Activity Series: Introduction
Various metals are found on earth; they have many uses in daily life, from constructing steel bridges to jewellery and medicine. These metals naturally vary in their physical and chemical properties. They react differently with oxygen, water or other elements. This difference in reactivity is used to extract them from their natural sources in high concentration. Calcination, roasting and reduction are a few methods used to extract metals of low reactivity.
Activity Series
Activity series or reactivity series of metals is an arrangement where metals are ordered from highest reactivity to the lowest, i.e., an activity series of metals is a series of metals in descending order of their reactivity.
The reactivity of metals is determined by their electronic configuration and the incomplete outer orbital. Metals are electropositive as they readily lose electrons from the valence shell. Higher metals owing to their larger size, have a lesser hold on the outer electron and tend to lose them more quickly, this results in their higher reactivity.
The reactivity of the metal is measured relative to hydrogen. So, hydrogen, even though not a metal, has been placed in the series as a reference. Here is a table of Activity Series of metals
Methods of Extraction
The metals positioned at the top of the series, such as sodium, potassium, and lithium, are highly reactive and can easily be oxidised. They can displace the metals placed below them lower in the series.
The lower metals in the series are less reactive. In the series, moderately reactive metals like zinc, iron, tin, and lead are found as carbonates or sulphides in their ore. These metals can be extracted by converting them into their oxides and reducing them with aluminium, sodium, calcium and carbon.
Conversion of ores into their corresponding metal oxides can be done by
Calcination
Roasting
The process to be adapted depends on the nature of their ore. Carbonate ores are converted by calcination, and sulphide ores are converted by roasting to their respective metal oxides.
Calcination is a process in which the ore is heated rapidly in the absence of air. For example, Zinc Carbonate (ZnCO3) in the calamine ore is converted into Zinc Oxide (ZnO) by calcination. ZnCO3, upon heating strongly in the absence of air, decompose into ZnO and carbon dioxide (CO2)
ZnCO3 → → ZnO + CO2
Roasting is a process in which the ore is heated rapidly in the presence of air. For example, zinc is found in zinc blend ore in the form of Zinc Sulphide. The zinc blende ore is heated strongly in the presence of air, and the roasting converts ZnS to ZnO and produces sulphur dioxide (SO2).
2ZnS + 3O2 → 2ZnO + 2SO2
Reduction of Metal Oxides
The resultant metal oxides are reduced with the help of carbon, aluminium etc. ZnO is treated with carbon or coke and heated in a furnace- this is a carbon reduction process. Iron, copper and lead are also reduced in this manner.
ZnO + C → Zn + CO
High reactivity metals like aluminium can displace less reactive metals and act as a reducing agent. This method is employed for metal oxides that cannot be reduced by carbon, such as manganese oxide.
Manganese oxide (MnO2) is treated with Aluminium which displaces the metal from its oxide, and free metal is formed.
3MnO2 + 4Al → 3Mn + 2Al2O3 + Heat
Metals which are at the bottom of the activity series are the least reactive metals which are similarly roasted to form metal oxides. This metal can be retrieved from its oxides by heating it in the air. For example, mercury, a very less reactive metal, is found in a sulphide ore called cinnabar, where mercury is present as mercury(II) sulphide, HgS.
Extraction of mercury is done by roasting it in air and converting it into mercury oxide, which is then heated at around 300 oC to decompose into free mercury metal.
Roasting: 2HgS + 3O2 → 2HgO + 2SO2
Reduction: 2HgO → 2Hg + O2
Interesting Facts
Metals are mostly shiny, hard and solid. They are good conductors of electricity and heat.
Mercury is the only metal that is liquid at room temperature.
Key Features
Activity series of metal is a series of metals arranged in descending order of their reactivity.
Topmost metals are the most reactive metals.
Metals in the middle of the series are moderately reactively, while those occupying the bottom are least reactive.
Moderately reactive metals are converted into oxides and subsequently reduced to obtain the free metal.
Least reactive metal oxides can be directly reduced upon heating.
Carbonate ores are decomposed by calcination; sulphide ores are decomposed by roasting.
Calcination is heating the ore in absence of air, while roasting is burning the ore in the presence of air.
FAQs on Extracting Metals Low in the Activity Series
1. What is the reduction reaction?
The reduction reaction is one-half of the redox reaction (reduction-oxidation reaction). According to the classical concept, reduction is a reaction where the addition of hydrogen or electropositive elements takes place, or the removal of oxygen takes place. Metal oxides are converted to their simplest form by reduction using a reducing agent.
2. What is a Thermite reaction?
Thermite reaction is a type of reduction reaction where the metal oxide is reduced to metal by using aluminium powder. The aluminium powder acts as a reducing agent, and the reaction occurs when the metal oxide is heated with aluminium powder. For example, in the extraction of manganese, the thermite reaction is used to reduce Manganese oxide using aluminium powder.
3. What is the Displacement reaction?
Displacement reaction is a type of oxidation-reduction reaction, where an ion or an element moves out of one compound, and another element takes its place, i.e., the incoming element displaces the existing element in the compound and takes its place. When a single element is displaced it's called a single displacement reaction. In metal extraction, a metal of low reactivity can be knocked out of its oxide by a metal of higher reactivity, thereby enriching the less reactive metal.