Date: Mon, 9 Apr 2001 14:09:02 -0700
Reply-To: Matthew Pollard <poll7356@UIDAHO.EDU>
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From: Matthew Pollard <poll7356@UIDAHO.EDU>
Subject: Re: Coolant Temps
In-Reply-To: <5.0.2.1.2.20010409151112.03e77008@#pop1.attglobal.net@192.168.0.1>
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Hold a second here. How about some physical chemistry background first...
test on friday.
The amount that the freezing point (or boiling point
too) changes from that of a pure substance (say pure water) to that of a
mixture (say water w/ table salt in it) has to do with how well the two
(or more) components of the mixture react with each other. ie: water
freezes at 0C and if you add some salt (NaCl) the freezing point goes
down. This is called positive deviation from Raoult's Law by the way...
This works for two liquid solutions mixed together. If you mix water and
ethylene glycol the freezing point of the mixture may not be
somewhere between their individual freezing points but could be higher or
lower than the individual components. So, looking at the the freezing
point of the individual components is not relevent, but must instead look
what's in our bus. In my bus i have a 1:1 volume:volume mixture of water
and coolant. To read the freezing point depression chart you need a mole
fraction (X1/X2) which is like a volume ratio but in molecule ratios. See
below.
I think what really matters here is the heat capicity because according to
DOW they both have roughly the same freezing point for a mixture. Water has
a huge heat capiciaty- (heat cap. is how easy it is to heat something-
water has a large # so it is hard to heat- versus alumnium which is easy
to heat.) and ethylene glycol has a heat capiciaty that is roughly half
of that of water and propylene glycol is even smaller.
> Per info on the site, thermal capacity is similar to 50/50 eg mix (caveat
> -- numbers given in xx/mole -- I'm assuming that a mole of mix is similar
> amount to a mole of pg).
>
xx/mole is a mole fration. So a 1:1 per/vol mix of h2o/ethylene glycol you
would need the density to convert from vol to mass and then molecular
weight to get the # of moles.... here: a 1:1 volume ratio of H2O: Ethylene
Glycol is the same as a 4:1 mole ratio of water to ethylene glycol (check
it: density of water=1g/ml. density of eg=0.8902. Formula weight of H2O
is 18.02 and eg=64.515)
Ok, now back to MY chemistry! Test on friday.
-Matthew
Matthew Pollard http://www.uidaho.edu/~poll7356
Dept. of Chemistry http://www.chem.uidaho.edu
University of Idaho http://www.uidaho.edu
On Mon, 9 Apr 2001, David Beierl wrote:
>
> Ethylene glycol freezing point -12.7C -- Propylene glycol f.p. -60C -- you
> have to use more pg than eg in an aqueous mix because it doesn't depress
> the freezing point as much; but eg reaches a minimum and starts rising
> again while pg either doesn't rise at all or else not nearly as much.
>
