Question

Consider the following half-reactions:

$$\begin{gathered} {\mathbf{Pt}}^{\mathbf{2}\mathbf{+}}\mathbf{+}\mathbf{2}{\mathbf{e}}^{\mathbf{-}}\mathbf{\to }\mathbf{Pt}\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }{\mathbf{E}}_{\mathbf{0}}\mathbf{=}\mathbf{1}\mathbf{.}\mathbf{188}\mathbf{V} \\ {\mathbf{PtCl}}_{\mathbf{4}}^{\mathbf{2}\mathbf{-}}\mathbf{+}\mathbf{2}{\mathbf{e}}^{\mathbf{-}}\mathbf{\to }\mathbf{Pt}\mathbf{+}\mathbf{4}{\mathbf{Cl}}^{\mathbf{-}}\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }{\mathbf{E}}_{\mathbf{0}}\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{73}\mathbf{V} \\ {\mathbf{NO}}_{\mathbf{3}}^{\mathbf{-}}\mathbf{+}\mathbf{4}{\mathbf{H}}^{\mathbf{+}}\mathbf{+}\mathbf{3}{\mathbf{e}}^{\mathbf{-}}\mathbf{\to }\mathbf{NO}\mathbf{+}\mathbf{2}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }{\mathbf{E}}_{\mathbf{0}}\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{96}\mathbf{V} \end{gathered}$$

Explain why platinum metal will dissolve in aqua regia (a mixture of hydrochloric and nitric acids) but not in either concentrated nitric or concentrated hydrochloric acid individually.

Short answer

The reaction of platinum withconcentrated nitric or concentrated hydrochloric

acid individually is a non-spontaneous reaction since the value of E° is negative. While the value of E° is positive in the case of aqua regia so it is a spontaneous reaction

Step-by-step solution

Explaining why Platinum metal will not dissolve in hydrochloric acid

Writing the two half equations:

$$\begin{gathered} {\mathrm{PtCl}}^{2-}+2{\mathrm{e}}^{-}\to \mathrm{Pt}+{\mathrm{Cl}}^{-}\left(\mathrm{E}°=0.755\mathrm{V}\right) \\ 2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2\left(\mathrm{E}°=0\mathrm{V}\right) \\ \mathrm{E}°=-0.755+0=-0.755\mathrm{V} \end{gathered}$$

Since E° is negative, hence the reaction is nonspontaneous.

Demonstrating why Platinum metal will not dissolve in concentrated nitric acid

Writing the two half equations:

$$\begin{gathered} \mathrm{N}{{\mathrm{O}}^{-}}_3+4{\mathrm{H}}^{+}+3{\mathrm{e}}^{-}\to \mathrm{NO}+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{E}°=0.96\mathrm{V}\right) \\ {\mathrm{Pt}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{Pt}\left(\mathrm{E}°=1.188\mathrm{V}\right) \\ \mathrm{E}°=-1.188+0.96=0.228\mathrm{V} \end{gathered}$$

Since E° is negative, hence the reaction is nonspontaneous.

Explaining why Platinum metal will dissolve in aqua regia (mixture hydrochloric and nitric acid

Writing the equation:

$3\mathrm{Pt}+12{\mathrm{Cl}}^{-}+2\mathrm{N}{{\mathrm{O}}^{-}}_3+4{\mathrm{H}}^{+}\to 3\mathrm{PtC}{{\mathrm{l}}^{2-}}_4+2\mathrm{NO}+2{\mathrm{H}}_2\mathrm{O}$

Writing the two half equations:

role="math" localid="1663751997157" $$\begin{gathered} \mathrm{Pt}+4{\mathrm{Cl}}^{-}\to \mathrm{PtC}{{\mathrm{l}}^{2-}}_4+2{\mathrm{e}}^{-}\left(\mathrm{E}°=0.755\mathrm{V}\right) \\ \mathrm{N}{{\mathrm{O}}^{-}}_3+2{\mathrm{H}}^{+}+3{\mathrm{e}}^{-}\to \mathrm{NO}+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{E}°=0.96\mathrm{V}\right) \\ \mathrm{E}°=0.96--0.755=0.205\mathrm{V} \end{gathered}$$

It is a spontaneous reaction and hence, platinum metal will dissolve in aqua regia

(a mixture of hydrochloric and nitric acids) but not in either concentrated nitric or concentrated hydrochloric acid individually