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 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. 1 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 2+ + - 2NH4Cl(s) + Ba(OH)2(s) 2NH3(g) + Ba (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. 2 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. 3 .