Electrochemical Series Characteristics, Metal Extraction
The electrochemical series of the elements is the set of elements (Metals and non-metals) arranged according to the algebraically increasing reduction potential. If begins with the most negative (Lowest algebraic) electrode potential of Lithium and increases gradually to 0V for Hydrogen, the reduction potential of the elements in the series are potential of the elements in the series up to the most positive value. The electrochemical series is arranged according to the decreasing oxidation potential of the element. In this tutorial, we will discuss how electrochemical series is used in different applications in chemistry.
Characteristics of electrochemical series
The ability of losing electrons decreases from top to bottom of the electrochemical series. Therefore oxidation ability of metals decreases. (The oxidation ability of non-metals also decreases.)
- Among metals Li is the strongest reducing agent while among non-metals F2 is the strongest oxidizing agent.
- Reducing properties of metals decreases as you go down the series.
- Reducing properties of non-metals decreases as you go down the series.
Metals at the top of the series can displace metals available at the bottom of the series from their salts.
Examples:
ZnCl2(aq) + Mg(s) → MgCl2(aq) + Zn(s)
Magnesium is located at the top of the table than zinc. Therefore, magnesium can replace zinc +2 ion from its salt.
MgCl2(aq) + Zn(s) → X (No reaction)
Zinc is located at the bottom of the table than magnesium. Therefore, zinc cannot replace magnesium +2 ion from its salt.
Any non-metal at the bottom of the electrochemical series can displace a non-metal at the top of the series from their salts
Cl2(aq) + 2KI(aq) → 2KCl(aq) +I2(s)
Chlorine is located at the bottom of the table than iodine. Therefore, chlorine cannot displace (oxidize) iodide ion from its salt and chlorine is reduced to chloride ion.
2KCl(aq) + I2(s) → X (No reaction)
iodine is located at the top of the table than chlorine. Therefore, iodine cannot displace chloride ion from its salt.
Reaction with oxygen and air
- Fe and the other metals above Fe comfortably reacts with oxygen /air (Powder from reacts much easily). Reactivity decreases as you go from top to bottom of the series.
- Metals between Hg and Pb do not undergo combustion such easily. However they from a tight layer of oxides with O2 and they tend to prevent further oxidation of the metal.
- Metals like Ag, Pt and Au do not oxidize at all.
Reaction with water
- Na and other metals above sodium rapidly react with water to give off H2 gas and heat. (Exothermic reaction) Mg is inert towards cold water. However Mg reacts slowly with hot water and reacts at a considerable rate with steam.
- Metals in between Al and Au do not react with cold water under any conditions.
- Metals below Fe are slightly reactive towards steam.
Reaction with acids
All the metals lying above H2 in the electrochemical series can displace H2 with dilute HCl. The rate of reaction increases as you go from bottom to top of the series.
Na(s) +cold. H2O(l) → NaOH(aq) + H2(g)
Other metals below H2 do not react with dilute HCl. However they react with concentrated mineral acids such as HNO3 and H2SO4 to form complex products.
Cu(s) + conc. HNO(aq) → Cu(NO3)2(aq) + NO2(g) + H2O(l)
Reaction with oxides
Oxides of metals below Fe are much easy to reduce with H2 to obtain the metals from its ore. However the oxides of metals above Zn cannot be reduced with H2 to form its metal from its ore.
ZnO(s) + H2(g) → X No reaction
CuO(s) + H2(g) → Cu(s) + H2O(l)
Thermal stability of compounds according to the electrochemical series
Thermal stability of hydroxides
Decomposition of hydroxides of metals up to Cu from the top except Na and K are much easier. They produce their oxides and water.
2 and Ca(OH)2.jpg "thermal decomposition of Mg(OH)2 and Ca(OH)2")
The rate of decomposition increases as you go down the series.
KOH and NaOH are thermally stable.
Hydroxides of metals below Hg are extremely unstable and they do not exist under normal conditions.
Thermal stability of carbonates
Decomposition of carbonates of metals up to Cu are much sensitive to heat. They decompose to form CO2 and their oxides. Descending the series the thermal stability decreases.
Carbonates of Na and K are extremely thermally stable.
Carbonates of metals below Hg are thermally unstable and decomposes to form its metal, CO2 and O2 gases.
Thermal stability Of nitrates
Metal nitrates between Li and Cu in the series are thermally unstable. These metal nitrates except KNO3 and NaNO3 form their oxides NO2 and O2.
However NaNO3 and KNO3 form their nitrite and O2 during their decompositions.
Nitrates of metals below mercury produce the metal, NO2 and O2.
2 AgNO3 NaNO3 MgNO3.jpg "thermal decompositin of metal nitrates Hg(NO3)2 AgNO3 NaNO3 MgNO3")
Natural occurrence of metals according to the electrochemical series
In general, metals at the top of the electrochemical series are available as their chlorides. Metals at the middle of the electrochemical series exists as carbonates, oxides or sulphides. Metals at the bottom of the series are available in free form of the metals.
Metal extraction
According to the position of a metal in the electrochemical series, we can decide how they are extracted from their sources.
- Metals such as Li, K, Na, Ca, Mg (Metals above Fe) could be isolated by electrolysis of fused solution of metal chlorides. (Metals cannot be discharged by the electrolysis of an aqueous solution of metal chlorides.)
- Most of the metals available at the series cannot be recovered through electrolysis.
- These metals can easily be extracted through reduction with carbon or carbon monoxide.
Metals at the bottom of the series are extracted using different methods. The extraction of Au and Ag could be performed as follows.
Copper (Cu) extraction
Copper (Cu) is obtained through a self-reduction method. In this process CuS (copper pyrites) is heated with excess O2 and the product oxides is used as the reducing agent.
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