Question

Gold exists in two common positive oxidation states, \(+1\) and \(+3\). The standard reduction potentials for these oxidation states are $$ \begin{aligned} \mathrm{Au}^{+}(a q)+\mathrm{e}^{-}-\longrightarrow \mathrm{Au}(s) & E_{\mathrm{red}}^{\circ}=+1.69 \mathrm{~V} \\ \mathrm{Au}^{3+}(a q)+3 \mathrm{e}^{-}-\ldots \mathrm{Au}(s) & E_{\mathrm{red}}^{\circ}=+1.50 \mathrm{~V} \end{aligned} $$ (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 goldcontaining 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 $$ \begin{array}{r} 4 \mathrm{Au}(s)+8 \mathrm{NaCN}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(g)-\longrightarrow \\ 4 \mathrm{Na}\left[\mathrm{Au}(\mathrm{CN})_{2}\right](a q)+4 \mathrm{NaOH}(a q) \end{array} $$ 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

Gold does not tarnish in air because its reduction potentials (+1.69 V and +1.50 V) are higher than the oxygen reduction potential (+0.40 V). Strong oxidizing agents that can oxidize gold include fluorine gas (F2), chlorine gas (Cl2), and potassium permanganate (KMnO4). In the gold cyanide reaction, gold (Au) is oxidized to gold(I) ion (Au+) while oxygen (O2) is reduced to hydroxide ions (OH-). To recover gold using Zn dust, zinc (Zn) is oxidized to zinc ion (Zn2+), and gold(I) ion (Au+) is reduced back to gold metal (Au) with the reaction: \(Zn(s) + 4Au^+(aq) \rightarrow Zn^{2+}(aq) + 4Au(s)\).

Step-by-step solution

a) Why Gold Does Not Tarnish in Air?

When a metal tarnishes, it means it reacts with a substance in the air, typically oxygen. In the case of gold, we must compare its reduction potentials with that of the oxygen reduction potential to determine if a reaction is likely to occur. The half-reaction of oxygen and water to form hydroxide ions (reduction of oxygen) is given by: \(O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq), \) with \(E_{red}^\circ = +0.40 V\) Comparing this with the given reduction potentials of gold can help us understand the stability of gold. Gold(I) has a reduction potential of +1.69 V, and gold(III) has a reduction potential of +1.50 V. Since both reduction potentials are higher than that of oxygen reduction, it implies that gold has a lower tendency to be oxidized by oxygen than to be reduced back to its metallic form. Thus, gold remains stable and does not tarnish in the air.

b) Strong Oxidizing Agents for Gold

To identify substances that can oxidize gold, we will be looking for strong oxidizing agents with reduction potentials higher than gold's reduction potentials. Some examples are: 1. Fluorine gas (F2): \(E_{red}^\circ = +2.87 V\) 2. Chlorine gas (Cl2): \(E_{red}^\circ = +1.36 V\) (Note: This can only oxidize Au to Au+) 3. Potassium permanganate (KMnO4): \(E_{red}^\circ = +1.68 V\) (in acidic conditions) These oxidizing agents have higher reduction potentials than gold, and thus possess the tendency to oxidize gold metal.

c) Oxidation and Reduction in Gold Cyanide Reaction

For the reaction: \(4Au(s) + 8NaCN(aq) + 2H_2O(l) + O_2(g) \rightarrow 4Na[Au(CN)_2](aq) + 4NaOH(aq)\) We can identify the oxidation and reduction half-reactions: Oxidation half-reaction: \(4Au(s) \rightarrow 4Au^+(aq) + 4e^-\) Reduction half-reaction: \(O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq)\) In this reaction, gold metal (Au) is being oxidized to gold(I) ion (Au+) and oxygen gas (O2) is being reduced to hydroxide ions (OH-).

d) Redox Reaction for Recovery of Gold using Zn Dust

To recover gold from the basic aqueous product solution containing Na[Au(CN)2] with Zn dust, the redox reaction is as follows: Oxidation half-reaction: \(Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-\) Reduction half-reaction: \(4Au^+(aq) + 4e^- \rightarrow 4Au(s) \) Combining both half-reactions gives the balanced redox equation: \(Zn(s) + 4Au^+(aq) \rightarrow Zn^{2+}(aq) + 4Au(s)\) In this reaction, zinc metal (Zn) is being oxidized to zinc ion (Zn2+), and gold(I) ion (Au+) is being reduced back to gold metal (Au).

Key concepts

Standard Reduction Potential

Understanding the standard reduction potential is crucial when studying chemistry, especially when analyzing redox reactions and the stability of metals. Gold, known for its lack of reactivity, is a great example to explore these ideas. The standard reduction potential measures the tendency of a chemical species to acquire electrons and thereby be reduced. It is recorded in volts and a higher positive value indicates a stronger tendency to gain electrons.

For instance, when comparing the reduction potential of gold's oxidation states, +1.69 V for gold(I) and +1.50 V for gold(III), to that of oxygen's +0.40 V, we see that gold's potential is much higher. This tells us that gold is more likely to gain electrons and return to its metallic state rather than react with oxygen. Consequently, gold does not tarnish in the air, maintaining its characteristic luster over time. In metallurgy, the consideration of reduction potentials allows for the prediction and control of reactions involving metals, which is fundamental for extracting and refining metals, including gold.

Tarnishing of Metals

Tarnishing is a familiar concept when it comes to metals; it's essentially when the surface of a metal dulls or discolors, often due to exposure to air or other chemicals. This phenomenon is a chemical reaction—usually an oxidation process—wherein the metal surface reacts with sulphur compounds, oxygen, or other substances in the environment.

The fact that gold does not tarnish easily is rooted in its high standard reduction potentials, +1.69 V for Au(I) and +1.50 V for Au(III). These potentials indicate that gold has a lower tendency to undergo redox reactions with the common substances it encounters in the air, such as oxygen. Hence, protective coatings, which are required for many metals to prevent tarnishing, are unnecessary for gold. Understanding this concept of tarnishing is vital, not just for those interested in jewelry, but also for applications involving metal surfaces in technology and industry.

Redox Reactions in Metallurgy

Redox reactions are the heart of metallurgy, which is the branch of science dealing with the extraction and processing of metals. These reactions involve the transfer of electrons between substances and play a pivotal role in both the recovery and purification of metals.

For the case of gold, miners immerse gold-containing ores in a solution of sodium cyanide. The reaction that ensues forms a soluble gold cyanide complex. Gold is oxidized while oxygen is reduced, highlighting the movement of electrons integral to redox processes. Subsequently, the addition of zinc dust to the solution triggers another redox reaction: zinc is oxidized and the gold cyanide complex is reduced back to pure gold, which then precipitates out. By harnessing these redox reactions, metallurgists can isolate metals like gold from their ores effectively, which is essential for producing the pure metals that are then used in various industries around the world.