Consider the following half-reactions: $$ \begin{aligned} \mathrm{Pt}^{2+}+2 \mathrm{e}^{-} \longrightarrow \mathrm{Pt} & \mathscr{E}^{\circ} &=1.188 \mathrm{~V} \\ \mathrm{PtCl}_{4}^{2-}+2 \mathrm{e}^{-} \longrightarrow \mathrm{Pt}+4 \mathrm{Cl}^{-} & \mathscr{E}^{\circ} &=0.755 \mathrm{~V} \\ \mathrm{NO}_{3}^{-}+4 \mathrm{H}^{+}+3 \mathrm{e}^{-} \longrightarrow \mathrm{NO}+2 \mathrm{H}_{2} \mathrm{O} & \mathscr{E}^{\circ} &=0.96 \mathrm{~V} \end{aligned} $$ 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.

Here, we will determine the overall redox reactions taking place when Pt metal react with a) aqua regia, b) nitric acid, and c) hydrochloric acid. a) In aqua regia, the half-reactions involved are the reduction of Pt²⁺ to Pt, and the reduction of NO₃⁻ to NO. We will add the half-reactions to obtain the overall redox reaction. b) In nitric acid, the half-reactions involved are the reduction of Pt²⁺ to Pt, and the reduction of NO₃⁻ to NO. c) In hydrochloric acid, the half-reactions involved are the reduction of Pt²⁺ to Pt, and the reduction of PtCl₄²⁻ to Pt and Cl⁻.

In this step, we will calculate the standard cell potentials (E₀) of the redox reactions to understand which reaction is favorable in each condition by using the Nernst equation: E₀(cell) = E₀(reduction) - E₀(oxidation) a) In aqua regia, E₀ = E₀(Pt²⁺ → Pt) - E₀(NO₃⁻ → NO) = 1.188 V - 0.96 V = 0.228 V b) In nitric acid, E₀ = E₀(Pt²⁺ → Pt) - E₀(NO₃⁻ → NO) = 1.188 V - 0.96 V = 0.228 V c) In hydrochloric acid, E₀ = E₀(Pt²⁺ → Pt) - E₀(PtCl₄²⁻ → Pt) = 1.188 V - 0.755 V = 0.433 V

Based on the standard cell potentials calculated above, we can analyze the favorability of the redox reactions in each condition: a) In aqua regia, the standard cell potential E₀ = 0.228 V, indicates that the redox reaction is favorable, and platinum metal will dissolve in aqua regia. b) In nitric acid, the standard cell potential E₀ = 0.228 V, is positive but not large enough to facilitate the dissolution of platinum metal. This indicates that the reaction with nitric acid alone is not sufficient to dissolve platinum-metal. c) In hydrochloric acid, the standard cell potential E₀ = 0.433 V, indicates that the reaction is not favorable, and platinum metal will not dissolve in hydrochloric acid. In conclusion, platinum metal dissolves in aqua regia due to the combined effect of the nitric and hydrochloric acids that create a favorable redox reaction. Individually, neither nitric nor hydrochloric acid can create a sufficient standard cell potential to dissolve platinum metal.