The case of aluminium metal in single replacement REDOX reactions

Displacement reactions are an essential way to demonstrate the reactivity series of metals. I have tried many different methods to demonstrate or perform displacement reactions over the years with mixed results regarding one particular metal, aluminium. Based upon my experience, the behavior of aluminium in displacement reactions often confuses students. In light of this, I have introduced a trick involving aluminium displacement reactions that hopefully helps students understand and remember (‘stick in their mind’ so to speak) these reactions.

To carry out the reaction, a small aluminium boat is constructed from a square piece of foil (about 5 cm²). Dry copper nitrate powder (about 1 g, quantities don’t need to be exact) is added on top. A few drops (approx. 5 or 6) of water is then dripped from a plastic pipette in order to wet the powder. No reaction is observed, even after waiting one or two minutes (Figure 1, left). The experiment is repeated using copper chloride instead of copper nitrate. Almost immediately, the aluminium foil breaks apart, releasing a gas (fizzing) and water vapor into the air (Figure 2, center)! The reaction is very exothermic and sudden. A red/brownish ppt. (copper metal) is left behind:¹

2 Al _(s)  +   3 CuCl_{2 (aq)}   →  2 AlCl_{3 (aq)}  +  3 Cu_(s)

      silver        green-blue         colorless         red/brown

The fizzing is due to the formation of hydrogen gas in the reaction between the aluminium foil with hydrogen ions present in solution:¹

2 Al _(s)   +   6 H⁺ _(aq)   →   2 Al³⁺ _(aq)   +  3 H_2 (g)

For the trick, another piece of foil is produced but in this case tin foil (which looks very similar to aluminium foil) is used. Before carrying out the trick, I pose a few questions: Why does the copper nitrate not react? Was it the aluminium foil? Was it that a different copper salt was used? To address the questions, the copper nitrate experiment is repeated (this time with the tin foil, however) and sure enough an exothermic reaction ensues within a few seconds (Figure 1, right). So much heat is generated that the nitrate might decompose releasing brown toxic nitrogen dioxide gas and it can even catch fire if the boat shape is rolled into a cigar shape.

Sn_(s)  +   Cu(NO₃)_2 (aq)   →   Sn(NO₃)_{2 (aq)}  +  Cu_(s)

          silver            blue                        colorless            red/brown

In addition, hydrogen gas is evolved as before, presumably due to the reaction between tin and protons:

2 Sn_(s)   +   4 H⁺_(aq)    →    2 Sn²⁺_(aq)   +   2 H_2 (g)

Figure 1 - Aluminum foil with copper II nitrate solution (left) and with

copper II chloride solution (center). Tin foil reacting with copper II nitrate solution (right). 

By now, I have many perplexed faces staring at me. I then reveal ‘the trick’: I reacted tin with the copper nitrate, not aluminium! After disclosing this to the students, the real detective work begins. I explain to the students that aluminium metal has an outer layer of very unreactive aluminium oxide,² while tin does not have such a layer. The oxide layer in aluminium must be penetrated in order for the aluminium metal to react.² Chloride ions (bromide ions work, too) act as a catalyst that allows the aluminium oxide layer to be penetrated, allowing the reaction with copper ion to occur.

The video below displays the effect of adding water to a mixture of Cu(NO₃)_2 (s) with Al_(s), CuCl_2 (s) with Al_(s), and Cu(NO₃)₂ with Sn_(s).

The case of aluminium metal in single replacement redox reactions video from ChemEd Xchange on Vimeo.*

   * The formula for copper II nitrate under the reaction on the right should read Cu(NO₃)₂.

REFERENCES

1. Flinn Scientific ChemFax, Foiled Again: Aluminum Loses to Copper, 2017.

2. Flinn Scientific ChemFax, Foiled Again: Single Replacement Reactions, 2016.