7/24/2019 2. Diffusion of Gases - Ammonia and Hydrogen Chloride

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Diffusion of gases ammonia and hydrogen chloride

Demonstration

Concentrated ammoniasolution is placed on a pad in one end of a tube and concentrated hydrochloric acidon a pad at theother. After about a minute the gases diffusefar enough to meet and a ring of solid ammonium chlorideis formed.

Lesson organisation

This demonstration is best performed in a fume cupboard. A black background, such as a sheet of black sugar paper, behind thedemonstration helps the white ring to be seen more clearly. Actually performing the demonstration takes only a few minutes.

Apparatus Chemicals

For one demonstration:

Eye protection (goggles)Access to a fume cupboardProtective gloves, preferablynitrile

A length of glass tube abouthalf a metre long with an insidediameter of about 2 cm (Note 1)Retort stands with bosses andclamps, 2Small wads of cotton wool, 2Bungs, to fit into the ends of

the glass tube, 2Strip of universal indicatorpaper (optional)

Concentrated hydrochloric acid(CORROSIVE), a few cm3(Note2)880 ammonia solution(CORROSIVE, DANGEROUS

FOR THE ENVIRONMENT), a fewcm3 (Note 2)Acetone (FLAMMABLE), a fewcm3(optional)(Note 1)Refer to Health & Safety andTechnical notes section belowfor additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

The demonstrator should wear goggles and protective gloves.

Concentrated hydrochloric acid, HCl(aq), (CORROSIVE) - see CLEAPSS Hazcard. Produces hydrogen chloride gas, HCl(g),(TOXIC, CORROSIVE) - see CLEAPSS Hazcard.

880 ammonia solution, NH3(aq), (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard. Producesammonia gas, NH

3(g), (TOXIC) - see CLEAPSS Hazcard. Care should be taken when opening the bottle of ammonia solution,

particularly on hot days when pressure can build up in the bottle. If the bottle of ammonia is kept for a long time, its concentrationmay decrease which will lessen the effectiveness of the demonstration.

1It is very important that the tube is clean and completely dry for this experiment. If necessary, the tube can be dried by pushinga cotton wool pad soaked in acteonethrough the tube and leaving it for a few minutes.

2The concentrated hydrochloric acidand the 880 ammonia solution are easier to handle in small bottles than in Winchesters(large bottles) for this demonstration.

Unit 2. Particles and Atomic Structure

Syllabus Ref. 1 (S)

http://www.rsc.org/learn-chemistry/wiki/Substance:Hydrochloric_acidhttp://www.rsc.org/learn-chemistry/wiki/Substance:Ammonium_chloridehttp://www.rsc.org/learn-chemistry/wiki/Substance:Acetonehttp://www.rsc.org/learn-chemistry/wiki/Substance:Hydrochloric_acidhttp://www.rsc.org/learn-chemistry/wiki/Substance:Hydrochloric_acidhttp://www.rsc.org/learn-chemistry/wiki/Substance:Acetonehttp://www.rsc.org/learn-chemistry/wiki/Substance:Ammonium_chloridehttp://www.rsc.org/learn-chemistry/wiki/Substance:Hydrochloric_acid

7/24/2019 2. Diffusion of Gases - Ammonia and Hydrogen Chloride

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Procedure

aWorking in the fume cupboard, clamp the glass tube at either end, ensuring that it is horizontal.

bOpen the bottle of ammonia solution cautiously, pointing the bottle away from both you and the audience. Open the bottle ofhydrochloric acid and hold the stopper near the mouth of the ammonia bottle. Note the white clouds of ammonium chloride thatform.

cPut one of the cotton wool wads in the mouth of the ammonia bottle and carefully invert it to soak one side of it. Push thesoaked end into one end of the glass tube. Replace the lid on the bottle.

dRepeat this procedure quickly with a second wad of cotton wool and hydrochloric acid. Put the cotton wool wad into the otherend of the glass tube.

ePutting bungs into the ends of the glass tube will reduce the quantity of the gases which escape and therefore the smell. Onceassembled, the tube can be removed from the fume cupboard.

fWatch the tube and observe a ring of white powder forming near the middle of the tube. This is ammonium chloride.

Teaching notes

The reaction which is taking place is:

ammonia+ hydrogen chloride ammonium chloride

NH3(g) + HCl (g) NH4Cl (s)

The exact time taken for the ring to form will depend on the dimensions of the tube, the amount of the solutions which are put onthe cotton wool wads and the temperature of the room.

The ring usually forms nearer to the hydrochloric acid end of the tube because hydrogen chloride diffuses more slowly thanammonia. This is because hydrogen chloride has almost twice the molecular weight of ammonia, and the rate of diffusion isinversely proportional to the square root of the molecular mass of the gas.

It is worth noting that the rate of diffusion is not the same as the speed at which the gas molecules travel (which is hundreds ofmeters per second). The gas molecules follow a zig-zag path through the tube as they collide with the air molecules in the tube.

The purpose of the glass tube is to eliminate air currents and to see if the gas molecules will move on their own.

Health & Safety checked, September 2014

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015

http://www.rsc.org/learn-chemistry/wiki/Substance:Ammoniahttp://www.rsc.org/learn-chemistry/wiki/Substance:Hydrochloric_acidhttp://www.rsc.org/learn-chemistry/wiki/Substance:Ammonium_chloridehttp://www.rsc.org/learn-chemistry/wiki/Substance:Ammonium_chloridehttp://www.rsc.org/learn-chemistry/wiki/Substance:Hydrochloric_acidhttp://www.rsc.org/learn-chemistry/wiki/Substance:Ammonia