Problem 94 - Chemistry The Science in Context

Question

Gold does not dissolve in concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) but readily dissolves in \(\mathrm{H}_{2} \mathrm{SeO}_{4}\) (selenic acid). Which acid is the stronger oxidizing agent?

Step-by-Step Solution

Answer

Answer: Selenic acid is a stronger oxidizing agent compared to sulfuric acid. This is because selenic acid has a higher standard reduction potential, allowing it to accept electrons from gold and dissolve it, whereas sulfuric acid cannot due to its lower standard reduction potential.

Step 1: Identify relevant redox half-reactions

Oxidation of gold from Au(0) to Au(III) is the relevant redox half-reaction for gold dissolution: Au(s) -> Au\(^{3+}\)(aq) + 3e\(^-\) Now let's examine the possible reduction half-reactions for both acids. Sulfuric acid: \(\mathrm{H}_{2} \mathrm{SO}_{4}\)(aq) + 2e\(^-\) -> SO\(_2\)(g) + 2H\(^+\)(aq) Selenic acid: \(\mathrm{H}_{2} \mathrm{SeO}_{4}\)(aq) + 2e\(^-\) -> SeO\(_2\)(g) + 2H\(^+\)(aq)

Step 2: Compare the standard reduction potentials

The standard reduction potential is the tendency for a chemical species to get reduced. The higher the standard reduction potential values, the more likely the species will get reduced. For Au\(^{3+}\)/Au(0) (oxidation of gold), standard reduction potential is +1.50 V. For \(\mathrm{H}_{2} \mathrm{SO}_{4}\)/SO\(_2\), the standard reduction potential is -0.17 V. For \(\mathrm{H}_{2} \mathrm{SeO}_{4}\)/SeO\(_2\), the standard reduction potential is +0.74 V.

Step 3: Compare the ability of each acid to oxidize gold

According to the standard reduction potential values, we can see that \(\mathrm{H}_{2} \mathrm{SeO}_{4}\)/SeO\(_2\) has a positive standard reduction potential (+0.74 V) which is higher than \(\mathrm{H}_{2} \mathrm{SO}_{4}\)/SO\(_2\) (-0.17 V) and Au\(^{3+}\)/Au(0) (+1.50 V). This means that \(\mathrm{H}_{2} \mathrm{SeO}_{4}\) can accept electrons from gold and get reduced to SeO\(_2\), while \(\mathrm{H}_{2} \mathrm{SO}_{4}\) cannot accept electrons from gold due to its lower standard reduction potential.

Step 4: Determine the stronger oxidizing agent

Since the standard reduction potential of \(\mathrm{H}_{2} \mathrm{SeO}_{4}\)/SeO\(_2\) is higher than \(\mathrm{H}_{2} \mathrm{SO}_{4}\)/SO\(_2\) and has a positive value, \(\mathrm{H}_{2} \mathrm{SeO}_{4}\) (selenic acid) is the stronger oxidizing agent compared to \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (sulfuric acid). Therefore, gold dissolves in selenic acid but not in concentrated sulfuric acid because selenic acid is a stronger oxidizing agent.

Key Concepts

Reduction Potential

Reduction potential is a key concept in understanding chemical reactions, especially redox reactions. It helps determine the tendency of a chemical species to gain electrons and undergo reduction. The standard reduction potential is typically represented in volts (V) and is measured against a standard hydrogen electrode.
- A higher standard reduction potential indicates a greater likelihood of the species accepting electrons and being reduced.- This value can be used to predict the direction of electron flow in a redox reaction.
In the context of gold dissolution, comparing the reduction potentials of different substances allows us to identify which oxidizing agent is stronger. In this exercise, the standard reduction potentials given are:- \(\text{For } \mathrm{H}_{2} \mathrm{SO}_{4}/\mathrm{SO}_2 : -0.17\,\text{V}\) and- \(\text{For } \mathrm{H}_{2} \mathrm{SeO}_{4}/\mathrm{SeO}_2 : +0.74\,\text{V}\).
The more positive value of selenic acid suggests it's a better oxidizing agent than sulfuric acid.

Redox Reactions

Redox reactions are a fundamental type of chemical reaction that involve the transfer of electrons between two substances. These reactions comprise two key processes: oxidation and reduction.
- **Oxidation:** This is the loss of electrons from a substance. In the exercise, gold (Au) undergoes oxidation as it loses electrons: \[ \text{Au(s)} \rightarrow \text{Au}^{3+}(\text{aq}) + 3\text{e}^- \]- **Reduction:** This involves the gain of electrons by a substance. The acids (sulfuric and selenic) accept electrons in this process, which can be written as half-reactions.
By pairing an oxidation half-reaction with a reduction half-reaction, a complete redox reaction can be described. In our exercise, the reduction potentials tell us whether a substance can act as an oxidizing agent or not. Selenic acid, with a higher potential, effectively accepts electrons more readily, thus being the better agent to oxidize gold.

Gold Dissolution

Gold's chemical stability makes it resistant to many common acids, such as sulfuric acid. However, it can still be dissolved in certain strong oxidizing environments. One specific reaction where gold dissolves readily is with selenic acid, an effective oxidizing agent.
The mechanism follows fundamental principles of redox chemistry, where:- Gold loses electrons (oxidation), resulting in \(\text{Au} \rightarrow \text{Au}^{3+} + 3\text{e}^-\).- Selenic acid accepts electrons (reduction), facilitating the process.
Key points about gold dissolution include:- **Stability of Au:** Despite being a noble metal, when a strong oxidizing agent is present, gold can be dissolved.- **Role of the oxidizing agent:** The capacity of selenic acid to dissolve gold underscores its ability to effectively accept electrons due to its higher standard reduction potential.
Understanding this process illustrates why selenic acid, not sulfuric acid, can dissolve gold, showcasing the interplay of oxidation and reduction reactions in creating different environments where metals like gold can interact differently.