Copper can be extracted by hydrometallurgy but not zinc. Explain.

Explanation

The ` E^(@)` of zinc ` (Zn^(2 + ) //Zn = - 0*76V) ` or iron ` (Fe^( 2 + )//Fe = - 0*44V )` is lower than that of copper ` (Cu^( 2 + ) //Cu = + 0 * 34V) ` and hence both zinc and Fe, can displace Cu from solutions of ` Cu^( 2 + ) ` ions . Although zinc is a stronger reducing agent than iron, yet iron scrap is chiefly used in hydrometallurgy of copper because it is much cheaper than zinc.
`" " Fe (s) + Cu^( 2 + ) (aq) to Fe^( 2 + ) (aq) + Cu (s) `
In contrast, to displace zinc from solution of ` Zn ^( 2 + ) ` ions, we need a more reactive metal than it, i.e.,
` Al( E ^(@)""_(Al^(3 + )//Al) = - 1*66V), Mg (E ^(@)""_(Mg^(2 + )//Mg) = -2*37V)`,
`Ca (E^(@)""_(Ca^(2+)//Ca) = - 2*87V), K(E ^(@)""_(K^(+)//K) = - 2*93V)`, etc.
But all these metals react with water `[ 2 H_2 O + 2 e ^(-) to H _ 2 (g) + 2 O H ^( - ) (aq) , E ^(@) = - 0 *83 V ] ` forming their corresponding ions with the evolution of ` H _2 ` gas.
Thus, Al, Mg, etc., cannot be used to displace zinc from solution of ` Zn^(2+)` ions. Thus, copper can be extracted by hydrometallurgy but not zinc.