Question

Gold exists in two common positive oxidation states, $+1$ and $+3$. The standard reduction potentials for these oxidation states are $$\begin{array}{rl}A{u}^{\ast }(aq)+{\mathrm{e}}^{-}& ⟶Au(s)E_{\text{red }}^o=+1.69\mathrm{V}\\ A{u}^{3+}(aq)+3{\mathrm{e}}^{-}⟶Au(s)E_{\text{red }}^o=+1.50\mathrm{V}\end{array}$$ (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 aqucous solution of sodium cyanide. A very soluble complex ion of gold forms in the aqueous solution because of the redox reaction $$\begin{array}{c}4\mathrm{Au}(s)+8\mathrm{NaCN}(aq)+2{\mathrm{H}}_2\mathrm{O}(l)+{\mathrm{O}}_2(g)\\ ⟶4\mathrm{Na}[\mathrm{Au}(\mathrm{CN}{)}_2](aq)+4\mathrm{NaOH}(aq)\end{array}$$ 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 $\mathrm{Zn}$ dust to get gold metal. Write a balanced redox reaction for this process. What is being exidized, and what is being reduced?

Short answer

(a) Gold does not tarnish in the air because its standard reduction potentials are higher than that of oxygen, preventing it from being oxidized by O2. (b) Strong oxidizing agents that can oxidize gold include Fluorine (F2), Ozone (O3), Chlorine (Cl2), and Potassium permanganate (KMnO4). (c) In the given redox reaction, gold (Au) is being oxidized, and molecular oxygen (O2) is being reduced. (d) The balanced redox reaction for gold extraction using Zn dust is: $2Na[Au(CN{)}_2](aq)+Zn(s)⟶2Au(s)+Zn(CN{)}_2(aq)+2NaCN(aq)$. In this reaction, gold ions are being reduced, and zinc (Zn) is being oxidized.

Step-by-step solution

a): Explaining why gold does not tarnish in the air

In air, the most common oxidizing agent is molecular oxygen (O2). The standard reduction potential of oxygen is: $O_2(g)+4H^{+}(aq)+4e^{-}⟶2H_{2}O(l)$ with $E_{red}^o=+1.23V$ Gold, in its two oxidation states +1 and +3, has standard reduction potentials of +1.69 V and +1.50 V respectively. As the standard reduction potential of gold is higher than that of oxygen, it will not get oxidized in the air. Hence, gold does not tarnish in the air.

b): Strong oxidizing agents to oxidize gold

Since gold has standard reduction potentials of +1.69 V and +1.50 V, any oxidizing agent with a reduction potential higher than these values will be able to oxidize gold. Some examples of strong oxidizing agents are: - Fluorine (F2) - Ozone (O3) - Chlorine (Cl2) - Potassium permanganate (KMnO4)

c): Oxidation and reduction in the given redox reaction

The given redox reaction is: $$4Au(s)+8NaCN(aq)+2H_{2}O(l)+O_2(g)⟶4Na[Au(CN{)}_2](aq)+4NaOH(aq)$$ In the reaction, the gold (Au) metal loses electrons, changing its oxidation state from 0 to +1, which means it is being oxidized. On the other hand, molecular oxygen (O2) gains electrons, changing its oxidation state from 0 to -2 (in H2O), which means it is being reduced.

d): Balanced redox reaction for gold extraction using Zn dust

Gold miners react the product solution from part (c) with Zn dust. The balanced redox reaction for this process is: $$2Na[Au(CN{)}_2](aq)+Zn(s)⟶2Au(s)+Zn(CN{)}_2(aq)+2NaCN(aq)$$ In this reaction, gold ions gain electrons, changing its oxidation state from +1 to 0, which means it is being reduced. On the other hand, the zinc (Zn) metal loses electrons, changing its oxidation state from 0 to +2, which means it is being oxidized.

Key concepts

Standard Reduction Potential

Understanding 'Standard Reduction Potential' or SRP is key to mastering redox reactions. It's a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Expressed in volts (V), this value is determined under standard conditions: a solute concentration of 1 molar, a pressure of 1 atmosphere for gases, and a temperature of 25°C.

In the context of gold, which can exist in oxidation states of +1 and +3, its reduction potentials are extremely high: +1.69 V and +1.50 V respectively. These positive values indicate a strong tendency to gain electrons and revert to the elemental form, Au(s). Consequently, we can deduce that gold is quite resistant to oxidation; this characteristic explains why gold does not tarnish in the air. Even the oxygen we breathe, with an SRP of +1.23 V, isn't strong enough to oxidize gold, emphasizing gold's noble nature.

Oxidizing and Reducing Agents

Identifying Oxidizing and Reducing Agents

A central aspect of redox chemistry is understanding the role of oxidizing and reducing agents. An oxidizing agent, or oxidant, gains electrons and is reduced in a chemical reaction. Conversely, a reducing agent, or reductant, loses electrons and is oxidized. The strength of these agents is rooted in their standard reduction potentials: stronger oxidizing agents have higher positive SRPs, and stronger reducing agents have lower or negative SRPs.

For gold to be oxidized, an agent with a higher SRP than gold's +1.50 V or +1.69 V is needed. Substances like fluorine (SRP = +2.87 V), ozone, chlorine, and potassium permanganate are potent enough to serve as oxidizing agents capable of oxidizing gold metal due to their high SRPs. The choice of oxidizing agents also determines the products and the feasibility of various industrial and chemical processes, including gold extraction and refinement.

Redox Reaction

The Mechanics of Redox Reactions

Understanding redox reactions is crucial for several industrial processes, including the extraction of metals like gold. A redox reaction consists of two half-reactions: oxidation (loss of electrons) and reduction (gain of electrons). These reactions are essential in energy production, metabolism in living organisms, and industrial processes such as electroplating and battery operation.

In gold mining, the redox reaction involves converting solid gold (Au) into a soluble complex ion via oxidation and then reducing it back to solid gold using another reductant. For example, solid gold reacts with sodium cyanide and oxygen to form a gold cyanide complex. This is an oxidation process where gold loses electrons. Later, zinc dust is used to reduce the gold cyanide complex to solid gold, with zinc being oxidized in the process. The interplay of oxidizing and reducing agents drives these transformations, underlying the concepts of electron transfer and chemical change in redox chemistry.