Student Worksheet for the Demonstration
Dancing Flames
Aluminum is very reactive and when exposed to air is quickly coated with a hard, nearly impervious layer of aluminum oxide (Al2O3).
Aluminum is one of the more active metals in the activity series. It is so active that it can replace hydrogen in water. And yet, we never see aluminum dissolve in water. The reason is that the surface of aluminum is coated with a thin layer of aluminum oxide which protects the metal from contacting air or water. This oxide is very unreactive and even protects aluminum from dissolving in some acids. However, in the presence of certain ions, such as the chloride ion (Cl–), aluminum will suddenly react vigorously with water and the ions of less active metals.
Copper(II) chloride is green in color but when it is added to water, the solution is blue. In this demonstration 3.5 g of copper(II) chloride dihydrate (CuCl2·2H2O) (a green solid) is added to 50 mL of 1 M hydrochloric acid (HCl) to make a blue solution. Next, a30 cm by 6 cm piece aluminum foil is submerged in the mixture. To demonstrate the presence of one of the chemical products, a flame is brought to the mouth of the flask. The demonstration is best enjoyed in a darkened room.
Questions
In this demonstration one chemical reaction that occurs involves aluminum (Al) and copper(II) chloride (CuCl2). Predict the products of this reaction and write a balanced chemical equation.
What physical evidence do you have for the reaction between Al and CuCl2? Describe the observations that show this reaction is taking place.
One important observation involves a color change. What does this color change tell you about the limiting reactant in the reaction between CuCl2 and Al?
Aluminum undergoes another chemical reaction in this demonstration. Predict the products of the reaction between hydrochloric acid (HCl) and aluminum and write a balanced chemical equation.
What observations of the demonstration show that the reaction between Al and HCl has taken place?
Most or all of the aluminum foil is gone. What happened to it? Where is it?
The demonstration includes a dramatic demonstration of the flammability of the gaseous product of one of the reactions. Write a balanced chemical reaction to account for this flame.
The amount of copper(II) chloride dihydrate (CuCl2·2H2O) used in this demonstration is 3.5 g. Since it is a limiting reactant, how many grams of aluminum are consumed by reacting with all of the copper(II) chloride?
Also, how many grams of copper form?
The hydrochloric acid solution has a concentration of 1.0 M (1.0 moles per liter). A volume of 50 mL of the solution is used in the demonstration. How many grams of hydrogen gas are formed when this amount of acid reacts with aluminum?
Given the data in the questions, 0.369 g of aluminum are consumed by reaction with the 3.5 g of copper(II) chloride dihydrate; 1.305 g of copper form, and 0.050 g of hydrogen form.
This demonstration is based on a demonstration by by Declan Fleming, published on the Royal Society of Chemistry (RSC) web site. In case you missed the classroom demonstration there is a video which you can watch at the RSC site: https://eic.rsc.org/exhibition-chemistry/dancing-flames/2000045.article.
Based on 3.5 g of copper(II) chloride dihydrate 0.369 g Al is required. For 50 mL 1 M HCl, 0.450 g Al is required. The total of 0.819 g should be exceeded but only by a little bit to make sure the water has no trace of blue in it but no visible Al remains. A piece of standard (not heavy duty) Al foil about 6 cm wide will do the trick. A standard roll in the US is a bit over 30 cm wide so this should make a piece 6 x 30 cm.
If you ignore the water of hydration the calculated amount of Al is 0.468 g, which is what some students may calculate since hydrated salts may not have been covered.
