[pizzadude]

Hi guys.  Newbie here seeking wisdom.

I am confused why a metal displacement reaction like the following happens when its product (MgSO4) is soluble in water?

1)  Mg(s) + CuSO4(aq)  ----->   MgSO4  +  Cu

I know that in double replacement, no reaction takes place when the product is soluble in water or not a precipitate like the following equation:

2)  KNO3(aq)  +  CuSO4(aq)  ----->  nothing happens because KSO4 & CuNO3 are soluble.

For equation #2, could it be that the reaction actually does take place, we just may not see it because no precipitate forms, but the product, say, CuNO3 actually does form and is floating around in the solution?

[Mr Peanut]

Precipitation reactions are not the only type of aqueous phase reactions.

Your first reaction is a special type of reaction called and oxidation-reduction reaction. The magnesium gives up electrons (it is oxidized) and the copper accepts those electrons (it's reduced). Solubility is not the driving force. Oxidation potential is:
(Mg(metal) :: Mg--     and     Cu-- :: Cu(metal))

In your second reaction both K and Cu are already both oxidized. Other types of reactions (such as acid/base or precipitation) don't occur either.

[pizzadude]

Thank you Mr Peanut for your reply.  What you said makes a lot of sense.  I am also curious about one more thing relating to this topic & hope that you or any other in this forum can help me with.  In equation 1:

Mg(s) + CuSO4(aq)  ----->   MgSO4  +  Cu

When I add Mg(s) to an aqueous solution of CuSO4, would Mg react with water as well to form a base since Mg has higher oxidation potential than Hydrogen in Water; thus can displace Hydrogen.  So, would the product be a mixture of MgSO4 and Mg(OH)2?  If that is the case, in laboratory works, how can we make Mg displace Cu only and not with Hydrogen?

[wilson]

Magnesium will only react with steam. I doubt it does with cold water. If not, the rate of reaction is negligible. The product formed are only magnesium sulphate and copper.

[Mr Peanut]

Two more things Pizzadude.

Acid will promote the oxidation (loss of electrons) of magnesium. If present, Mg will be oxidized and hydrogen will be reduced. However, the kinetics of a neutral water/magnesium reaction (as Wilson points out) are negligible.

BUT!... SO4-- will undergo hydrolysis in water yielding some HSO4- and OH-. The OH- and any Mg++ present in solution could precipitate Mg(OH)2. Buffering or pH adjusting the solution will prevent this.