G420: Graham's Law Of Diffusion – NH3 And HCl Diffusion

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G420: Graham’s Law of Diffusion – NH3 and HCl DiffusionG420: Graham’s Law of Diffusion – NH3 and HCl Diffusion

Introduction

A cotton swab is dipped into concentrated hydrochloric acid (producing hydrogen chloride gas) while a second on is dipped into concentrated aqueous ammonia (producing ammonia gas).  Both cotton swabs are simultaneously inserted into opposite ends of a long glass Graham’s Law apparatus and placed on an overhead projector.  A white ring of solid ammonium chloride forms where the two gases meet inside the tube.

NH3 + HCl →  NH4Cl

To Conduct Demonstration

 If necessary, protect non-glass surfaces of an overhead projector with clear plastic.

1. Dip one Q-tip into concentrated hydrochloric acid (HCl) and a second cotton swab into concentrated aqueous ammonia (NH3).
2. Simultaneously insert both cotton swabs into the ends of the glass apparatus securing the rubber septa.
3. Place the glass apparatus on the overhead projector.  Wait until a while ring of solid ammonium chloride forms where the hydrogen chloride and ammonia gases meet.  The ammonia gas, having a lower molecular weight than the hydrogen chloride, will diffuse faster and travel a greater length of the tube.  Consequently, the white ring of ammonium chloride will form much closer to hydrochloric acid end of the tube.

Reaction Time: 10 min

Safety 

Wear safety goggles and latex gloves to protect the eyes and hands from the strong acid and base used in this demonstration.  Keep the containers of concentrated hydrochloric acid and aqueous ammonia covered when not dipping the cotton.  Work in a well ventilated area and avoid breathing the concentrated vapors of either reagent. 

Notes

1.             This demo is more easily seen on an overhead projector than on a document camera with a dark background because of glare which is created by the camera on the dark background.
2.       Do not store concentrated acid and base in the box with the apparatus and the swabs.  The chemicals interact coating everything in the box with a 

References

L.R. Summerlin and J.L. Ealy Jr., Chemical Demonstrations: A Sourcebook for Teachers, 1985.

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