8.5. Chemical reactions are processes in which atoms are rearranged into different combinations of molecules. As a basis for understanding this concept:
a. Students know reactant atoms and molecules interact to form products with different chemical properties.
b. Students know the idea of atoms explains the conservation of matter: In chemical reactions the number of atoms stays the same no matter how they are arranged, so their total mass stays the same.
c. Students know chemical reactions usually liberate heat or absorb heat.
The students should know many different concepts before the demonstration. This demonstration allows students to utilize what they know and apply multiple concepts into one reaction; this works well as a review on chemical reactions. Prior to the demonstration, students should understand the concepts of density, exothermic/endothermic reactions, as well as different rates of reactions. This can be modified for your classroom. The multitude of concepts presented allows you to present the concept that you wish to teach.
Under normal conditions aluminum and water do not react because there is a protective layer of Al2O3 preventing the reaction from occurring.
2 Al + 6 H2O --> 2 Al(OH)3 + 3 H2
Al(OH)3 + NaOH --> Na+ + [Al(OH)4]-
Adding NaOH disrupts the aluminum oxide layer and the chemical reaction is activated.
Al2O3 + 2 NaOH + 3 H2O --> 2 Na+ + 2 [Al(OH)4]-
If you touch the flask during the reaction you can feel the heat given off. Students will understand that this is an exothermic reaction because heat is being released, not absorbed. Do not breathe in fumes, the vapor is harmful because it contains NaOH. After letting the reaction happen for some time, the balloon fills with hydrogen gas.
Adding hydrogen peroxide with manganese dioxide in another flask, manganese dioxide acts as a catalyst for hydrogen peroxide to form oxygen and water. This is one way to add oxygen to the balloon and show the students how the reaction changes when oxygen is in direct contact with the hydrogen before the energy is added. The more MnO2 is added, the faster the reaction will go.
2 H2O2 (aq) O2 (g) + 2 H2O(l)
1. What will happen to the reaction if the same amount of aluminum foil is added to the H2O and NaOH solution, but the surface area of the aluminum is greater in one solution than the other?
A: The aluminum foil with the larger surface area will react faster than the aluminum foil with the smaller surface area because more particles of aluminum are exposed to the water and sodium hydroxide.
2. What effect will oxygen have on the chemicals in the balloon if it is added to the balloon prior to the explosion?
A: If the oxygen is mixed with the hydrogen in the balloon, the molecules will be next to each other and the reaction will occur more quickly.
3. What is a potential use for this reaction?
A: Hydrogen powered cars (mobility powered by hydrogen energy).
See attachments below.
4.4.10 Reaction of Aluminum with Water and Sodium Hydroxide.
http://www2.uni-siegen.de/~pci/versuche/english/v44-10.html
Franek, J. (2000). Genie in a Bottle. The University of Minnesota
http://www.chem.umn.edu/services/lecturedemo/info/genie.htm
Petrovic, J. & Thomas, G. (2008). Reaction of Aluminum with Water to Produce
Hydrogen: A Study of Issues Related to the Use of Aluminum for On-Board
Vehicular Hydrogen Storage. http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/aluminium_water_hydrogen.pdf