Side Reactions in Displacement Reactions?

[jimmydasaint]

I don't expect many hits on this Thread but I have not found the answer to this enquiry.

 

Imagine the scene - an excited Yr 9 class is following up displacement reactions where a more reactive metal will displace a less reactive metal from its compound. For example, the students add magnesium metal to copper sulfate and immediately notice that the copper is precipitated.

 

However, a lot of groups also notice streams of bubbles coming off various reactions and these are discounted as 'side reactions.' by me.

 

Does anyone know if I am right, and if so, what are the side reactions?

 

Any clues will be appreciated.

[gcol]

I very hesitantly suggest that the "mystery gases" probably first dissolve in the solute and only become apparent when saturation occurs? If so, and the proportions of chemicals are hit-and-miss or more likely in tiny ammounts, the reaction might complete before saturation. perhaps altering the temperature of the spent solution will produce visible/collectable bubbles?

[UC]

Magnesium is an alkaline earth metal and will happily react with water to generate hydrogen.

 

I think this is because the copper is laid down on the magnesium and forms a galvanic cell, which should predispose the magnesium to rapid reaction in water

 

If you put some aluminum foil in saturated NaCl brine, you get no reaction. but it is rather vigorous upon the addition of a catalytic amount of copper salt. This is the same principle, and credit to Woelen for the experiment.