4.1 Obtaining and using metals Flashcards
Reactivity of metals
The chemistry of the metals is seen by comparing their characteristic reactions.
Based on these reactions a reactivity series of metals can be produced.
The series can be used to place a group of metals in order of reactivity based on the observations of their reactions with water, acids and salts.
Reactivity series
Potassium, sodium, lithium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold.
Reaction with water
In general, when a metal reacts with water it produces a metal hydroxide and hydrogen gas.
Ca (s) + 2H2O (l) ⟶ Ca(OH)2 (aq) + H2 (g)
calcium + water ⟶ calcium hydroxide + hydrogen
Reaction with dilute acids
Only metals above hydrogen in the reactivity series will react with dilute acids.
The more reactive the metal then the more vigorous the reaction will be.
Metals that are placed high on the reactivity series such as potassium and sodium are very dangerous and react explosively with acids.
metal + acid ⟶ salt + hydrogen
Reactivity with metal salts
The reactivity between two metals can be compared using displacement reactions in salt solutions of one of the metals.
This is easily seen as the more reactive metal slowly disappears from the solution, displacing the less reactive metal.
Mg + CuSO4→ MgSO4 + Cu
Oxidation
In all these reactions the more reactive metals lose electrons to become cations.
The more reactive the metal the more easily it becomes a cation.
The loss of electrons is oxidation.
The higher up the metal is in the reactivity series the more easily it will undergo oxidation.
Unreactive metals are therefore more resistant to oxidation.
Redox
Redox reactions occur when electrons are transferred between substances.
* Oxidation is a loss of electrons. OIL
* Reduction is a gain of electrons RIG.
Both oxidation and reduction happen at the same time, hence the term redox. Oxidation and reduction can also be defined in terms of loss or gain of oxygen but on this page, they’re referring to the transfer of electrons.
Sources of metal ores
The Earth’s crust contains metals and metal compounds such as gold, copper, iron oxide and aluminium oxide.
Useful metals are often chemically combined with other substances forming ores.
A metal ore is a rock that contains enough of the metal to make it worthwhile extracting.
They have to be extracted from their ores through processes such as electrolysis, using a blast furnace or by reacting with more reactive material.
In many cases the ore is an oxide of the metal, therefore the extraction of these metals is a reduction process since oxygen is being removed.
Common examples of oxide ores are iron and aluminium ores which are called haematite and bauxite respectively.
Extracting metals
The most reactive metals are at the top of the series.
The tendency to become oxidised is thus linked to how reactive a metal is and therefore its position on the reactivity series.
Metals higher up are therefore less resistant to oxidation than the metals placed lower down which are more resistant to oxidation.
The position of the metal on the reactivity series determines the method of extraction.
Higher placed metals (above carbon) have to be extracted using electrolysis as they are too reactive and cannot be reduced by carbon.
Lower placed metals can be extracted by heating with carbon which reduces them.
Sources of unreactive metals
Unreactive metals do not have to be extracted chemically as they are often found as the uncombined element.
They are known as native metals.
This occurs as they do not easily react with other substances due to their chemical stability.
Examples include gold and platinum which can both be mined directly from the Earth’s crust.
Bioleaching & Phytomining
Extraction of metal ores from the ground is only economically viable when the ore contains sufficiently high proportions of the useful metal, such as iron ores and aluminium ores.
For low grade ores (ores with lower quantities of metals) other techniques are being developed to meet global demand.
This is happening in particular with nickel and copper as their ores are becoming more and more scarce.
Phytoextraction and bioleaching (bacterial) are two relatively new methods of extracting metals that rely on biological processes.
Both of these methods avoid the significant environmental damage caused by the more traditional methods of mining.
Traditional mining involves a great deal of digging, moving and disposing of large amounts of rock.
Biological methods are, however, very slow and also require either displacement or electrolysis to purify the extracted metal.
Both techniques are also used to extract metals from mining wastes, which may contain small quantities of metals or toxic metals that need to be removed from that environment.
Phytomining
This process takes advantage of how some plants absorb metals through their roots.
The plants are grown in areas known to contain metals of interest in the soil.
As the plants grow the metals are taken up through the plants vascular system and become concentrated in specific parts such as their shoots and leaves.
These parts of the plant are harvested, dried and burned.
The resulting ash contains metal compounds from which the useful metals can be extracted by displacement reactions or electrolysis.
Bioleaching
Bioleaching is a technique that makes use of bacteria to extract metals from metal ores.
Some strains of bacteria are capable of breaking down ores to form acidic solutions containing metals ions such as copper(II).
The solution is called a leachate which contains significant quantities of metal ions.
The ions can then be reduced to the solid metal form and extracted by displacement reactions or electrolysis.
This method is often used to extract metals from sulfides e.g. CuS or Fe2S
Although bioleaching does not require high temperatures, it does produce toxic substances which need to be treated so they don’t contaminate the environment.
Bioleaching is not only used for the primary extraction of metals, but it is also used in mining waste clean up operations.
Recycling metals
Everyday materials such as metals are produced from natural but finite sources.
Some products made from these materials can be reused which saves energy and decreases the environmental impact.
Metals can be melted and recast into new shapes.
Sometimes the materials being recycled need to be kept separate, depending on what the use of the recycled material will be.
Economical benefits to recycling metals
It is economically beneficial to recycle metals, especially those that are costly to extract such as aluminium.
Recycling is fast becoming a major industry and provides employment which feeds back into the economy.
Environmental impact of recycling metals
Mining and extracting metal from ores has detrimental effects on the environment and ecosystems.
It is much more energy efficient to recycle metals than it is to extract them as melting and re-moulding requires less energy.
Recycling decreases the amount of waste produced, hence saving space at landfill sites and energy in transport.
There is a limited supply of every material on Earth.
As global populations increase there is greater need for effective recycling methods to attain sustainable development.
Mining and extraction use up valuable fossil fuels, which contributes to climate change.
Disadvantages of recycling metals
Collection and transport of material to be recycled requires energy and fuel.
Workers, vehicles and worksites need to be organised and maintained.
Materials need to be sorted before they can be recycled which also requires energy and labour.
Products made from recycled materials may not always be of the same quality as the original.
Life cycle assessment
A life cycle assessment (LCA) is an analysis of the overall environmental impact that a product may have throughout its lifetime.
The cycle is broken down into four main stages which are:
Raw Materials
Manufacture
Usage
Disposal
A life cycle assessment is carried out using the data of a given product and the criteria of the assessment.
Rarely is there a perfect product with zero environmental impact, so often a compromise is made between environmental impact and economical factors.
Raw materials
Obtaining the necessary raw materials has an impact on the environment which may include:
Using up limited resources such as ores and crude oil.
Damaging habitats through deforestation or mining.
Manufacturing
Manufacturing processes also have an impact on the environment which may include:
Using up land for factories.
The use of fossil fuelled machines for production and transport.
Usage
Usage of a product may also affect the environment although it depends on the type of product.
For example, a wooden desk has very little impact whereas a car will have a significant impact (air pollution).
Disposal
The disposal of outdated products has an impact on the environment which may include:
Using up space at landfill sites.
Whether the product or its parts can be recycled.
