Endothermic and Exothermic Reactions|
This demonstration illustrates how chemical reaction can either give off heat (exothermic) or absorb heat (endothermic). The crystallization of a supersaturated sodium acetate solution is an exothermic process that is available commercially is the form of hand warmers. This demonstration can also be carried out in a large flask by seeding the supersaturated solution with a small sodium acetate crystal.
To demonstrate an endothermic process, Barium hydroxide (octahydrate) and Ammonium nitrate are mixed (in a 2:1 ratio) in a small beaker. This reaction displays the endothermic process and illustrated the interaction between changes in enthalpy and entropy is spontaneous chemical reactions. For a process to take place spontaneously at constant temperature and pressure, the change in free energy must be negative. An endothermic reaction may thus be spontaneous at constant pressure if the positive value of the heat absorbed is offset by a sufficient increase in entropy (randomness). In the reaction between barium hydroxide octahydrate and ammonium nitrate, the large increase in entropy is related to the increase in the number of particles present and their states (remember that two solids are combining to form a solid product and some liquid). As the reaction below shows, there are three molecules that combine to form 13 product molecules.
With this reaction, temperatures of -20o C can be achieved. Both of these demos can be done hands-on by volunteers from the audience.
- Flask of supersaturated sodium acetate
- Sodium acetate seed crystals
- Handwarmers (one for each or every other student)
- Approximately 32g of barium hydroxide octahydrate
- Approximately 17g of ammonium nitrate
- 125 mL Erlenmeyer flask
- Glass stirring rod
- Small piece of cardboard
- Squeeze bottle of water
Handwarmers can be passed out among the audience (for smaller groups there should be enough for each or at least every other student). Have the student click the metal disk inside the handwarmers to activate the crystallization. They should notice that the crystallization will begin at the metal disk and spread outward though the whole package. As an alternative, the large flask containing the sodium acetate solution can be used. To activate the crystallization, add a single seed crystal of sodium acetate. If the solution has been regenerated properly you should observe a long crystalline spike run out from the seed crystal and eventually spread through the whole solution. As the solution crystallizes, the system will give off heat.
Place a pre-weighed amount of solid barium hydroxide octahydrate and ammonium nitrate into a Erlenmeyer flask and stir the two components together. Within about 30 seconds the odor of ammonia will become evident and the liquid product will begin to form.
Within a few minutes the temperature of the flask will drop to about -20o or -30o C. If a small amount of water is placed on a small piece of cardboard, the flask will freeze the water and the cardboard will stick to the flask after a minute or two of contact. Alternatively, this may be son with a small wooden block. The temperature of the flask may be measured directly if a thermometer is available.
- It should be noted that the hand warmers can be recharged by boiling them in water for six to ten minutes.
- Be sure to collect all of the hand warmers when you are done, the have a tendency to disappear.
- Ask the students see if they can come up with any practical applications for both endothermic and exothermic reactions. They may suggest something like a car battery heater or a chemical ice pack for injured muscles.
- Be aware that the temperatures achieved in the endothermic reaction are well below the normal body temperature and the reaction flask should not be handled for prolonged periods of time.
- Inhalation of concentrated ammonia vapor can cause edema of the respiratory tract, spasm of the glottis, and asphyxia. Show the students how to properly smell a reaction flask.
- Soluble barium salts are poisonous if ingested. Upon contact with the skin, barium and ammonium salts may produce minor irritations or cause allergic reactions. If the flask is spilled or broken, its contents should be flushed down the drain with copious amounts of water.
- Handwarmers do tend to get very warm. It is possible that someone who is sensitive to heat or cold could be burned.
- Due to the ammonia vapor given off, the endothermic demonstration should not be done in a room with poor ventilation.
- Always were safety goggles when working with these chemicals. Be sure to provide safety goggles for any of the audience members who may be helping you or who are handling these demonstrations.