Design a pocket handwarmer

A perfect experiment for winter term.

This experiment should take one to two hours.

Equipment

Materials per group

• Sodium ethanoate trihydrate (Low hazard), 25 g 
• Deionised water and samples of supersaturated sodium ethanoate solution prepared in advance

Materials for class demonstration

• Sodium ethanoate trihydrate, 250 g 
• Deionised water

Equipment per group

• Boiling water bath or beaker of hot water on a hot plate
• Boiling tubes with stoppers
• Wash bottles
• Heavy-duty plastic bags
• Small size miscellaneous household items that might be useful in designing the handwarmer – eg small plastic bottles or other containers
• ‘Klippits’
• Rubber bands
• Paper clips etc
• Safety glasses

Equipment for class demonstration

• Boiling water bath or beaker of hot water on a hot plate
• Large boiling tube or conical flask (with stopper)

Health, safety and technical notes 

• Read our standard health and safety guidance here.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• All chemicals are of low hazard.
• Eye protection may be desirable when dealing with the hot liquid. 
• Consider using lab coats, or aprons in order to protect clothing.

Commentary

Hand warmers based on the transfer of heat in various different chemical reactions are available commercially. The best design will be reusable – ie it will be possible either to regenerate the reaction or to ‘recharge’ the device by heating it up. 

Possible approach

The ‘rechargeable’ type requires a closed cycle in which an exothermic process releases heat at a low temperature and the reverse endothermic process takes in heat at a higher temperature.

It is suggested that the students are shown the exothermic crystallisation of supersaturated sodium ethanoate described below. The handwarmers sold in many shops use this phenomenon. If the students are encouraged to pursue this approach several samples of supersaturated solutions need to be prepared in advance.

The method of initiating recrystallisation will be challenging to students. They should remember that a bump or shock could be sufficient!

Demonstration 

Crystallisation from supersaturated solutions of sodium ethanoate

1. To 250 g of sodium ethanoate trihydrate in a large boiling tube, add 100 cm3 of deionised water.
2. Set up a boiling water bath or set up a large beaker of boiling water on a hot plate.
3. Heat the mixture in the water bath with occasional swirling until a clear solution is obtained.
4. Using a wash bottle, carefully rinse the glass surface at the top of the tube.
5. Insert a stopper and allow the tube and its contents to cool to room temperature (this will take 1–3 hours). The process can be speeded up by placing the tube in a large beaker and cooling it with running water.
6. To start recrystallisation remove the stopper and carefully drop a single crystal of sodium ethanoate trihydrate into the tube. Crystallisation occurs with the evolution of heat.
7. The tube can be reheated, and the process repeated. 

An alternative approach involves a mixture of iron powder, sodium chloride, vermiculite and water in a closed plastic bag. An exothermic reaction occurs as the iron is oxidised. The reaction can be controlled by restricting the amount of air entering the bag. 

Extension 

1. The design of cold packs could also be considered. These can be based on the absorption of heat when ammonium nitrate dissolves in water.
2. The reaction described in the commentary could be the basis of a chemical heat pump

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.