Question

A reaction cyclefor 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.

Short answer

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

Step-by-step solution

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,

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

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,

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

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,

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

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,

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

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,

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

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,

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

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,

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