Question

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.

Short answer

In aqua regia, a mixture of nitric acid (contributing nitrate ions, NO₃⁻) and hydrochloric acid (contributing chloride ions, Cl⁻), the combined presence of nitrate and chloride ions creates strong oxidizing conditions that drive the reaction forward, overcoming its unfavorable thermodynamic equilibrium. This results in the dissolution of platinum metal in aqua regia. In contrast, the strong oxidizing conditions necessary to dissolve platinum are not present in concentrated nitric or hydrochloric acid individually, and the reverse reaction (platinum precipitating from the solution) remains thermodynamically favorable, preventing the dissolution of platinum in either of these acids alone.

Step-by-step solution

Analyze the given half-reactions

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 \(\mathscr{E}^° = 0.96 V\)

Determine the overall reaction for dissolving platinum in aqua regia

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: \(Pt + NO_{3}^{-} + 4H^{+} + 4Cl^{-} \rightarrow PtCl_{4}^{2-} + NO + 2H_{2}O\)

Determine the overall cell potential for the aqua regia reaction

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.

Explain why platinum dissolves in aqua regia but not in concentrated nitric or hydrochloric acid individually

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.

Key concepts

Half-reaction

In electrochemistry, a half-reaction is a part of an overall chemical reaction that involves the transfer of electrons between two species. Each half-reaction explains either the oxidation or the reduction process separately. For example, when platinum \((Pt)\) is involved, you might see its oxidation half-reaction as \(Pt \rightarrow Pt^{2+} + 2e^{-}\). This shows the platinum donating electrons, which is an oxidation process.

Half-reactions are crucial because they allow us to understand and calculate the electron transfer steps in redox reactions.

• Oxidation Half-reactions: These involve a species losing electrons, as with the Pt example mentioned above.
• Reduction Half-reactions: These involve a species gaining electrons, such as nitrate ions \((NO_3^{-})\) gaining electrons in the reduction process.

Understanding how to write and balance these half-reactions is key to solving complex redox equations, and are often used to calculate the potentials of a chemical cell.

Redox potential

Redox potential, also known as reduction potential, measures the tendency of a chemical species to acquire electrons and thereby be reduced. Represented by the symbol \(\mathscr{E}\), it can help predict the direction of a redox reaction. A higher redox potential indicates a greater affinity for electrons and therefore a stronger tendency to be reduced.

When dealing with electrochemical reactions like those involving metal dissolution, the redox potentials can be combined to determine the overall cell potential. This impacts whether a reaction will proceed spontaneously. Here's how you can interpret it:

• A positive overall redox potential suggests that the reaction can proceed spontaneously.
• A negative value indicates the reaction is thermodynamically unfavorable under standard conditions.

In the context of our example with aqua regia, even a reaction with a negative redox potential can occur due to the overcoming of kinetic barriers, highlighting the energetic complexity of redox processes.

Standard electrode potential

The standard electrode potential \((\mathscr{E}^°)\) is a specific type of redox potential measured under standard conditions, meaning 25°C, 1 M concentration, and 1 atm pressure for gases. The values are useful for understanding how a specific substance behaves as a part of a redox reaction when compared to a standard hydrogen electrode, which is assigned a potential of zero volts.

For example, the standard electrode potential for the reduction of \(Pt^{2+}\) to \(Pt\) is \(1.188 \, V\), making it more likely to occur compared to other reactions with lower electrode potentials.

• A higher \(\mathscr{E}^°\) indicates a greater tendency for the reduction to proceed.
• Values can be added to determine the possible spontaneity of an electrochemical cell.

Understanding these potentials is vital for predicting which half-reactions will dominate under given conditions, such as in the dissolving of platinum in aqua regia.

Aqua regia

Aqua regia, meaning "royal water" in Latin, is a corrosive mixture of concentrated nitric acid and hydrochloric acid, typically in a volume ratio of 1:3. This potent combination is known for its ability to dissolve noble metals like gold and platinum, which do not normally react with single acids.

The effectiveness of aqua regia comes from its unique chemistry:

• Nitrate ions \((NO_3^{-})\) from nitric acid act as oxidizing agents, enabling the oxidation of the metal.
• Chloride ions \((Cl^{-})\) stabilize the metal ions as complexes, allowing it to dissolve efficiently.

In practice, while thermodynamic calculations may imply that platinum should resist dissolution, aqua regia's highly reactive nature and the formation of complex ions accelerate the process, explaining why it can dissolve platinum when neither nitric nor hydrochloric acid can do so separately.