Problem 100 - Chemistry The Central Science In Si Units

Question

Gold exists in two common positive oxidation states, +1 and +3 . The standard reduction potentials for these oxidation states are $$ \begin{array}{l} \mathrm{Au}^{+}(a q)+\mathrm{e}^{-} \quad \longrightarrow \mathrm{Au}(s) \quad E_{\mathrm{red}}^{\circ}=+1.69 \mathrm{~V} \\ \mathrm{Au}^{3+}(a q)+3 \mathrm{e}^{-} \longrightarrow \mathrm{Au}(s) \quad E_{\mathrm{red}}^{\circ}=+1.50 \mathrm{~V} \end{array} $$ (a) Can you use these data to explain why gold does not tarnish in the air? (b) Suggest several substances that should be strong enough oxidizing agents to oxidize gold metal. (c) Miners obtain gold by soaking gold-containing ores in an aqueous solution of sodium cyanide. A very soluble complex ion of gold forms in the aqueous solution because of the redox reaction $$ \begin{aligned} 4 \mathrm{Au}(s)+8 \mathrm{NaCN}(a q) &+2 \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(g) \\ & \longrightarrow 4 \mathrm{Na}\left[\mathrm{Au}(\mathrm{CN})_{2}\right](a q)+4 \mathrm{NaOH}(a q) \end{aligned} $$ What is being oxidized, and what is being reduced in this reaction? (d) Gold miners then react the basic aqueous product solution from part (c) with \(\mathrm{Zn}\) dust to get gold metal. Write a balanced redox reaction for this process. What is being oxidized, and what is being reduced?

Step-by-Step Solution

Answer

Gold does not tarnish due to its high reduction potential. \( \text{Cl}_2 \), \( \text{F}_2 \) are strong oxidizers for gold. In the miner's reaction, Au is oxidized and \( \text{O}_2 \) is reduced; with Zn, Au is reduced, and Zn is oxidized.

Step 1: Understand why gold does not tarnish in air

Gold does not tarnish in air because air does not contain strong enough oxidizing agents to oxidize gold. The standard reduction potential of gold indicating high resistance to oxidation is given as \( E_{\text{red}}^{\circ} = +1.69 \text{ V} \) and \( E_{\text{red}}^{\circ} = +1.50 \text{ V} \) for +1 and +3 states, respectively. Most agents in the air cannot overcome this potential enough to oxidize gold, thus preventing tarnishing.

Step 2: Identify strong oxidizing agents for gold

Potential oxidizing agents that can oxidize gold metal have reduction potentials higher than that of gold. These include agents like \( \text{Cl}_2 \), \( \text{F}_2 \), and \( \text{Br}_2 \), which have higher reduction potentials compared to gold, making them capable of oxidizing gold metal.

Step 3: Analyze redox reaction of miners obtaining gold

In the reaction with sodium cyanide, gold (Au) is being oxidized from its elemental form to a complex ion, turning \( \text{Au} \rightarrow \text{Au}^{+} \). Concurrently, oxygen (\( \text{O}_2 \)) reduces to form hydroxide ions (\( \text{OH}^- \)); thus, \( \text{O}_2 \) is reduced.

Step 4: Write and balance redox reaction for retrieving gold with Zn

The reaction with zinc dust involves gold ions being reduced back to elemental gold, while zinc is oxidized. The balanced redox reaction is: \[ 2 \text{Na} [\text{Au}(\text{CN})_2](aq) + \text{Zn}(s) \rightarrow 2 \text{Au}(s) + \text{Na}_2[\text{Zn(CN)}_4](aq) \]. Here, zinc is oxidized from Zn to \( \text{Zn}^{2+} \), whereas gold is reduced from a complex ion back to its elemental form.

Key Concepts

Oxidation States

Understanding oxidation states is fundamental in identifying the charge or oxidation level of a particular element in a compound. For gold, the oxidation states are commonly +1 and +3, indicating that it can lose one or three electrons to form a stable ion. These states help us understand the possible reactions gold can undergo.
Gold's noteworthy feature is its reluctance to react, which is tied to these oxidation states. They reveal how much energy is needed to remove electrons during redox reactions. Since gold has high reduction potentials in both its +1 and +3 states, as indicated by the positive values of +1.69 V and +1.50 V, it resists oxidation by agents with lower reduction potentials, thus explaining its non-tarnishing nature in ordinary environments like air.
In chemistry, tracking oxidation states aids in predicting and balancing chemical equations, particularly in redox reactions where changes in electron count define the transformation of reactants to products.

Reduction Potential

Reduction potential, represented often as standard reduction potential \(E_{ ext{red}}^{ ext{\circ}}\), measures the tendency of a chemical species to gain electrons and thereby be reduced. The higher the potential, the greater the substance's ability to act as an oxidizing agent, meaning it wants to capture electrons and become reduced itself.
Gold's standard reduction potentials for its +1 and +3 oxidation states, at +1.69 V and +1.50 V respectively, are quite high. These values show that gold can easily gain electrons, thus being a poor candidate to give up electrons to less powerful oxidizers. This high resistance to oxidation contributes to its lack of tarnishing.
Understanding reduction potential is crucial when predicting reactions. It helps in determining whether a redox process will occur spontaneously. For instance, substances with a reduction potential higher than gold, like fluoride \((\text{F}_2)\), chlorine \((\text{Cl}_2)\), and bromine \((\text{Br}_2)\), would be able to oxidize gold because they can effectively pull electrons away from gold.

Oxidizing Agents

Oxidizing agents are substances that accept electrons during a chemical reaction, effectively getting reduced in the process. In a redox equation, identifying the oxidizing agent is key as it drives the reaction by pulling electrons from other substances.
In the context of gold’s reactions, strong oxidizing agents would be those with higher reduction potentials than gold. This makes substances like chlorine \((\text{Cl}_2)\), fluorine \((\text{F}_2)\), and bromine \((\text{Br}_2)\) capable of oxidizing gold, as their desire for electrons surpasses gold's tendency to keep its own.
Knowing the right oxidizing agents helps chemists control and predict the outcomes of redox reactions. It brings a level of predictability to chemical processes, making it possible to harness these reactions in practical applications like mining. For instance, in a gold recovery process, identifying an appropriate oxidizing agent is essential to effectively transform gold into its soluble ionic form for further processing.