Q: How do you calculate the coefficient of ammonia diffusion in air?

A: Ammonia Diffusion

This Investigation should follow a demonstration of the diffusion of ammonia and hydrogen chloride along a tube.  The position and shape of the “smoke ring” so produced indicates…

·         Different gases diffuse at different speeds.  This links in to the rate of diffusion being related to molar mass.

·         The shape of the ring shows ammonia to be less dense than hydrogen chloride, since it slants in such a way as to indicate that ammonia floats over the top of the hydrogen chloride.  Leaving the ring to develop illustrates this clearly.

·         Also, observing the ring closely shows small convection currents which can be stimulated by holding two fingers below the ring and waiting for heat to diffuse through the glass.

This gives pupils imagination to tackle the investigation…

“Does Ammonia gas diffuse equally fast in all directions?”

·         The question prompts a discussion on the density of gases – brighter pupils can calculate that ammonia is less dense than air (approximates to nitrogen or oxygen/nitrogen mix?)  Others can look up data in a book, or simply be told.

·         Diffusion works well in boiling tubes, typically taking 5 to 15 minutes to cover the distance from the neck to base, depending on conditions.

·         Progress of the gas is measured by using thin, pink litmus sensors.  (Long litmus strips give a diffuse boundary which makes taking measurements harder.)

·         Diffusion (boiling) tubes can be set up along Cartesian axes, and brighter pupils may want to further divide the angles.  This should not be encouraged too much, especially initially.

·         There is scope for a reasonable, simple theory and prediction.  Complicated or simple plans to suit pupils’ abilities are soon ready allowing lots of results and graphs to be produced.  As differences are random and not directional, there is plenty of scope for evaluations, explanations and suggested improvements.

·         Some considerations for key factors include: amounts of ammonia solution used, water inside the tube reabsorbing ammonia gas, holding the tubes and making one warmer than another,  ammonia already in the air…

In spite of the difficulties, this investigation provides a short, simple assessment that even the most challenged can attempt and the brightest can score maximum marks in 2 – 3 weeks.  It also teaches that experiments do not always work the way you expect, but for GCSE that does not matter providing it is realized that the experiment has not worked as expected – ideally with an explanation.

Pupils will assume the ammonia advances down the tube with a flat diffusion front perpendicular to the direction of diffusion.  This is clearly not so.  When difficulties arise, a demo of dropping a few cm3 of milk into a 250 cm3 beaker of still water illustrates the misconception and answers some of the more confusing anomalous results.