specific heat of dry ice?

[phleg...@yahoo.com]

various Net searches produce no result for the specific heat of dry
ice. Even the 2 dedicated dry-ice sites I found, don't include this
number. I am physically away from brick-mortar libraries.

is the specific heat of dry-ice less than that of water-ice?

Ie, does a kg of water-ice-chilled-to-dry-ice-temp, have more cooling
power than a kg of dry ice?

The common apparatus of using a tank of liquid CO@ to produce dry-ice
on-site, the resulting product will appear at exactly the freezing
temperature of CO2? Or super-chilled dry-ice can be obtained?

[Martin Hogbin]

<phleg...@yahoo.com> wrote in message news:1123051109....@o13g2000cwo.googlegroups.com...

It is the latent heat of sublimation that you want, try this link:

http://www.answers.com/topic/latent-heat

or search using the above terms.

Martin Hogbin

[rek...@gmail.com]

The specific heat is the heat capacity per unit weight (instead of
molar). If you can find heat capacity in J/mol.K, it can be converted
to specific heat by dividing by the molecular weight of CO2. It does
give you information about the amount of heat something holds, but note
that it is a function of temperature. So if you find the specific heat
capacities of water and CO2 at the same temperature, it will tell you
relative heat-holding capability per mass, as the name 'heat capacity'
suggests.

Thermodynamic data, particularly of non-standard states of substances
(CO2's standard state is a gas), can be difficult to find from just a
web search. The web does NOT have all answers in life, contrary to
popular belief. A good deal of data that chemical engineers and
chemists need doesn't exist, it needs to be measured (this won't be the
case for dry ice). I searched the CRC and it doesn't appear to be there
for dry ice. Go to a university library and look at chemical data
reference books, eg ASHRAE handbook might be a good start.

When you make dry ice from a tank of CO2, there is no particular reason
for it to form at the freezing point. Heat is removed from the
emerging gas by a throttling process (Joule-Thompson effect). The
final temperature of the dry ice (initially) will depend on the
pressure in the tank, as throttling involves rapid decompression at
constant enthalpy. However unless you have very good insulation, the
dry ice should rather quickly equilibrate to the freezing point, and
you know the surface of the block is at the freezing point when you see
sublimation of vapors.

[Bennett Price]

About 0.7 see
http://www.engineeringtoolbox.com/carbon-dioxide-24_974.html

Kelvin / Celsius conversion at, e.g.,
http://www.sengpielaudio.com/calculator-fahrenheit.htm
175k=-98C

[Uncle Al]

phleg...@yahoo.com wrote:
>
> various Net searches produce no result for the specific heat of dry
> ice. Even the 2 dedicated dry-ice sites I found, don't include this
> number. I am physically away from brick-mortar libraries.
>
> is the specific heat of dry-ice less than that of water-ice?

Per mass, near their respective phase transition temps, obviously
yes. Can you find the specific heat of water ice at -78 C?

> Ie, does a kg of water-ice-chilled-to-dry-ice-temp, have more cooling
> power than a kg of dry ice?

You are confusing specific heat with latent heat of transition. The
latent heat of sublimation of dry ice is substantially larger than the
latent heat of fusion of water - and it happens at a much lower
temperature.

<http://www.continentalcarbonic.com/dryice/specs.php>
<http://www.linde-gas.com/International/Web/LG/COM/likelgcom30.nsf/DocByAlias/nav_dryice_characteristics>

> The common apparatus of using a tank of liquid CO@ to produce dry-ice
> on-site, the resulting product will appear at exactly the freezing
> temperature of CO2? Or super-chilled dry-ice can be obtained?

Google
"carbon dioxide" "phase diagram" 11,700 hits

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf

[Repeating Rifle]

On 8/2/05 11:38 PM, in article
1123051109....@o13g2000cwo.googlegroups.com,
"phleg...@yahoo.com" <phleg...@yahoo.com> wrote:

On theoretical grounds, the specific heat of dry ice will be smaller than
that of water. If quantum effects are ignored, Each particle stores the same
thermal energy. With CO2 and H2O, each molecule two substances would store
equal additional amounts of thermal energy for every degree of added
temperature. Even the stored vibrational energies would be the same. Because
CO2 has a MW=44 and H2O has a MW=18, CO2 would have less than 1/2 of the
specific heat of H2O.

My guess is that quantum effects are smaller for CO2. A measure of that is
the Debye temperature. But you should look things up for yourself.

When it comes to latent heat of vaporization, again I would expect CO2 to
have a lower latent heat. Roughly, at the transition from a solid to a
liquid, the disorder for the two substances will be the same. Thus the
entropy changes will be the same. To get the same change of entropy,
Delta S = Delta H/T, the latent heat will be proportional to the melting
temperature (per mol). Thus, you have MW and melting temperature working
against you when using CO2.

If someone has the initiative to follow this through, I would like to use
those results to see how well the facts fit this loose model I describe.

Bill

[ienc.s...@gmail.com]

Consider this a basic answer, but the thermal transfer of cooling into the ice H2O is going to be thermodynamically efficient in both cooling and in heating. As such H2O chilled by dry ice with give back the same cooling energy as was delivered. The entropy of H2O from solid to liquid can be ignored for the moment. This is due to the practicality in assuming that the application is in keeping water based items frozen.

As such, I offer that the escaping gas vapors from CO2 sublimation will cause dry ice to be less thermally effective. Therefore supercooled ice, as suggested will seemingly provide more cooling from the point of equalization in the transfer of energy