Electromotive Series – What is the significance? – Best Tips!

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electromotive series

Electromotive series is the list of atoms, ions, and molecules in order of their tendency to lose or gain electrons by oxidation or reduction, represented in volts, and measured by using the Hydrogen electrode as the reference which standardly takes as the voltage of zero.

Electromotive Series is also known as the Electrochemical Series.

This series places the metallic elements in the order of their electropositivity and is a useful way of classifying them. In assessing the electropositivity of a given metal, it has conventionally agreed to use a hydrogen electrode as the standard of comparison for all others. Hydrogen electrode assigned as zero potential.

Standard Electrode Potential

The hydrogen electrode consists essentially of a platinum plate immersed in a molar solution of hydrogen, ions(from an acid). It Also has hydrogen gas continually bubbling round it. The electrode potential of a given metal I contact with a molar solution of its ions then measured against this hydrogen electrode as zero.

  • If the potential is negative, the metal is electropositive.
  • If the potential is positive, the metal is electronegative.

Of common metals, the most electropositive is Lithium(-3.03Volts). Others follow in the order shown. Below hydrogen, the metals are electronegative with reference to it, e.g., copper, 0.34Volts.

Full Electromotive Series of Metals

MetalPotential (Volts)
Li+ + e ↔ Li(s)-3.03 (Most Electropositive)
K+ + e ↔ K(s)-2.92
Ca++ + 2e ↔Ca(s)-2.87
Na+ + e ↔ Na(s)-2.71
Mg++ + 2e ↔ Mg(s)-2.37
Al+++ + 3e ↔ Al(s)-1.66
Zn++ + 2e ↔ Zn(g)-0.76
Fe++ + 2e ↔ Fe(s)-0.44
Pb++ + 2e ↔ Pb(s)-0.13
2H+ + e ↔ H2(s)0
Cu++ + 2e ↔ Cu(s)+0.34
Ag+ + e ↔ Ag(s)+0.80
Au+++ + 3e ↔ Au(s)+1.50 (Least Electropositive)
Electrochemical Series of Metals

 The greater the negative potential is shown for a metal (i.e., the more electropositive its character), the higher is its tendency to ionize.

So, according to the above series, Lithium is the metal that is most likely to be oxidized. Because Lithium is the element that has the highest Electropositive value (-3.03Volts).

In the cases of iron, tin, and mercury, the figures stated concerning the -ous states of oxidation of the metals.

Calculate Net Ionic Equation of Any Element – Net Ionic Equation Calculator

Many of the significant properties and relations of the metals and their compounds follow fairly strictly the order of this series. So, let’s have a look at the following examples.

What is the Significance of the Electromotive Series?

The significance of the electrochemical series can describe under the main 5 categories. They are,

  1. Displacement of one metal by another
  2. Products if electrolysis
  3. Kind of oxides; their reduction
  4. Nature of carbonates
  5. Nature of nitrates. The action of heat

1. Displacement of one metal by another

Provided it does not attack water and is not “passive” by superficial oxidation. A more electropositive metal in the electromotive series will displace a less electropositive metal from a solution of its salt. So, This displacement takes place by electron transfer from the more electropositive metal (which is oxidized) to the ion of the less electropositive metal. (which is reduced). The ion discharged, and the corresponding metal precipitates. The more electropositive metal passes into solution as ions, e.g., zinc displaces copper from a solution of copper sulfate.

Zn + Cu++ –> Zn++ + Cu

A considerable separation n the series is necessary for rapid displacement.

electromotive series displacement metal

The displacement of hydrogen from water or dilute acid is merely a special case of this reaction. So, All metals which are more electropositive than hydrogen displace it from water from some acid, as stated below.

  • K, Ca, Na – These metals displace hydrogen from cold water. E.g.,

2Na + 2H+ -> 2Na+ + H2

  • Mg, Al, Zn, Fe – These metals displace hydrogen from steam when heated, and from some dilute acid. E.g.,

Zn + 2H+ -> Zn++ + H2

2Al + 6H+ -> 2Al3+ + 3H2

  • Cu, Hg, Ag – They are less electropositive than hydrogen and never displace it directly from acids. They are attacked by oxidizing acids only. i.e., by nitric acid or hot, concentrated sulphuric acid.
electromotive series for Experiment for the displacement of metals
Experiment for the displacement of metals

2. Products of  electrolysis

Metallic ions in solution will tend to accept electrons and precipitate at the cathode during electrolysis in the order of the electromotive series.

