Chapter U4, Problem 16STP

Interpretation Introduction

Interpretation:

The best reagent to precipitate copper, Cu²+ ion from a solution of copper nitrate, Cu(NO₃)₂ needs to be identified.

Concept introduction:

A precipitation reaction occurs between two or more reagents to produce a chemical species that is insoluble in water. The best way to remove Cu²+ from a solution of Cu(NO₃)₂ is by converting Cu²+ to a chemical compound that is insoluble in water.

Answer to Problem 16STP

The second option, option (B) is the correct answer.

Explanation of Solution

Reason for Correct Option:

Cu(NO₃)₂ is highly water soluble and ionizes in aqueous solutions to produce Cu²+ ions as below.

$\text{Cu}{\left({\text{NO}}_3\right)}_2\left(\text{aq}\right)\to {\text{Cu}}^{2+}\left(\text{aq}\right)+2{\text{ NO}}_3^{-}\left(\text{aq}\right)$

KOH ionizes in an aqueous solution to hydroxide, OH- ion as below.

$\text{KOH}\left(\text{s}\right)\to {\text{K}}^{+}\left(\text{aq}\right)+{\text{OH}}^{-}\left(\text{aq}\right)$

The Cu²+ ion combines with the OH- ion to produce copper (II) hydroxide, Cu(OH)₂ that remains insoluble in water and thus, Cu²+ can be quantitatively removed from the aqueous solution.

${\text{Cu}}^{2+}\left(\text{aq}\right)+2{\text{ OH}}^{-}\left(\text{aq}\right)\to \text{Cu}{\left(\text{OH}\right)}_2\left(\text{s}\right)$

Conclusion

Reasons for Incorrect Options:

All chloride, Cl- salts except those of silver (Ag), mercury (Hg) and lead (Pb) are soluble in water. Thus, copper (II) chloride, CuCl₂ is water-soluble and hence, Cu²+ ions cannot be removed from a solution of Cu(NO₃)₂ by adding solid sodium chloride, NaCl.

All nitrate, NO₃- salts are water soluble and hence, solid magnesium nitrate, Mg(NO₃)₂ cannot remove Cu²+ ions from an aqueous solution of Cu(NO₃)₂.

Copper carbonate, CuCO₃ is insoluble in water and hence, carbonate, ${\text{CO}}_3^{2-}$ ion is an excellent reagent for removing Cu²⁺. However, all carbonates except those of the alkali metal and ammonium, ${\text{NH}}_4^{+}$ are insoluble in water and thus, lead carbonate, PbCO₃ cannot be brought in solution to react with Cu(NO₃)₂.