Extracting metals and equilibria Revision Notes

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.

Reactivity Series and Extraction Methods

The reactivity series of metals determines the chemical method suitable for extracting a metal from its ore. Metals less reactive than carbon (e.g., iron, copper) can be extracted by heating their oxides with carbon. Carbon acts as a reducing agent, removing oxygen from the metal oxide. Metals more reactive than carbon (e.g., aluminium, potassium, sodium, calcium, magnesium) cannot be extracted by heating with carbon because carbon is not reactive enough to displace them. These metals generally require electrolysis for extraction.

Cost of Extraction Methods

The cost of extracting a metal is a major factor in determining the chosen method. Heating with carbon is generally a less expensive process as it requires less energy compared to electrolysis. Electrolysis uses a significant amount of electrical energy to melt the ore and pass a current through it, making it much more costly. This economic consideration influences which extraction method is commercially viable for different metals, with more reactive metals incurring higher extraction costs due to their reliance on energy-intensive electrolytic processes.

Examples: Iron and Aluminium

Iron is less reactive than carbon. It can therefore be extracted from its iron oxide ore by heating with carbon. This is a relatively inexpensive method. Aluminium is more reactive than carbon. It cannot be extracted by heating its oxide with carbon. Instead, aluminium is extracted from its ore (bauxite) using electrolysis, which is a much more expensive process due to the high energy demands of melting the ore and maintaining the large electric current required.

Extraction of Most Metals

Most metals exist in the Earth's crust within ores. Ores are rocks containing metal compounds in sufficient quantities to make extraction economically viable. These metal compounds need chemical processing, often reduction, to obtain the pure metal. Examples include iron from iron oxide and aluminium from bauxite.

Unreactive Metals

Unreactive metals are those that do not easily react with other elements like oxygen or sulfur. Due to their low reactivity, these metals are found in the Earth's crust in their uncombined, elemental form. This means they do not need to be extracted from compounds; instead, they are physically separated from other materials. Gold, silver, and platinum are common examples of such metals.

The Reactivity Series

The reactivity series is an ordered list of metals based on their chemical reactivity. Generally, more reactive metals are at the top, and less reactive metals are at the bottom. This order reflects how easily a metal loses electrons to form positive ions (cations).

Oxidation and Reactivity

Oxidation is the loss of electrons by a substance. For metals, this means reacting with oxygen, water, or acids. Metals higher in the reactivity series readily lose electrons, so they are easily oxidised and are less resistant to oxidation. Metals lower in the reactivity series do not lose electrons easily; thus, they are more resistant to oxidation.

Examples and Trends

Highly reactive metals like potassium and sodium oxidise very quickly, even in air, and are often stored under oil. Moderately reactive metals like iron oxidise slower but corrode over time (e.g., rusting). Unreactive 'noble' metals such as gold and platinum show very high resistance to oxidation and corrosion, maintaining their lustre for long periods. This relationship allows us to predict how different metals will react under various conditions.