Download Teacher Demo: Hot Ice

SCH 4U: Energy Changes and Rates of Reaction
Teacher Demo: Hot Ice
Topics
Exothermic reactions
Solubility
Timing
preparation: 20 minutes + approx. 30 min
cooling time
demonstration/activity: 10 minutes
Specific Expectations SCH4U
Introduction
Heat transfers in physical and chemical reactions have many practical applications.
Exothermic reactions release heat to the surroundings, resulting in an increase in
surrounding temperature. Endothermic reactions require heat from the
surroundings, resulting in a decrease in surrounding temperature. The most
obvious application of exothermic reactions is a combustion reaction, but there are
many exothermic reactions that are used in our everyday lives.
A sodium acetate solution is found in reusable hand warmers and hot packs. These
are safe and non-toxic, and depending on the size of the hot pack, the heat produced
can last for over an hour. Boiling the hot packs will dissolve the sodium acetate in
the water, and once cooled, they can be used again.
The purpose of this demonstration is to illustrate an example of an exothermic
reaction that has a practical application.
In this demonstration, sodium acetate trihydrate is dissolved in water to produce the
supersaturated solution. As it recrystallizes, energy is released in the form of heat.
CH3COONa•3H2O(aq)  CH3COONa•3H2O(s) + heat
Equation 1. Recrystallization of sodium acetate trihydrate, producing heat energy.
Materials
sodium acetate trihydrate, CH3CO2Na•3H2O
heat resistant gloves or tongs
500 mL Pyrex Erlenmeyer flask
Parafilm, plastic wrap, or a watch glass larger than the
flask opening.
electronic balance
weighing boat
distilled water
glass stirring rod
hot plate
graduated cylinder
watch glass
safety goggles
heat resistant gloves
Safety Considerations
Wear safety goggles. Sodium acetate is slightly toxic and should not be inhaled or
ingested. Use tongs or wear heat resistant gloves while handling hot glassware.
Hazardous Materials Identification System Rating
(0-minimal 1-slight 2-moderate 3-serious 4-severe)
Sodium acetate trihydrate
Procedure
Preparation
1. Measure 150 g of sodium acetate trihydrate and place in a 500 mL
Erlenmeyer flask.
2. Measure 28 mL of distilled water and add it to the sodium acetated trihydrate
in the flask.
3. Heat the mixture on a hot plate, stirring frequently, until the solid is dissolved
(about 10-15 min). Ensure there are no remaining solid crystals on the side
or mouth of the flask.
4. Remove the flask from the hot plate, cover with plastic wrap, Parafilm, or an
inverted watch glass.
5. Allow the flask to cool to room temperature.
6. Place a few small crystals of solid sodium acetate trihydrate on a watch glass
immediately before performing the demonstration.
Predict/Explain
7. Organize your class into groups of 2-4 students per group. Ask each group to
predict what will happen when the solution is poured onto the watch glass
containing a few solid crystals of the same substance. Each group should also
attempt to come up with a rationale for their prediction.
8. Very slowly pour the solution directly onto the solid crystals on the watch
glass.
Observe
9. Conduct the demonstration and provide time for students to record their
observations.
Explain
10. Ask the groups to reconvene and revise their explanations, if necessary.
Disposal
The solid product can be reheated and reused several times. The liquid solution can
be neutralized with baking soda (sodium bicarbonate) and discarded. Follow the
employer’s protocol for chemical disposed.
What happens?
As the liquid is slowly poured onto the crystals on the watch glass, it solidifies
immediately into a translucent white compound. Depending on the speed at which it
is poured, a tall tower will be formed. The solid feels warm, as heat is released.
How does it work?
When a supersaturated solution of sodium acetate trihydrate comes into contact
with a solid crystal of sodium acetate trihydrate, it will recrystallize. This
recrystallization is exothermic, so it releases a significant amount of heat.
A saturated solution is a solution that contains the maximum amount of solute
dissolved in a solvent. A supersaturated solution is a solution that contains more
than the maximum amount of solute dissolved in a solvent. When using a solid
solute and liquid solvent such as water, heating the solvent will create a
supersaturated solution. Increasing the temperature will increase the maximum
solubility of many solutes, so when the solution is cooled back down, it will contain
more than the maximum amount of dissolved solute. Supersaturated solutions are
unstable and will recrystallize with agitation or the addition of even just one crystal.
Teaching Suggestions/Hints
1. Students could be asked why a chemical reaction can be suitable for either a
hot pack or a cold pack.
2. Anhydrous sodium acetate does not work as well as the trihydrate form.
3. Tap water will not work as well as distilled water.
4. To speed up the cooling process, run room temperature or colder water over
the outside of the flask, ensuring no water gets inside the flask.
5. A clean burette could be used to add the solution to the crystal to control the
flow.
6. A variation of this demonstration is adding the solid crystals to the flask to
show the formation of the solid within the flask.
7. The proportions can easily be scaled up or down depending on how the
demonstration will be performed.
Next Steps
This demonstration can be used to introduce the concept of exothermic reactions. It
can also be used to review the concept of solubility from SCH 3U. A natural next step
would be for students to perform or even plan an inquiry lab investigating specific
heat capacity and enthalpy changes in reactions.
Additional Resources
1. A great demonstration example and video from the Royal Society of Chemistry:
http://www.rsc.org/Education/EiC/issues/2013July/hot-ice-stalagmite-sodiumacetate-solution.asp
2. Video demonstrations of an endothermic reaction:
http://www.dnatube.com/video/4882/Demonstration-on-Endothermic-Reaction
3. Video demonstrations of thermite, an exothermic reaction:
http://www.dnatube.com/video/12093/Iron-Video--Periodic-Table-of-Videos and
http://www.dnatube.com/video/4892/Exothermic-IronOxide-and-AluminumReaction
Specific Expectations
SCH4U
A1.1 formulate relevant scientific questions about observed relationships, ideas,
problems, or issues, make informed predictions, and/or formulate educated
hypotheses to focus inquiries or research
D3.1 compare the energy changes resulting from physical change (e.g., boiling
water), chemical reactions (e.g., bleaching a stain), and nuclear reactions (e.g.,
fission, fusion), in terms of whether energy is released or absorbed
Return to top