The least electropositive metals precipitate first.

electromotive series electrolysis

Thus, in copper sulfate solution containing hydrogen ions from water and copper ions, copper precipitates at the cathode. No hydrogen discharged.

Cu++ + 2e -> Cu

In potassium sulfate solution, the less electropositive hydrogen discharge. Potassium ions do not discharge.

2H+ + 2e -> H2

This simple situation is, however, modified such factors as the nature of the electrode and the relative concentrations of the ions in the solution. For example, at a mercury cathode, sodium ions discharge from aqueous solution in preference to hydrogen ions. Zinc can discharge from aqueous zinc sulfate solution by electrolysis though zinc is more electropositive than hydrogen.

3. Kind of oxides; their reduction

  • K, Ca, Na, Mg – Oxides of these metals are strongly basic and all form alkalis in water except MgO, which has insufficient solubility.  E.g.,

Ca++.O + H2O->Ca++.2OH

None of these oxides reduce by hydrogen, and they are stable towards heat.

  • Al, Zn – Oxides of these metals do not form alkalis. They are insoluble in water. They are predominantly basic but with some acidic character as well. I.e., They are amphoteric.

Basic –   ZnO + H2SO4 -> ZnSO4 + H2O

Acidic – ZnO + 2NaOH -> Na2ZnO2 + H2O

These oxides are not reduced by heating in hydrogen. So, they are stable towards heat.

  • Fe, Pb, Cu, Hg, Ag – Oxides of these metals are basic. They are insoluble in water. Also, Do not form alkalis. PbO is amphoteric. It is acting with acidic character. It can form plumbites (acidic ion, PbO2) with caustic alkalis.

So, All the metals Al, Zn, Fe, and Pb, enter into the formation of acidic ions. For example,

  • AlO2 (Aluminate ion)
  • ZnO2(Zincate ion)
  • FeO2(Ferrite ion)
  • PbO2(Plumbite ion)

Most importantly, Metals preceding and following them in the electromotive series do not process this property.

4. Nature of carbonates

  • K, Na – Carbonates of these metals are soluble in water and stable to heat.
  • Ca, Mg, Al, Zn, Fe, Pb, Cu – Carbonates of these metals are insoluble in water. They decompose by heat to the oxide of the metal. E.g.,

ZnCO3 -> ZnO +CO2

Aluminum forms no carbonate. Copper forms no true carbonate, CuCO3, but has a stable carbonate, CuCO3.Cu(OH)2.

  • Hg – Forms no carbonate.
  • Ag – Formas an insoluble carbonate. It decomposes by heat, leaving the metal because silver oxide is unstable to heat.

2Ag2CO3 -> 4Ag + 2CO2 + O2

5. Nature of nitrates; the action of heat

When heated,

  • K, Na – Nitrates of these metals decompose to nitrates and oxygen.

2KNO3 -> 2KNO2 + O2

  • Ca, Mg, Al, Zn, Fe, Pb, Cu – Nitrates of these metals decompose to oxides, nitrogen dioxide, and oxygen. E.g.,

2Cu(NO3)2 -> 2CuO + 4NO2 + O2

4Al(NO3)3 -> 2Al2O3 + 12NO2 + 3O2

  • Hg, Ag – Nitrates of these metals decompose to the metals, nitrogen dioxide, and oxygen. (The oxides of these metals are unstable to heat. So, Nitrates of mercury would leave no residue if heated strongly because mercury vaporizes at 3500C.)

2AgNO3 -> 2Ag + 2NO2 + O2

2Hg(NO3)2 -> 2Hg + 4NO2 + 2O2

Conclusion of Electromotive Series

So, from this article, you have gained good knowledge about the definition of the Electromotive series, electromotive chart, and what is its significance as well. Read more. Stay with us!

How do you tell which element is most easily oxidized?

Higher Electropositivity means easily oxidize. Lithium most easily oxidize.

Which metal is most easily oxidized Ag, Co, Cu or Mg ?

Mg. Because Mg has the highest electropositivity than Ag, Co and Cu.

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