[helloweili]

An answer to the two questions in a classical general chemistry laboratory demonstration of the displacement reactions:

Questions: (1) Why does Al react with CuCl2 in aqueous solution to produce Cu metal? (2) Why does Al not react with CuSO4 or Cu(NO3)2?

Answers (ref. 3): The reason is that only CuCl2 among them causes significant pitting corrosion of Al, and the redox reaction actually happens between atomic H and Cu2+ ions, instead of between Al and Cu2+ ions (ref. 1 and 2).

Ref. 1: Hutchings, K. Classic Chemistry Experiments (Royal Society of Chemistry, London, 2000).

Ref. 2: Summerlin, L. R., Borgford, C. L., and Ealy, J. B. Chemical Demonstrations: A Sourcebook for Teachers (American Chemical Society, Washington, DC, 1988).

Ref. 3: Wei Li, Thomas Cochell, and Arumugam Manthiram, Activation of aluminum as an effective reducing agent by pitting corrosion for wet-chemical synthesis, Scientific Reports (a new open access multidisciplinary journal published by Nature Publishing Group) 3 (2013), p.1229.

[antimatter101]

Here is a pattern.

The chloride Cl^- ion is non-resonating, which means that the extra electron is not delocalised. The nitrate and sulphate ions, however, are resonating, the extra electron(s) being shared between atoms.

[helloweili]

Hi antimatter101,

You are right. resonating could be a reason.

As we know, the metallic Al is covered with a dense film of Al oxide. The key point for reacting with Al is how to remove the protecting Al oxide. From the view of corrosion, Cl^- significantly enhances the pitting corrosion of Al metallic, which is covered with a dense film of Al oxide. On the contrary, SO₄^2- and NO₃^- do not cause significant pitting corrosion. With a long time and high concentration, there could be some Cu produced in the aqueous solution of CuSO4 or Cu(NO3)2.

Please read our paper at the link below if you are interested.
http://www.nature.com/srep/2013/130206/srep01229/full/srep01229.html