Consider the following half-reactions: Pt^2++2 e^- ⟶Pt ℰ^∘=1.188 V PtCl4^2-+2 e^- ⟶Pt

step 1 So if you look at potassium in an HCL solution, you look at the two relevant half reactions. If

Consider the following half-reactions: Pt^2++2 e^- ⟶Pt ...

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

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

Key Concepts

Standard Reduction Potentials

Standard reduction potentials are a measure of the tendency of a chemical species to gain electrons (undergo reduction). They allow us to determine if a reaction is thermodynamically favorable. In the context of dissolving platinum, comparing the potentials of platinum’s oxidation states with those of the acids’ redox couples helps us understand why certain acids can oxidize platinum while others cannot.

Redox Reaction Mechanism

Redox reactions involve the transfer of electrons between species. For platinum to dissolve, it must be oxidized, meaning it loses electrons. Concentrated nitric acid is a strong oxidizing agent, but its oxidation potential may not be sufficient on its own relative to the complex equilibria involved in Pt chemistry, whereas the mixture in aqua regia provides conditions where platinum can be oxidized effectively.

Complex Ion Formation

Complex ion formation is the process where central metal atoms bind to surrounding ligands, such as chloride ions. In aqua regia, chloride ions from hydrochloric acid coordinate with the oxidized platinum to form soluble complex ions (e.g., PtCl4^2?), which stabilizes the oxidized form of platinum and drives the dissolution reaction forward.

Synergistic Effects in Acid Mixtures

The effectiveness of aqua regia arises from the synergistic action of its components: nitric acid acts as the oxidizing agent to convert platinum to its ionic form, while hydrochloric acid supplies chloride ions that form stable complexes with the platinum ions. This dual mechanism makes the overall reaction thermodynamically favorable, enabling platinum to dissolve, a process that does not occur when either acid is used individually.

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