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.

The half-reactions given are: 1) \(Pt^{2+} + 2e^{-} \rightarrow Pt\) with \(\mathscr{E}^° = 1.188 V\) 2) \(PtCl_{4}^{2-} + 2e^{-} \rightarrow Pt + 4Cl^{-}\) with \(\mathscr{E}^° = 0.755 V\) 3) \(NO_{3}^{-} + 4H^{+} + 3e^{-} \rightarrow NO + 2H_{2}O\) with \(\mathscr{E}^° = 0.96 V\)

Aqua regia is a mixture of nitric acid (contributing nitrate ions, NO₃⁻) and hydrochloric acid (contributing chloride ions, Cl⁻). Since the platinum metal reacts with both nitrate and chloride ions, the overall reaction can be determined by combining the appropriate half-reactions. We will combine half-reaction 1 (in reverse) with half-reaction 3 to represent the reaction with nitrate ions, and half-reaction 1 (in reverse) with half-reaction 2 (in reverse) to represent the reaction with chloride ions. Reverse half-reaction 1: \(Pt \rightarrow Pt^{2+} + 2e^{-}\) with \(\mathscr{E}^° = -1.188 V\) Reverse half-reaction 2: \(Pt + 4Cl^{-} \rightarrow PtCl_{4}^{2-} + 2e^{-}\) with \(\mathscr{E}^° = -0.755 V\) Reaction with nitrate ions: \(Pt + NO_{3}^{-} + 4H^{+} + 3e^{-} \rightarrow Pt^{2+} + NO + 2H_{2}O + 2e^{-}\) Reaction with chloride ions: \(Pt + 4Cl^{-} + 2e^{-} \rightarrow Pt^{2+} + PtCl_{4}^{2-} + 2e^{-}\) Now, add these two reactions to obtain the overall reaction in aqua regia. Overall reaction in aqua regia: \(Pt + NO_{3}^{-} + 4H^{+} + 4Cl^{-} \rightarrow PtCl_{4}^{2-} + NO + 2H_{2}O\)

The overall cell potential can be calculated by combining the appropriate half-reaction potentials: \(\mathscr{E}^°_{overall} = \mathscr{E}^°_{nitrate} + \mathscr{E}^°_{chloride}\) \(\mathscr{E}^°_{overall} = (-1.188 V) + (-0.755 V) + 0.96 V = -0.983 V\) The overall cell potential for the platinum metal dissolving in aqua regia is negative, indicating that the reverse reaction, where platinum precipitates from the solution, is thermodynamically favorable. However, it is important to recognize that this is just an equilibrium analysis, based on standard reaction conditions. In practice, aqua regia is a highly reactive solution that can dissolve platinum by creating strong oxidizing conditions that drive the reaction forward.

The thermodynamic analysis above suggests that platinum should not dissolve in aqua regia. However, in practice, aqua regia can dissolve platinum because the combined presence of nitrate and chloride ions creates strong oxidizing conditions that drive the reaction forward, overcoming the unfavorable equilibrium. In other words, the kinetics of the reaction in aqua regia is much faster than the thermodynamics would suggest. On the contrary, in concentrated nitric or hydrochloric acid individually, the strong oxidizing conditions necessary to dissolve platinum are not present, and the reverse reaction (platinum precipitating from the solution) remains thermodynamically favorable, in accord with the equilibrium analysis. As a result, platinum will not dissolve in either concentrated nitric or hydrochloric acid individually.