Question

In \(1899,\) the German chemist Ludwig Mond developed a process for purifying nickel by converting it to the volatile nickel tetracarbonyl \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\left(\right.\) b.p. \(\left.=42.2^{\circ} \mathrm{C}\right)\) : $$\mathrm{Ni}(s)+4 \mathrm{CO}(g) \rightleftarrows \mathrm{Ni}(\mathrm{CO})_{4}(g)$$ (a) Describe how you can separate nickel and its solid impurities. (b) How would you recover nickel? \(\left[\Delta H_{\mathrm{f}}^{\circ}\right.\) for \(\mathrm{Ni}(\mathrm{CO})_{4}\) is \(-602.9 \mathrm{~kJ} / \mathrm{mol} .\)

Short answer

Separate nickel as \(\text{Ni(CO)}_4\), then decompose it by heating to recover pure nickel.

Step-by-step solution

Understanding Nickel Purification

The process involves converting solid nickel to a gaseous form by reacting it with carbon monoxide to form nickel tetracarbonyl. This reaction is reversible and the product is a volatile compound that can be separated easily by heating.

Identifying the Reaction Conditions

The reaction to form nickel tetracarbonyl is as follows: \(\text{Ni}(s) + 4 \text{CO}(g) \rightleftharpoons \text{Ni(CO)}_4(g)\). This reaction occurs at a temperature where nickel tetracarbonyl can exist in its gaseous state thus separating nickel from any solid impurities.

Separating Nickel Tetracarbonyl

At a temperature slightly below its boiling point of \(42.2^{\circ} \mathrm{C}\), nickel tetracarbonyl exists as a gas, while other solid impurities remain solids. This allows for separation of \(\text{Ni(CO)}_4\) from the impurities through a gas-solid separation method.

Recovering Pure Nickel

Once nickel tetracarbonyl is separated, it can be decomposed back into nickel metal by heating it above its boiling point. This will cause the decomposition of \(\text{Ni(CO)}_4\) into \(\text{Ni}(s)\) and \(\text{CO}(g)\). The nickel deposits as a pure solid when this decomposition occurs.

Energy Considerations

The enthalpy of formation \([-602.9 \mathrm{~kJ} / \mathrm{mol}]\) indicates that the reaction to form nickel tetracarbonyl is exothermic. The gaseous \(\text{Ni(CO)}_4\) can be condensed or decomposed into nickel by reversing the conditions (heating to decompose it).

Key concepts

Ludwig Mond

Ludwig Mond was a German chemist known for his significant contribution to the nickel purification process. He developed a groundbreaking method to refine nickel by leveraging its chemical properties to form a gas.
In 1899, Mond discovered that nickel could be purified through a reversible process that involved reacting it with carbon monoxide to form nickel tetracarbonyl. This discovery was pivotal because it allowed for the efficient separation of nickel from its impurities in a volatile form, setting the stage for modern purification methods.
Mond's method capitalized on the chemical tendency of nickel to form a complex with carbon monoxide, effectively enhancing the purity of the metal by enabling its transformation into a gaseous state. His work not only advanced metallurgy but also influenced later developments in chemical engineering.

Nickel Tetracarbonyl

Nickel tetracarbonyl, \( ext{Ni(CO)}_4\), is a volatile compound that plays a crucial role in the purification of nickel. This gas is produced when nickel reacts with carbon monoxide, enabling the nickel to be separated from its solid impurities.
Characterized by its low boiling point of \(42.2^{\circ} \text{C}\), nickel tetracarbonyl easily transitions into a gaseous state within operational temperatures. This property is advantageous because it allows the nickel to be extracted in a volatile form, leaving behind unwanted solid impurities.
Once in gas form, the nickel tetracarbonyl can be condensed or heated to decompose back into pure nickel and carbon monoxide. This unique chemical behavior facilitates both the separation and recovery of nickel in an efficient manner. Nickel tetracarbonyl's volatility and reactivity are central to the effectiveness of Mond's purification process.

Gas-Solid Separation

Gas-solid separation is a technique utilized in the nickel purification process to segregate gas from solid impurities. When nickel reacts with carbon monoxide to form nickel tetracarbonyl, this mixture is exposed to conditions that favor the gaseous state of the nickel complex.
At temperatures slightly below the boiling point of nickel tetracarbonyl, the compound vaporizes, while the impurities remain solid. This differentiation in physical state allows for an effective separation, as the gaseous nickel tetracarbonyl can be easily transported away from the solid contaminants.
This process is crucial for ensuring the purity of nickel because it exploits the unique chemical traits of nickel tetracarbonyl to achieve a clean division between valuable metal and unwanted materials.

Enthalpy of Formation

The enthalpy of formation for nickel tetracarbonyl is \([-602.9 \text{ kJ/mol}]\). This value indicates that the reaction forming nickel tetracarbonyl from nickel and carbon monoxide is exothermic, meaning it releases energy.
Understanding enthalpy of formation is important in the nickel purification process because it provides insight into the energy changes involved. The exothermic nature of the formation of nickel tetracarbonyl suggests that the reaction is thermodynamically favorable, allowing it to proceed efficiently under appropriate conditions.
This energy release must be managed, especially when reversing the reaction to recover pure nickel, as energy input will be necessary to decompose the nickel tetracarbonyl back into its elements. Knowing the enthalpy change helps in designing processes that are both energy-efficient and effective in achieving high purity levels for nickel.