Extracting metals and equilibria Revision Notes

What is Reduction in Extraction?

Reduction is the removal of oxygen from a compound. In metal extraction, many metal ores are metal oxides. These are reduced so the metal is separated from oxygen, allowing it to be obtained in its pure form.

Common Methods of Reduction

Some metals, like iron, are extracted from their ores by heating with carbon, which acts as a reducing agent. More reactive metals often require methods like electrolysis for reduction, while less reactive metals can sometimes be extracted by heating alone.

Temperature in the Haber Process

The Haber process uses a temperature of approximately 450 °C. This is a compromise temperature. While a lower temperature would favour a higher equilibrium yield of ammonia (as the forward reaction is exothermic), a higher temperature is needed to achieve a sufficient rate of reaction. Too low a temperature would make the process uneconomically slow.

Pressure and Catalyst in Haber Process

A high pressure of around 200 atmospheres (atm) is employed in the Haber process. High pressure shifts the equilibrium to the side with fewer gas molecules, which is the product side (ammonia). An iron catalyst is also used. The catalyst increases the rate of both the forward and reverse reactions equally, ensuring that equilibrium is reached more quickly without affecting the final yield. This helps speed up the industrial production of ammonia.

Understanding Displacement Reactions

A displacement reaction occurs when a more reactive element takes the place of a less reactive element in a compound. For metals, the reactivity series helps predict which metal will displace another. The element that displaces undergoes oxidation, and the ion that is displaced undergoes reduction.

Redox in Displacement

All displacement reactions are redox reactions, meaning they involve both reduction and oxidation. Oxidation is the loss of electrons (OIL), and reduction is the gain of electrons (RIG). In a metal displacement, the more reactive metal atom loses electrons (is oxidised) and the less reactive metal ion gains these electrons (is reduced), forming a neutral metal atom. This electron transfer is the core characteristic of a redox process.

Half-Equations for Displacement

To illustrate the electron transfer in displacement, half-equations can be used. For example, in the reaction between zinc and copper sulfate: Zinc atoms (Zn) lose electrons to become zinc ions (Zn²⁺), represented as Zn → Zn²⁺ + 2e⁻ (oxidation). Copper ions (Cu²⁺) gain these electrons to become copper atoms (Cu), represented as Cu²⁺ + 2e⁻ → Cu (reduction). The overall ionic equation shows the displacement and electron transfer clearly.

Stages of a Life Cycle Assessment (LCA)

A Life Cycle Assessment involves evaluating a product's environmental impact from its inception to its disposal. This comprehensive study covers several key stages: raw material extraction and processing, manufacturing, product use, and end-of-life disposal. Each stage contributes to the overall environmental footprint, encompassing factors like energy use, resource depletion, and pollution.

Considerations at Each Stage

During raw material acquisition, the focus is on the environmental impact of obtaining resources, such as mining metals or drilling for oil. Manufacturing involves assessing energy use, waste products, and emissions during production. The use phase examines the energy consumed and maintenance required by the product, as well as any emissions during its operational life. Finally, disposal considers the environmental effects of landfill, recycling, or incineration of the product components once it is no longer useful.