Endothermic solid–solid reactions

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 may not be 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.

Equipment

Apparatus

• Eye protection (goggles)
• Beaker, 100 cm³
• Watch glass
• Thermometer, reading to –30°C (note 3)
• Top-pan balance
• Fume cupboard (optional)

Chemicals

• Barium hydroxide-8-water (CORROSIVE), 32 g 
• Ammonium chloride (HARMFUL), 10 g
• Concentrated hydrochloric acid (CORROSIVE)
• Universal indicator (or litmus) paper, 1 strip

Health, safety and technical notes

• Read our standard health and safety guidance
• Wear eye protection (goggles) throughout.
• Work in a fume cupboard unless the room is well ventilated.
• Consider using a thermocouple type thermometer which can be connected to a large display or computer monitor.
• Barium hydroxide-8-water, Ba(OH)₂.8H₂O(s), (CORROSIVE) – see CLEAPSS Hazcard HC010B. Avoid lumps as far as possible when weighing out the required amount.
• Ammonium chloride, NH₄Cl(s), (HARMFUL) – see CLEAPSS Hazcard HC009a. Avoid lumps as far as possible when weighing out the required amount.
• Concentrated hydrochloric acid, HCl(aq), (CORROSIVE) – see CLEAPSS Hazcard HC047a. Use a small stock bottle to provide fumes for the ammonia test.

Procedure

1. Stand the beaker on a watch glass containing a few drops of water, so that the base of the beaker is touching the water.
2. Note the room temperature.
3. 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.
4. 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.
5. 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.

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 ∆S_total or ∆G for the reaction and confirm that the process is spontaneous. A value of ∆S_system of +591 J mol^–1 K^–1is 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).