4.144_CP - Chemistry: Molecular Nature Of Matter And Change

Question

A reaction cycle for an element is a series of reactions beginning and ending with that element. In the following copper reaction cycle, copper has either a 0 or a _2 oxidation state. Write balanced molecular and net ionic equations for each step in the cycle.

(1) Copper metal reacts with aqueous bromine to produce a green-blue solution.

(2) Adding aqueous sodium hydroxide forms a blue precipitate.

(3) The precipitate is heated and turns black (water is released).

(4) The black solid dissolves in nitric acid to give a blue solution.

(5) Adding aqueous sodium phosphate forms a green precipitate.

(6) The precipitate forms a blue solution in sulfuric acid.

(7) Copper metal is recovered from the blue solution when zinc metal is added.

 

Step-by-Step Solution

Answer

The chemical equations are balanced for all steps in the cycle.

Step 1: Balanced molecular and net ionic reaction for step (1)

The molecular equation for the given reaction is,

$\mathbf{Cu}\left(s\right)\mathbf{+}{\mathbf{Br}}_{\mathbf{2}}\left(\mathrm{aq}\right)\mathbf{\to }{\mathbf{CuBr}}_{\mathbf{2}}\left(\mathrm{aq}\right)$ 

 

The net ionic equation is,

 $\mathbf{Cu}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{+}{\mathbf{Br}}_{\mathbf{2}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{\to }{\mathbf{Cu}}^{\mathbf{2}\mathbf{+}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{+}\mathbf{2}{\mathbf{Br}}^{\mathbf{-}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}$

Step 2: Balanced molecular and net ionic reaction for step (2)

The molecular equation for the given reaction is,

 ${\mathbf{CuBr}}_{\mathbf{2}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{2}\mathbf{NaOH}\left(\mathrm{aq}\right)\mathbf{\to }\mathbf{Cu}{\left(\mathrm{OH}\right)}_{\mathbf{2}}\left(s\right)\mathbf{+}\mathbf{2}\mathbf{NaBr}\left(\mathrm{aq}\right)$

 

The net ionic equation is,

 ${\mathbf{Cu}}^{\mathbf{2}\mathbf{+}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{2}{\mathbf{OH}}^{\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{\to }\mathbf{Cu}{\left(\mathrm{OH}\right)}_{\mathbf{2}}\left(s\right)$

Step 3: Balanced molecular and net ionic reaction for step (3)

The molecular equation for the given reaction is,

$\mathbf{2}\mathbf{Cu}{\left(\mathrm{OH}\right)}_{\mathbf{2}}\left(s\right)\mathbf{\to }\mathbf{2}\mathbf{CuO}\left(s\right)\mathbf{+}\mathbf{2}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\left(l\right)$ 

 

The net ionic equation is,

$\mathbf{2}\mathbf{Cu}{\left(\mathrm{OH}\right)}_{\mathbf{2}}\left(s\right)\mathbf{\to }\mathbf{2}\mathbf{CuO}\left(s\right)\mathbf{+}\mathbf{2}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\left(l\right)$ 

Step 4: Balanced molecular and net ionic reaction for step (4)

The molecular equation for the given reaction is,

 $\mathbf{CuO}\left(s\right)\mathbf{+}\mathbf{2}{\mathbf{HNO}}_{\mathbf{3}}\left(\mathrm{aq}\right)\mathbf{\to }\mathbf{Cu}{\left({\mathrm{NO}}_3\right)}_{\mathbf{2}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\left(l\right)$

 

The net ionic equation is,

 $\mathbf{CuO}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{+}\mathbf{2}{\mathbf{H}}^{\mathbf{+}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{\to }{\mathbf{Cu}}^{\mathbf{2}\mathbf{+}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{+}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\left(}\mathbf{l}\mathbf{\right)}$

Step 5: Balanced molecular and net ionic reaction for step (5)

The molecular equation for the given reaction is,

$\mathbf{3}\mathbf{Cu}{\left({\mathrm{NO}}_3\right)}_{\mathbf{2}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{2}{\mathbf{Na}}_{\mathbf{3}}{\mathbf{PO}}_{\mathbf{4}}\left(\mathrm{aq}\right)\mathbf{\to }{\mathbf{Cu}}_{\mathbf{3}}{\left({\mathrm{PO}}_4\right)}_{\mathbf{2}}\left(s\right)\mathbf{+}\mathbf{6}{\mathbf{NaNO}}_{\mathbf{3}}\left(\mathrm{aq}\right)$ 

 

The net ionic equation is,

 $\mathbf{3}{\mathbf{Cu}}^{\mathbf{2}\mathbf{+}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{2}{\mathbf{PO}}_{\mathbf{4}}^{\mathbf{3}\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{\to }{\mathbf{Cu}}_{\mathbf{3}}{\mathbf{\left(}{\mathbf{PO}}_{\mathbf{4}}\mathbf{\right)}}_{\mathbf{2}}\mathbf{\left(}\mathbf{s}\mathbf{\right)}$

Step 6: Balanced molecular and net ionic reaction for step (6)

The molecular equation for the given reaction is,

${\mathbf{Cu}}_{\mathbf{3}}{\left({\mathrm{PO}}_4\right)}_{\mathbf{2}}\left(s\right)\mathbf{+}\mathbf{3}{\mathbf{H}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}\left(\mathrm{aq}\right)\mathbf{\to }{\mathbf{CuSO}}_{\mathbf{4}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{2}{\mathbf{H}}_{\mathbf{3}}{\mathbf{PO}}_{\mathbf{4}}\left(\mathrm{aq}\right)$ 

 

The net ionic equation is,

 ${\mathbf{Cu}}_{\mathbf{3}}{\left({\mathrm{PO}}_4\right)}_{\mathbf{2}}\left(s\right)\mathbf{\to }\mathbf{3}{\mathbf{Cu}}^{\mathbf{2}\mathbf{+}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{2}{\mathbf{PO}}_{\mathbf{4}}^{\mathbf{3}\mathbf{-}}\left(\mathrm{aq}\right)$

Step 7: Balanced molecular and net ionic equation for step (7)

The molecular equation for the given reaction is,

 ${\mathbf{CuSO}}_{\mathbf{4}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{Zn}\left(s\right)\mathbf{\to }{\mathbf{ZnSO}}_{\mathbf{4}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{Cu}\left(s\right)$

 

The net ionic equation is,

 ${\mathbf{Cu}}^{\mathbf{2}\mathbf{+}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{Zn}\left(s\right)\mathbf{\to }{\mathbf{Zn}}^{\mathbf{2}\mathbf{+}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{Cu}\left(s\right)$