Question

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

Platinum metal dissolves in aqua regia because the mixture of nitric and hydrochloric acids provides the necessary reaction environment for a complete redox reaction to occur. In aqua regia, chloride ions from HCl create a suitable environment for platinum dissolution, while nitrate ions from HNO₃ provide the required oxidation. However, when platinum is in contact with either concentrated HCl or HNO₃ individually, there is no suitable oxidation half-reaction available, preventing the dissolution of platinum.

Step-by-step solution

1. Identifying redox half-reactions

First, we need to identify which half-reactions will occur when platinum metal comes into contact with each acid individually and in aqua regia. Let's consider the first half-reaction: \( \mathrm{Pt}^{2+} + 2 \mathrm{e}^{-} \longrightarrow \mathrm{Pt} \\ \mathscr{E}^{\circ} = 1.188{\mathrm{V}}\) This is a reduction half-reaction where platinum ions will get reduced to solid platinum metal. Now let's consider the second half-reaction: \( \mathrm{PtCl}_{4}^{2-} + 2 \mathrm{e}^{-} \longrightarrow \mathrm{Pt} + 4 \mathrm{Cl}^{-} \\ \mathscr{E}^{\circ} = 0.755\,\mathrm{V} \) In this half-reaction, platinum ions in a complex with chlorine will get reduced to solid platinum while chlorine ions are released. Finally, let's consider the third half-reaction: \( \mathrm{NO}_{3}^{-} + 4 \mathrm{H}^{+} + 3 \mathrm{e}^{-} \longrightarrow \mathrm{NO} + 2 \mathrm{H}_{2} \mathrm{O} \\ \mathscr{E}^{\circ} = 0.96\,\mathrm{V}\) In this half-reaction, nitrate ions will get reduced to nitric oxide while water is produced. Now, let's analyze these half-reactions in the context of different acids.

2. Analyzing half-reactions with concentrated HCl

When platinum is in contact with concentrated hydrochloric acid (HCl), the relevant half-reaction can be the second one. It involves chlorine ions (producing platinum and chlorine ion release). However, we need an oxidation half-reaction for a complete redox reaction to happen; just having reductants (like Pt²⁺ and Cl⁻) doesn't lead to the dissolution of platinum.

3. Analyzing half-reactions with concentrated HNO3

When platinum is in contact with concentrated nitric acid (HNO₃), the relevant half-reaction can be the third one. It involves nitrate ions (producing nitric oxide and water). However, just like in the case of HCl, we need an oxidation half-reaction for a complete redox reaction to happen.

4. Analyzing half-reactions with aqua regia

Aqua regia is a mixture of nitric acid (HNO₃) and hydrochloric acid (HCl). When platinum is exposed to this mixture, we have the combination of half-reactions that involve both chloride and nitrate ions. The chloride ions \( (4\mathrm{Cl}^{-}) \), which are involved in the second half-reaction, act as the environment where platinum can dissolve; meanwhile, the nitrate ions \( (\mathrm{NO}_{3}^{-}) \) in the third half-reaction can provide the necessary oxidation for a complete redox reaction to happen. Since aqua regia is a mixture of both acids, it provides the necessary reaction environment for platinum to dissolve, which does not happen when platinum is in contact with concentrated HCl or HNO₃ individually.

Key concepts

Platinum Dissolution

Platinum is known for its remarkable resistance to corrosion and chemical attacks, but it can dissolve in a specific type of chemical mixture known as aqua regia. Platinum dissolution is fascinating because it defies the metal's characteristic inertness. Aqua regia, a potent mixture of nitric acid and hydrochloric acid, has the unique ability to dissolve platinum metal. This solution is renowned for its ability to dissolve noble metals such as gold and platinum, which often resist the action of most other acids. The combination of these acids makes a powerful solution that can react with platinum, enabling its dissolution.
Platinum dissolution in aqua regia occurs due to a unique set of redox (reduction-oxidation) reactions that enable the normally resistant platinum to become reactive. This reaction involves both oxidation and reduction processes, allowing platinum to dissolve into the acidic solution as a chloroplatinic acid complex.

Redox Reactions

Redox reactions play a crucial role in the process of platinum dissolution in aqua regia. These reactions involve the transfer of electrons between different chemical species. In the case of platinum dissolution, two half-reactions come into play: reduction of nitrate ions and oxidation involving chloride ions.

• The reduction half-reaction involves nitrate ions from nitric acid, whereby nitrate ions ( \(\mathrm{NO}_{3}^{-}\)) gain electrons and get reduced to nitric oxide, alongside the production of water.
• The oxidation half-reaction involves chloride ions from hydrochloric acid, which react with platinum to form \(\mathrm{PtCl}_{4}^{2-}\), a soluble complex, releasing electrons.

In aqua regia, these redox reactions occur simultaneously, causing the normally inert platinum to become soluble. This creates a complete redox system, with platinum undergoing oxidation and losing electrons, whilst the nitrate ions undergo reduction, completing the electronic exchange necessary for reaction.

Nitric Acid

Nitric acid is an important component of aqua regia. On its own, concentrated nitric acid is not sufficient to dissolve platinum, due to the lack of an accompanying simultaneous oxidation process. However, within aqua regia, nitric acid contributes to the dissolution process by primarily acting as an oxidizing agent.
Nitric acid produces nitrate ions (\(\mathrm{NO}_{3}^{-}\)) that play a vital role in the redox reaction. These ions are responsible for oxidizing other molecules, which, when coupled with the presence of hydrochloric acid, ensure that platinum dissolution can occur.

• Nitric acid contributes the necessary electrons that help reduce nitrate ions, thus fueling the reaction environment.
• This provides the crucial oxidation step that aids in dissolving platinum metal in the acidic solution.

Hydrochloric Acid

Hydrochloric acid is the other vital component in the aqua regia mixture that enables the dissolution of platinum. Though concentrated hydrochloric acid alone cannot oxidize and dissolve platinum, its presence in aqua regia is essential.
Hydrochloric acid facilitates the formation of chloride ions (\(\mathrm{Cl}^{-}\)). These ions are crucial for forming the chloroplatinic acid complex (\(\mathrm{PtCl}_{4}^{2-}\)), which makes platinum soluble.

• Chloride ions help create this compound, effectively converting solid platinum into a dissolved form.
• This interaction sets the stage for the redox reaction to proceed efficiently.

The chloride ions enable the platinum metal to transition into a dissolvable state, thus making hydrochloric acid indispensable in the aqua regia mixture, allowing the dissolution of metals that are otherwise resistant to chemical reactions.