Teacher Notes

Lab – Observing a Chemical Reaction

Summary

Students observe the dissolving of CuCl22H2O in water followed by the exothermic reaction between CuCl22H2O and aluminum. The products of the reaction are then separated by filtration. Time Frame: 45 minutes.

Chemistry Concepts: physical and chemical changes, quantitative and qualitative observations, endothermic and exothermic reactions, separation by filtration

Materials (per group)

2 100-mL beakersthermometer buret clampringring stand

glass stirring rodplastic spoonmagnifying glassfunnelfilter paper

copper(II) chloride dihydratealuminum foil, 8 cm 8 cmdistilled water wash bottle

Notes: Use reagent grade CuCl22H2O rather than technical grade for better results. Dispense in beakers at

each lab station. Each group will use about 5 g of the crystals. Precut the aluminum foil squares to 8 cm 8 cm.

Procedure Notes

While the quantities described in the procedure may seem approximate, it is important that students adhere to them in order to achieve the best results.

Demonstrate the proper filtration setup as described in steps 5 through 7. See the diagram to the right.

Safety Precaution:The beaker may become quite hot during the course of this exothermic reaction. Some spitting of solution may also occur, so it’s important that students heed the caution in step 4 to not look directly down into the beaker.

Expected observations:During the reaction with aluminum foil, students should observe the blue-green solution of CuCl2 change to colorless. A red-brown precipitate forms, heat is released, and H2 gas is produced. The equation for this redox reaction is as follows:

3 Cu2+(aq) + 4 Al(s) + 6 H+(aq) 3 Cu(s) + 4 Al3+(aq) + 3 H2(g)

Teacher Notes

Conclusions Answer Key

1. The only quantitative observations in this experiment were the temperature measurements.

2. Physical changes are a change in the form of a substance without changing its identity. Chemical changes involve a change in the identity of a substance; a new substance is formed. Signs of a chemical change include color change, formation of a precipitate, evolution of gas, and change in light or heat.

3. When CuCl22H2O was added to water, it dissolved. This was a physical change. Both of the substances retained their identities. The color of the crystals was simply imparted to the water, thus it was not an example of a “color change.” Also, the two substances could easily be separated by evaporating the water.

4. When the aluminum foil was added to the experiment, a chemical change occurred. There was a color change from blue-green to colorless, a precipitate (new solid) was formed, gas was evolved, and heat was produced. All of these are signs of a chemical change. Filtration of the resulting mixture yields new substances, not the original aluminum foil and CuCl2 solution.

5. Exothermic reactions produce heat and feel hot, while endothermic reactions absorb heat and often feel cold. Since this reaction produced heat and a rise in temperature, it would be classified as exothermic.

6. The larger particle size of the red-brown solid is what allowed it to be separated from the water by filtration. The water and dissolved substances were able to pass through the filter, while the solid was not able to do so.