Molecular Speed Demo (Racing Molecules)
Graham’s law is demonstrated using a drop of
hydrochloric acid and a drop of ammonia.
Materials
Where to Find
Dropper bottle of concentrated HCl
Dropper bottle of concentrated NH3
Pre-labeled petri dish with lid
2 latex gloves
Safety glasses
Fume hood
Fume hood
Drawer N3
Left of the sink
Drawer S4
Set Up
Bring out demonstration; check to make sure there is a working projector in
the classroom.
Safety
Be careful with the concentrated acid and base. Do not allow them to come
in contact with each other or with skin or eyes. Keep the cap on both bottles
to minimize fumes.
Disposal
Rinse the petri dish off in the sink. Make sure that both bottles are capped,
and place back in hood.
Professor/Lecturer
Chemistry
The molecular weight of HCl is 36.46 g/mol, while that of NH3 is 17.031
g/mol. Graham’s law states that the rate of effusion of a gas is inversely
proportional to the square root of the mass of its particles:
Thus, since HCl is about twice as massive as NH3, it will diffuse about √2
times slower than NH3.
Presentation
Place the petri dish on the overhead and turn it on. Place a drop of HCl and
Ammonia on top of the marks on the petri dish (if there are no marks, put
them equidistant from the line down the center). Fan the area above the sheet
to disperse the hot air to minimize convection. Then cover the petri dish with
the lid, and wait. A cloud will form when the HCl gas comes in contact with
the NH3. Since HCl diffuses slower than NH3, the cloud will form closer to
the HCl droplet.
Search Items
Graham’s law
Kinetic Molecular Theory
Acid/Base Reaction
Suitable for Recitation
Yes
Reference
http://en.wikipedia.org/wiki/Graham%27s_law
PS100 demo booklet