Extracting metals and equilibria Revision Notes

Bioleaching (Bacterial Extraction)

Bioleaching is a biological process that uses bacteria to extract metals from their ores. Specific bacteria convert metal compounds into soluble compounds, allowing the metal ions to dissolve in water. This creates a leachate solution containing the desired metal. The metal can then be recovered from this solution, often by displacement with a more reactive metal (e.g., iron for copper) or by electrolysis. Bioleaching is particularly useful for low-grade ores where traditional mining methods are uneconomical, but it is a relatively slow process.

Phytoextraction (Plant-based Extraction)

Phytoextraction is a method of metal extraction that involves growing plants in soil containing metal compounds. These 'hyperaccumulator' plants absorb metal ions through their roots and concentrate them in their shoots and leaves. Once the plants have matured and absorbed a significant amount of metal, they are harvested and then incinerated. The ash produced is rich in the desired metal, which can then be extracted. This method is environmentally friendly and can also be used for cleaning up contaminated land. However, like bioleaching, it is a slow process and depends on the growth rate and metal-accumulating capacity of the plants.

Advantages and Disadvantages of Biological Methods

Biological methods like bioleaching and phytoextraction offer several advantages over traditional smelting, including lower energy consumption, reduced air pollution (e.g., less sulfur dioxide and carbon dioxide emissions), and suitability for low-grade ores or contaminated sites. They are generally more environmentally friendly. However, key disadvantages include their slow rate of metal extraction compared to traditional methods, the need for large land areas (especially for phytoextraction), and the potential for leaching of other unwanted substances into the environment. The concentration of metal in the product (leachate or ash) may also be lower initially than from high-grade ores processed by smelting.

Metal Reactions with Water and Steam

Metals such as potassium and sodium react explosively with cold water, rapidly producing hydrogen gas and a hydroxide. Magnesium reacts slowly with cold water but much more quickly with steam, producing magnesium oxide and hydrogen. Iron does not react with cold water but will react slowly with steam, forming iron(II,III) oxide (Fe3O4) and hydrogen. Copper and silver do not react with water or steam, reflecting their low reactivity.

Reactions with Acids and Displacement Reactions

Reactive metals above hydrogen in the reactivity series, like magnesium and zinc, will react with dilute acids to give salt and hydrogen gas. The speed of this reaction shows their relative reactivity. Metals below hydrogen, such as copper, do not react with dilute acids. In solution, a more reactive metal will displace a less reactive metal from its salt solution. For example, placing zinc in copper(II) sulfate solution leads to zinc sulfate and copper being deposited.

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.

Obtaining the Raw Materials

Nitrogen for ammonia production is sourced from the atmosphere through fractional distillation of liquid air. Hydrogen is primarily obtained by reacting methane from natural gas with steam, often via a process called steam reforming.

Reversible Reaction and Ammonia Formation

The production of ammonia involves a reversible reaction: nitrogen and hydrogen react to form ammonia, but ammonia can also decompose back into nitrogen and hydrogen. The balanced chemical equation for this process is N₂(g) + 3H₂(g) ↔ 2NH₃(g).