There are no intermolecular attractive forces (because of this the
> nonexistent "ideal gas" cannot be liquefied).  This situation is only a
> decent approximation at very low pressures (where the molecules are very
> far apart and their individual volumes are insignificant in comparison to
> the total volume of the system and interactive forces, which decrease
> markedly with distance, are insignificant) and/or very high temperatures
> (where attractive forces are very small in comparison to the average
> kinetic energies of the molecules).  We are dealing with neither of these
> conditions inside this tank of N2O.  Here the temperature is well below
> the
> critical temperature and the pressure is well above the critical pressure.
> 
> (Definition {just in case you might need it}: critical temperature - the
> temperature above which a gas cannot be liquefied by pressure alone.
> Critical pressure - the minimum pressure required to liquify a gas which
> is
> at its critical temperature.)
> 
> >Remember, the tank exploded while
> >the car was sitting in the garage...the nitrous system was not actively
> >being used (other than the bottle heater, apparently).  Admittedly, there
> is
> >only a small volume of gas in a full tank of nitrous oxide (mostly
> liquid),
> >but even that small volume is subject to the Ideal gas law.
> 
> There must be some other explanation for the explosion.  A malfunctioning
> heater which caused quite high temperature?  A defective or damaged tank?
> Defective or damaged plumbing, etc?  A combination of more than one of
> these things?  Something else?
> 
> >Bob W.
> >
> >
> >
> ___
>    Bob
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