MadSci Network: Chemistry

Re: Why do temperatures of gases remain constant for a while when condensing?

Date: Tue Feb 8 21:47:51 2000
Posted By: Mike Conrad, Post-doc/Fellow, Microbiology, UNC
Area of science: Chemistry
ID: 948805067.Ch

Interesting question.  When a gas is cooling, why does the temperature 
stop falling during condensation?

To start with, it must be realized that heat is molecular motion.  This is a 
rather subtle concept, and previously heat was thought to be some kind of 
"caloric" fluid substance.  It wasn't until 1798, that Benjamin Thompson, 
(American born, with British sympathies and living in Bavaria), was watching 
a cannon being bored and he reasoned that the heat produced was "excited and 
communicated" by motion.  

Temperature is literally a measure of the speed of molecular motion. The 
higher the temperature the faster the molecules are traveling.  And as 
something cools, the molecules will be traveling slower and slower.  Thus, 
as that gas you mentioned cools, the molecules will be traveling slower and 
slower.  Incidently, at room temperature, the average air molecule is 
traveling about a thousand miles an hour!  

Molecules have attractive forces between them resulting from charge, dipole 
effects, van der Waals forces, etc.  As the temperature drops, the molecules 
will travel slower and slower until the attractive force gets the upper 
hand.  This would be at 100 C for steam.  At this point, a very drastic 
thing happens.  There is a "change of state" and the steam begins to 
condense into water.  As the steam, at 100 C, condenses into water, still 
at 100 C, the bonds that hold liquid water together start forming.  When 
these bonds form, heat is released.  This heat must must be removed for the 
water to form.  Therefore, heat must continue to be extracted until all the 
steam has condensed into water.  Only then does the water begin to cool 
below 100 C.  Now of course, I am speaking of an ideal system with instant 
mixing of the heat, so the whole system remains at 100 C.  In the real 
world, different parts of a vessel can be different temperatures, but the 
principles are still valid.

It turns out that you must remove 2300 joules worth of heat to turn one gram 
of steam (at 100 C) into one gram of water (also at 100 C).  Conversely, 
you must add back the 2300 joules if you want to turn that gram of water 
back into steam.  These 2300 joules are called the heat of evaporation.  
Water forms rather strong bonds between itself, thus it has a high heat of 
evaporation and a rather high boiling point.  Nitrogen has much weaker 
intermolecular attractions and has a heat of evaporation of 200 joules per 
gram.  And although nitrogen is a heavier molecule than water, nitrogen 
boils at -196 C.

Similarly, as heat continues to be removed from the liquid, it will cool 
until its attractive forces are strong enough to hold it together as a 
solid.  In other words, it will freeze.  Water freezes at 0 C.  And the 
liquid water will stay at 0 C until it is completely frozen.  One must 
remove 330 joules per gram to convert liquid water to ice.  Only after that, 
will the temperature fall below 0 C.  This means that the temperature of 
any ice/water system at equilibrium, whether there's just a little ice or 
it's almost completely ice, will be 0 C. 

Any good physics or general chemistry text should have a discussion about 
changes of state and about the heats of evaporation and melting.       		
	Mike Conrad.

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