I agree with Phil, with one possible exception... it depends what you are
looking at.
The rate of change of the solid will be constant only if the surface area
of the solid is a constant. An example of this would be a right circular
cylinder that is only exposed on one end, ie a glass full of a solid block
of dry ice. The area where the solid can sublime will remain fixed as the
material disappears.
If the solid is in a cube, the surface rate of sublimation will be the
same, ie per area of solid, BUT as the cube gets smaller, the apperant rate
of sublimation will appear to drop off.
For a cube, the surface area of the solid = f(mass), but rate of change
that the surface is a function of the mass of the solid remaining.
= f(mass) * d(surface area)/d(mass)
Grams of solid sublimed per second = rate constant /area * surface area of
the solid = rate constant /area * f(mass) * d(surface area)/d(mass)
Now a simple first order kinetics equation would be = k*conc; ie the rate
the solid is consumed is a function of the concentration (amount) of solid
present.
This cube calculation is going to make me do integration, and I don't do
integration without a charge number... But since the rate of change of the
surface area should change with the cube root of mass, I am going to guess
the order is something like 1.333, ie rate = k * concentration ^(1.333)
Other assumptions, heat flow into the material is a constant per surface
area, ie the flow of energy per area of solid is fixed. (BTU/sq.in.)
Material transport is diffusion, ie the air is still.
Last and largest assumption: I am remembering most of this correctly.
Oh and lastly, if the cube is sitting on a surface (table top), the exposed
surface area for sublimation on the cube is (Length * Length * 5), not
(Length * length * 6)
George Franck
Quality Assurance
703 295 2635
(Univ of Ill, BS ChemEng - 1978)
"Crepeau, Phil"
<phil.crepeau@NG To: [log in to unmask]
C.COM> cc:
Sent by: TechNet Subject: Re: [TN] Need a chemist or chemical engineer to answer
<[log in to unmask]
>
12/17/2003 02:52
PM
Please respond
to "TechNet
E-Mail Forum.";
Please respond
to phil.crepeau
hi,
i don't see this as a chemical reaction. i see it as a physical change in
state. assuming temperature and pressure remain constant, the rate of
change of solid into gas would remain constant and thus a '0' order
operation.
phil
-----Original Message-----
From: Bev Christian [mailto:[log in to unmask]]
Sent: Wednesday, December 17, 2003 10:57 AM
To: [log in to unmask]
Subject: [TN] Need a chemist or chemical engineer to answer
Technetters,
It has been almost 30 years since I took kinetics. I have dug out my old
text books, but they only help so much. I'm a preparative inorganic chemist
by training. My question is the following: If a single compound sublimes,
without decomposition, would you expect the kinetics to be zero or first
order? Thanks.
Bev Christian
Research in Motion
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
for additional information, or contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
for additional information, or contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------
|
