Endothermic
and Exothermic Reactions
Introduction:
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.
Materials:
- 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
Exothermic Reactions
Procedure:
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.
Endothermic Reactions
Procedure:
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
done with a small wooden block. Vigorous stirring will improve
the effectiveness of this. The temperature of the flask may
be measured directly if a thermometer is available.
Helpful Hints:
- 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.
Safety:
- 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.
- 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.
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