Magnesium + Copper (II) Sulfate: Lots going on here!

When we put solid Mg ribbon in the CuSO4 solution, and the reaction starts right away. Because Mg is more active than Cu the Mg replaces the Cu in CuSO4. This is a classic single displacement reaction.

Mg (s) + CuSO4 (aq) → MgSO4 (aq) + Cu (s)

If we wrote this as a net ionic equation --- really see what is happening.
The red substance, that’s the solid Copper. The Mg metal is now in the water. So this is also a redox reaction.

Mg (s) + Cu2+ (aq) → Mg2+ (aq) + Cu (s)

But what are those bubbles? There aren’t any gasses in our equation.

We actually have two reactions taking place. The single rep reaction and Magnesium reacting with water.

Mg + H2O = Mg(OH)2 + H2

Normally the reaction takes place slowly in water because a layer of MgOH2 forms on the Mg metal and slows the reaction. But the reaction of the Mg and CuSO4 is exposing Mg to react with the water. If we collected the H2 and lit it on fire it combust.

You might have noticed a change in the color of the solution. After a while all the Cu in CuSO4 has been replaced and it the red you see on the bottom. The solution is now clear. That’s because all of the aqueous Copper (II) ions are gone – the are now solid solid copper. The Mg ions dissolved in the water – they make a clear solution.

So it’s not as simple as they tell you in school!