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 does it mean for a metal to be 'low in the activity series'?
This means the metal is very unreactive. Metals like gold, silver, platinum, and mercury do not easily form compounds with other elements like oxygen or sulphur. Because of their stability, they are often found in their free or native state in nature and are easier to extract.
2. How are metals with low reactivity typically extracted from their ores?
The extraction process for low-reactivity metals is quite simple. It usually involves two main steps:
- Metals found as sulphide ores (like cinnabar, HgS) are first heated strongly in air. This process is called roasting, which converts the sulphide into an oxide.
- The metal oxide is then heated further. This heating alone is enough to reduce the oxide to its pure metal form.
3. Can you provide an example of extracting a low-reactivity metal like mercury?
Certainly. Mercury is extracted from its ore, cinnabar (HgS). First, the cinnabar is roasted by heating it in air, which turns it into mercuric oxide (HgO). Then, this mercuric oxide is simply heated to a higher temperature, causing it to decompose easily into pure liquid mercury (Hg) and oxygen gas.
4. Why are these low-reactivity metals often found as simple oxides or sulphides?
This is due to their chemical stability. Since these metals are not very reactive, they have a very weak tendency to form chemical bonds. When they do react, it is usually with common elements like oxygen or sulphur to form simple, unstable compounds. They are too unreactive to form complex ores like carbonates or nitrates, which are common for more reactive metals.
5. Why is simple heating enough to extract these metals, without needing a reducing agent?
The chemical bonds in the oxides of low-reactivity metals are quite weak and unstable. The metal atom does not hold on tightly to the oxygen atom. Therefore, just supplying heat energy provides enough energy to break these bonds and separate the pure metal. Stronger reducing agents, like carbon, are not necessary for this process.
6. How does the extraction of low-reactivity metals compare to that of highly reactive metals like sodium?
The methods are completely different because of the vast difference in reactivity:
- Low-reactivity metals (e.g., Mercury, Silver): Extracted by simple heating of their ores because their compounds are unstable.
- Highly reactive metals (e.g., Sodium, Aluminium): Form extremely stable compounds. They cannot be extracted by heating and require a powerful process called electrolysis to force the metal out of its molten ore using electricity.
7. What happens to the non-metal part, like sulphur, during the extraction process?
The non-metal part is typically removed as a gas. For instance, when a sulphide ore is roasted, the sulphur combines with oxygen from the air to form sulphur dioxide (SO₂). This gaseous byproduct is often collected to prevent air pollution and can be used in other industrial processes, such as the manufacturing of sulphuric acid.
