Question

Gold exists in two common positive oxidation states, $+1$ and $+3.$ The standard reduction potentials for these oxidation states are $$\begin{array}{l}{\mathrm{Au}}^{+}(aq)+{\mathrm{e}}^{-}⟶\mathrm{Au}(s)E_{\mathrm{red}}^{\circ }=+1.69\mathrm{V}\\ {\mathrm{Au}}^{3+}(aq)+3{\mathrm{e}}^{-}⟶\mathrm{Au}(s)E_{\mathrm{red}}^{\circ }=+1.50\mathrm{V}\end{array}$$ (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 $$\begin{array}{rl}4\mathrm{Au}(s)+8\mathrm{NaCN}(aq)+2& {\mathrm{H}}_2\mathrm{O}(l)+{\mathrm{O}}_2(g)\\ ⟶& 4\mathrm{Na}[\mathrm{Au}(\mathrm{CN}{)}_2](aq)+4\mathrm{NaOH}(aq)\end{array}$$ 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 Zn dust to get gold metal. Write a balanced redox reaction for this process. What is being oxidized, and what is being reduced?

Short answer

(a) Gold does not tarnish in the air because it has high standard reduction potentials for its oxidation states (+1.69 V and +1.50 V), making it less likely to react with non-metal ions in the environment. (b) F2, with a standard reduction potential of +2.87 V, can oxidize gold metal. (c) In the given redox reaction, gold (Au) is being oxidized, and oxygen (O2) is being reduced. (d) The balanced redox reaction for obtaining gold metal is: $2Zn(s)+4Na[Au(CN{)}_2](aq)\to 2Zn(CN{)}_2(aq)+4NaCN(aq)+4Au(s)$ Zinc (Zn) is being oxidized, and gold (Au) is being reduced.

Step-by-step solution

Part (a): Tarnishing of gold

Tarnishing happens when a metal reacts with a non-metal in the environment, forming a new compound on its surface. In the case of gold, the two common positive oxidation states are ${\text{Au}}^{+}$ and ${\text{Au}}^{3+}$. The standard reduction potentials of these oxidation states are +1.69 V and +1.50 V, respectively. The higher the reduction potential, the less likely the metal will oxidize, or react with other non-metal ions in the environment. Since gold has a high reduction potential, it is less likely to react with other non-metal ions in the air, such as oxygen or sulfur. Therefore, gold does not tarnish in the air.

Part (b): Substances that can oxidize gold

In order to oxidize gold, we need a strong oxidizing agent with a potential higher than that of gold. Some of the well-known oxidizing agents are F2 $(E_{\mathrm{red}}^0=+2.87\mathrm{V})$, Cl2 $(E_{\mathrm{red}}^0=+1.36\mathrm{V})$, $B{r}_2(E_{\mathrm{red}}^0=+1.07\mathrm{V})$, but note that only F2's standard reduction potential is higher than that of gold. Hence, F2 will oxidize gold metal.

Part (c): Identifying the components being oxidized and reduced

We are given the redox reaction: $$4\text{Au}(s)+8\text{NaCN}(aq)+2{\text{H}}_2\text{O}(l)+{\text{O}}_2(g)\to 4\text{Na}[\text{Au}(\text{CN}{)}_2](aq)+4\text{NaOH}(aq)$$ Here, gold ($\text{Au}$) is being oxidized (loses electrons) from $\text{Au}(s)$ to ${\text{Au}}^{+}$ in the complex ions, and oxygen (${\text{O}}_2$) is being reduced (gains electrons) from ${\text{O}}_2(g)$ to ${\text{OH}}^{-}$ in $\text{NaOH}$.

Part (d): Balanced redox reaction for obtaining gold metal

The product solution from part (c) reacts with Zn dust to obtain gold metal. The overall balanced redox reaction is: $$2\text{Zn}(s)+4\text{Na}[\text{Au}(\text{CN}{)}_2](aq)\to 2\text{Zn}(\text{CN}{)}_2(aq)+4\text{NaCN}(aq)+4\text{Au}(s)$$ In this reaction, zinc ($\text{Zn}$) is being oxidized (loses electrons) from $\text{Zn}(s)$ to $\text{Zn}(\text{CN}{)}_2^2$, and gold ($\text{Au}$) is being reduced (gains electrons) from ${\text{Au}}^{+}$ in the complex ions to $\text{Au}(s)$.

