Obtaining and using metals

• Reactivity of Metals

  The chemistry of the metals is seen by comparing their characteristic reactions
• Reactivity series of metals

  A series that 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
• Reaction with water

  • In general, when a metal reacts with water it produces a metal hydroxide and hydrogen gas
  • The reactions of potassium and sodium are covered in more detail in another section, but the reaction with calcium and water is given here for reference
  • The reactions with magnesium, iron and zinc and cold water are very slow
• 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
• Reaction with dilute acids

  Metal + acid ⟶ salt + hydrogen
• Acid-Metal Reactions

  • Mg + CuSO4 → MgSO4 + Cu
• Displacement reactions in salt solutions

  • The reactivity between two metals can be compared using displacement reactions in salt solutions of one of the metals
  • The more reactive metal slowly disappears from the solution, displacing the less reactive metal
• Oxidation

  The loss of electrons by a metal to become a cation
• Reduction

  The gain of electrons by a metal
• Deducing Redox Change in Displacement Reactions

  Identify which species undergoes oxidation and which species undergoes reduction
• Metal ores

  • 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
• Native metals

  Unreactive metals that do not have to be extracted chemically as they are often found as the uncombined element
• Extraction methods

  • Electrolysis
  • Heating with carbon
• Position of metal on reactivity series

  Determines the method of extraction
• Bioleaching & Phytomining

  • Extraction of metal ores from the ground is only economically viable when the ore contains sufficiently high proportions of the useful metal
  • For low grade ores (ores with lower quantities of metals) other techniques are being developed to meet global demand
  • Phytoextraction and bioleaching (bacterial) are two relatively new methods of extracting metals that rely on biological processes
  • Both techniques avoid the significant environmental damage caused by the more traditional methods of mining
  • Both techniques are also used to extract metals from mining wastes
• Bioleaching

  Technique that makes use of bacteria to extract metals from metal ores
• Phytomining

  Process that takes advantage of how some plants absorb metals through their roots
• Extraction of metal ores from the ground is only economically viable when the ore contains sufficiently high proportions of the useful metal
• For low grade ores, other techniques are being developed to meet global demand, in particular with nickel and copper as their ores are becoming more and more scarce
• Bioleaching and phytomining

  • Rely on biological processes
  • Avoid the significant environmental damage caused by more traditional mining methods
• Biological methods are very slow and also require either displacement or electrolysis to purify the extracted metal
• Bioleaching and phytomining are 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

  • 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

  • Some strains of bacteria are capable of breaking down ores to form acidic solutions containing metal 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 sulphides e.g. CuS or FeS
  • Although bioleaching does not require high temperatures, it does produce toxic substances which need to be treated so they don't contaminate the environment
• Phytoextraction and bioleaching are principally used for copper extraction due to the high global demand for copper, but these methods can be applied to other metals
• Recycling metals

  • 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
• Iron for example can be recycled together with waste steel as both materials can be added to a blast furnace, reducing the use of iron ore
• Advantages of recycling metals

  • It is economically beneficial, especially for costly to extract metals like aluminium
  • Recycling is fast becoming a major industry and provides employment which feeds back into the economy
  • Mining and extracting metal from ores has detrimental effects on the environment and ecosystems
  • It is much more energy efficient to recycle metals than 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, and 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
• Iron ore supplies can be conserved and will last longer if iron is recycled
• 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 (LCA)

  • An analysis of the overall environmental impact that a product may have throughout its lifetime
  • The cycle is broken down into four main stages: raw materials, manufacture, usage, and disposal
• Environmental impacts of LCA stages

  • Raw materials: Using up limited resources, damaging habitats
  • Manufacturing: Using land for factories, fossil fuelled machines for production and transport
  • Usage: Depends on the product, e.g. a wooden desk has little impact, a car has significant impact
  • Disposal: Using up space at landfill sites, whether the product or its parts can be recycled
• Rarely is there a perfect product with zero environmental impact, so often a compromise is made between environmental impact and economical factors
• Life cycle assessments are objective exercises as it is difficult to quantify each stage, so LCAs can therefore be biased
• Considering both life-cycle assessments, the plastic bag may be the better option. Even though they aren't biodegradable, they do have a much longer lifespan and thus are less harmful than paper bags
• Much depends on the usage of the item - if the paper bag is recycled then it could be more favourable to use it, if the plastic bag is used only once, then the argument for using plastic bags is less favourable