Technicians as demonstrators
Demonstration: Endothermic reaction with barium hydroxide
and ammonium chloride
Summary
This is an example of an endothermic reaction. It will cool down sufficiently to
freeze a small amount of water, which is between the beaker that chemicals are
reacting in and a small wooden block.
Requirements
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32g Barium Hydroxide (harmful)
11g Ammonium Chloride (harmful)
100cm3 beaker
block of wood 20x20x5cm or similar
wash bottle and stirring rod
You can use a datalogger with a temperature probe (which can be used to stir the
chemicals). Projected on a screen the temperature change is quite spectacular.
You need a temperature probe that goes down to at least -40c
This is a TEACHER-ONLY demonstration
Procedure
1. Wear eye protection. The room should be well ventilated, or the reaction
could take place in a fume cupboard, due to the ammonia gas produced.
2. Weigh the two chemicals and place them in the 100cm3 beaker.
3. Make a small pool of water on the surface of the wooden block and place
beaker on top of the pool.
4. Start to vigorously stir the chemicals together. Within 30 seconds they
should have reacted turning into slush and giving off ammonia (Toxic).
5. The mixture should now be getting cold and will freeze the water, sticking
the beaker to the block.
6. After the reaction has taken place the beaker can be lifted up and the block
should come with it.
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7. Place the solution in a bucket of water and pour the liquid down a foul-water
drain.
Don’t use very heavy wood, as the frozen water might not be sufficient to support
it!
Scientific Background
2NH4Cl(s) + Ba(OH)2(s)  2NH3(g) + Ba2+(aq) + 2NH4+(aq) + 2Cl-(aq) + 2H2O(l)
The endothermic reaction is driven forward by the large positive entropy change. It
has one of the largest positive entropy changes of any common reaction.
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References
Hazcards: 5: Ammonia; 9: ammonium chloride; 10: barium hydroxide
This practical guidance does not constitute a formal risk assessment, but
exist as guidance only. You should always complete your own Risk
Assessment, for your own context, if you carry out this activity (or one
developed from it) in your school or elsewhere.
For further details, contact CLEAPSS or SSERC as appropriate.
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