Key concepts

Oxidation States

In chemistry, the oxidation state, also known as the oxidation number, is a measure of the degree of oxidation of an atom in a substance. A higher oxidation state refers to more lost electrons, while a lower state indicates fewer lost electrons. Gold is unusual because it exists in two positive oxidation states:

• Gold (I): Known as Au⁺, this state implies that each gold atom has lost one electron.
• Gold (III): Denoted as Au³⁺, this indicates a loss of three electrons per gold atom.

These states affect how gold interacts with other substances in chemical reactions. Understanding gold's oxidation states helps explain why it resists tarnishing and informs processes like gold extraction. This knowledge is fundamental to both chemistry and industry.

Standard Reduction Potential

The concept of standard reduction potential (SRP) is crucial in assessing a substance's readiness to gain electrons, and consequently, its ease to be reduced. The higher the standard reduction potential, the stronger the tendency to gain electrons and be reduced. Gold has relatively high standard reduction potentials:

• Au⁺ to Au has an SRP of +1.69 V.
• Au³⁺ to Au has an SRP of +1.50 V.

These values indicate that gold strongly resists losing its electrons, making it less likely to react negatively when exposed to other elements in air, like oxygen or sulfur. This characteristic helps to explain why gold does not tarnish easily. Evaluating standard reduction potential data can help predict and understand redox reaction outcomes.

Tarnishing

Tarnishing is a chemical reaction that happens when a metal reacts with oxygen, sulfur, or other substances in the air, causing a film to develop on the surface. This film usually results in discoloration. Common metals like silver tarnish easily, forming compounds that change their appearance. Gold, however, is known for its resistance to tarnishing. This is primarily due to its high standard reduction potential. Since gold has high reduction potentials at both oxidation states (+1.69 V and +1.50 V), it is less likely to react with airborne substances. As a result:

• Gold remains shiny and does not form tarnish easily.
• The integrity of the gold in terms of its appearance and chemical properties remains intact over time.

The resilience against tarnishing makes gold highly valuable and desirable for use in jewelry and other applications.

Oxidizing Agents

Oxidizing agents play a crucial role in redox reactions as they accept electrons from other substances, causing those substances to oxidize. For an oxidizing agent to effectively oxidize another material, it must have a higher standard reduction potential than the material it is oxidizing. When it comes to gold, very few substances can oxidize it due to its high standard reduction potential (+1.69 V and +1.50 V). However, some potent oxidizing agents include:

• Fluorine (F₂) with an SRP of +2.87 V, is strong enough to oxidize gold.

These agents require a remarkably higher potential to overcome gold's reluctance to give up its electrons. Knowledge of oxidizing agents is essential in processes such as refining and recycling metals.

Redox Reaction

Redox reaction, a shorthand for reduction-oxidation reaction, describes a chemical process where one substance is reduced, and another is oxidized. Essentially, it deals with the transfer of electrons between chemical species.In the context of gold extraction, the redox reaction is represented as:$$4\text{Au}(s)+8\text{NaCN}(aq)+2{\text{H}}_2\text{O}(l)+{\text{O}}_2(g)\to 4\text{Na}[\text{Au}(\text{CN}{)}_2](aq)+4\text{NaOH}(aq)$$- Gold (Au) undergoes oxidation, losing electrons as it forms complex ions.- Oxygen (O₂) is reduced, gaining electrons to form hydroxide ions (OH^-).This redox system is a critical step in transforming gold ore into a usable product, as it facilitates the separation and recovery of gold from its natural sources.

Gold Extraction

Gold extraction involves separating gold from other materials within its ores. This complex process uses chemical reactions that highlight the unique properties of gold. Key steps in gold extraction include:

• Using sodium cyanide (NaCN): This compound creates a soluble gold complex ($$\text{Na}[\text{Au}(\text{CN}{)}_2]$$) that allows gold to dissolve away from the surrounding materials.
• Redox reactions: Gold is oxidized while cyanide and oxygen play their roles as reactants in the extraction equation.
• Reduction with zinc dust: Finally, gold extraction is completed by introducing zinc, which reduces the gold ions back to solid gold, releasing the metal from the solution.
  This is shown in the balanced reaction:$$2\text{Zn}(s)+4\text{Na}[\text{Au}(\text{CN}{)}_2](aq)\to 2\text{Zn}(\text{CN}{)}_2(aq)+4\text{NaCN}(aq)+4\text{Au}(s)$$

Every successful extraction and refinement stage capitalizes on these principles, awakening the gleaming allure of gold hidden within the ore.