Gold exists in two common positive oxidation states, +1 and +3 . The standard reduction potentials for these oxidation states are $$ \begin{array}{l} \mathrm{Au}^{+}(a q)+\mathrm{e}^{-} \quad \longrightarrow \mathrm{Au}(s) \quad E_{\mathrm{red}}^{\circ}=+1.69 \mathrm{~V} \\ \mathrm{Au}^{3+}(a q)+3 \mathrm{e}^{-} \longrightarrow \mathrm{Au}(s) E_{\mathrm{red}}^{\circ}=+1.50 \mathrm{~V} \end{array} $$ (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

01

Using reduction potentials to explain non-tarnishing of gold

The reduction potentials given are: $$ \mathrm{Au}^{+}(a q)+\mathrm{e}^{-} \longrightarrow \mathrm{Au}(s) \quad E_{\mathrm{red}}^{\circ}=+1.69 \mathrm{~V} \\ \mathrm{Au}^{3+}(a q)+3 \mathrm{e}^{-} \longrightarrow \mathrm{Au}(s) \quad E_{\mathrm{red}}^{\circ}=+1.50 \mathrm{~V} $$ The positive values of the reduction potentials mean that gold is readily reduced, but difficult to oxidize. It means that the gold metal itself is not easily oxidized by the common oxidizing agents in the air, such as oxygen or moisture, preventing it from tarnishing. (b) Suggest several substances that should be strong enough oxidizing agents to oxidize gold metal

02

Identifying strong enough oxidizing agents

To find substances that can oxidize gold metal, we must look for oxidizing agents with a standard reduction potential greater than those of gold, which are +1.69V and +1.50V. Examples of such strong oxidizing agents are fluorine (F2) and chlorine (Cl2), with standard reduction potentials of +2.87V and +1.36V, respectively. (c) Identify what is being oxidized and what is being reduced in the given redox reaction

03

Oxidation and reduction in the reaction

In the reaction: $$ 4 \mathrm{Au}(s)+8 \mathrm{NaCN}(a q) +2 \mathrm{H}_{2}\mathrm{O}(l)+\mathrm{O}_{2}(g) \longrightarrow 4 \mathrm{Na}\left[\mathrm{Au}(\mathrm{CN})_{2}\right](a q)+4 \mathrm{NaOH}(a q) $$ The gold metal (Au) is being oxidized, as it loses electrons and forms a complex ion with cyanide (\(\mathrm{Au}(\mathrm{CN})_{2}\)), and oxygen gas (O2) is being reduced, as it gains electrons and forms hydroxide ions (OH-) as part of sodium hydroxide (NaOH). (d) Write a balanced redox reaction for the process of obtaining gold metal using Zn dust and identify what is being oxidized and what is being reduced

04

Balanced redox reaction and identifying oxidized and reduced species

The balanced redox reaction for the process of obtaining gold metal using Zn dust is: $$ 2 \mathrm{Zn}(s) + 4\mathrm{Na}\left[\mathrm{Au}(\mathrm{CN})_{2}\right](a q) \longrightarrow 2 \mathrm{Zn}(\mathrm{CN})_{2}(a q) + 4 \mathrm{NaCN}(a q) + 4 \mathrm{Au}(s) $$ In this reaction, zinc (Zn) is being oxidized, as it loses electrons and forms zinc cyanide (Zn(CN)2), and the complex gold ion (\(\mathrm{Au}(\mathrm{CN})_{2}\)) is being reduced, as it gains electrons and forms gold metal (Au).

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