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TeacherExpt:Endothermic solid-solid reactions

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This procedure is designed as a demonstration to be performed by a teacher or other qualified person.

Solid hydrated barium hydroxide is mixed with solid ammonium chloride in a beaker. An endothermic reaction takes place to produce a liquid, with the evolution of ammonia. The temperature drops dramatically to about -20 °C.

Contents

Lesson organisation

Although the experiment can be safely carried out as a class experiment (with GCSE or A-level candidates in mind), it lasts only about 5 minutes and is probably not worth the extra time spent by students setting up and clearing away. Therefore it is recommended as being more suitable as a teacher demonstration. Students could be allowed to feel the outside of the very cold container.

Apparatus and chemicals

  • Eye protection: goggles
  • Fume cupboard (optional)
  • One demonstration will require:
  • Beaker (100 cm3)
  • Watch-glass
  • Thermometer, reading to -30 °C (see note 1)
  • Top-pan balance

Technical notes

Barium hydroxide-8-water (Corrosive) Refer to CLEAPSS® Hazcard 10B

Ammonium chloride (Harmful) Refer to CLEAPSS® Hazcard 9A

Concentrated hydrochloric acid (Corrosive) Refer to CLEAPSS® Hazcard 47A

  1. Consider using a thermocouple-type of thermometer which can be connected to a large display or computer monitor.
  2. Small stock bottle, to provide fumes for ammonia test.

Procedure

Health & Safety

Wear eye protection.

Procedure

Before the demonstration

Weigh out separately the barium hydroxide and the ammonium chloride. Avoid lumps as far as possible.

The demonstration

Work in a fume cupboard unless the room is well ventilated.

A. Stand the beaker on a watch-glass containing a few drops of water, so that the base of the beaker is touching the water.

B. Note the room temperature.

C. Mix the two solids in the beaker and stir with the thermometer. The mixture becomes slushy as a liquid is formed, together with a white suspension.

D. The presence of ammonia can be detected by smell, and confirmed by blowing fumes from the hydrochloric acid bottle across the beaker’s mouth and by using moist indicator paper.

E. Observe the drop in temperature, which is confirmed by the fact that the beaker freezes to the watch-glass.


Teaching notes

It helps to use a large thermometer display. The cold beaker can be passed around the class once the evolution of ammonia has stopped.

It is not possible to determine easily the exact barium compound or compounds produced in this reaction but the equation is usually represented as: Ba(OH)2.8H2O(s) + 2NH4Cl(s) -> 2NH3(g) + 10H2O(l) + BaCl2(s) or Ba(OH)2.8H2O(s) + 2NH4Cl(s) -> 2NH3(g) + 8H2O(l) + BaCl2.2H2O(s)

A-level students could be asked to calculate the value of the enthalpy and entropy changes for the reaction, using standard enthalpy changes of formation and standard entropy values obtained from a data book or from the table below.

chart from teaching notes from endothermic solid-solid reactions

An enthalpy change of +164 kJ mol–1 is obtained if the product is assumed to be BaCl2(s), and +135 kJ mol–1 if it is assumed to be BaCl2.2H2O(s). Students should be able to predict qualitatively that the entropy change for the system has a positive value because a gas and a liquid are formed from two solids. From the values above they could also be asked to calculate the actual entropy change for the system and the surroundings, and hence ΔS total or ΔG for the reaction and confirm that the process is spontaneous. A value of ΔS system of +591 J mol–1 K–1 is obtained if the product is assumed to be BaCl2(s) and +530 J mol–1 K–1 if it is assumed to be BaCl2.2H2O(s).

Reference

This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/endothermic-solid-solid-reactions,277,EX.html

Useful resource

This weblink shows a related endothermic reaction involving barium hydroxide: http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/ENDO2/PAGE1.HTM

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