Activity Series of Metals: The electrochemical, electromotive, or activity series of the elements is formed when the electrodes (metals and nonmetals) in contact with their ions are ordered on the basis of the values of their standard reduction potentials or standard oxidation potentials. A set of standard electrode potentials has been constructed by measuring the potentials of various electrodes vs the standard hydrogen electrode \(\left( {{\rm{SHE}}} \right)\).

Electrochemical Series or Activity Series

The standard potentials of electrodes for reduction half-reactions are tabulated according to international convention, reflecting the electrodes’ proclivity to behave as cathodes towards \({\rm{SHE}}\). Electrodes with positive \({\rm{E^\circ }}\) values for reduction half-reactions act as cathodes against \({\rm{SHE}}\), whereas those with negative E° values for reduction half-reactions act as anodes. The electrochemical series is listed in the table below. The standard state potential of a cell is its potential at standard state circumstances, which are approximated at \(1\) mole per litre \(\left( {1{\rm{ M}}} \right)\) concentrations and \(1\) atmosphere pressure at \(25^\circ {\rm{C}}\). The electrochemical series is given below.

Element	Electrode Reaction (Reduction)	Standard Electrode Reduction potential \({\rm{E^\circ }}\), volt
\({\rm{Li}}\)	\({\rm{L}}{{\rm{i}}^ + } + {{\rm{e}}^ – } \to {\rm{Li}}\)	\( – 3.05\)
\({\rm{K}}\)	\({{\rm{K}}^ + } + {{\rm{e}}^ – } \to {\rm{K}}\)	\( – 2.925\)
\({\rm{Ca}}\)	\({\rm{C}}{{\rm{a}}^{{\rm{2 + }}}}{\rm{ + 2}}{{\rm{e}}^{\rm{ – }}} \to {\rm{Ca}}\)	\( – 2.87\)
\({\rm{Na}}\)	\({\rm{N}}{{\rm{a}}^ + } + {{\rm{e}}^ – } \to {\rm{Na}}\)	\( – 2.714\)
\({\rm{Mg}}\)	\({\rm{M}}{{\rm{g}}^{2 + }} + {\rm{2}}{{\rm{e}}^ – } \to {\rm{Mg}}\)	\( – 2.37\)
\({\rm{Al}}\)	\({\rm{A}}{{\rm{l}}^{3 + }} + 3{{\rm{e}}^ – } \to {\rm{Al}}\)	\( – 1.66\)
\({\rm{Zn}}\)	\({\rm{Z}}{{\rm{n}}^{2 + }} + 2{{\rm{e}}^ – } \to {\rm{Zn}}\)	\( – 0.7628\)
\({\rm{Cr}}\)	\({\rm{C}}{{\rm{r}}^{3 + }} + 3{{\rm{e}}^ – } \to {\rm{Cr}}\)	\( – 0.74\)
\({\rm{Fe}}\)	\({\rm{F}}{{\rm{e}}^{2 + }} + 2{{\rm{e}}^ – } \to {\rm{Fe}}\)	\( – 0.44\)
\({\rm{Cd}}\)	\({\rm{C}}{{\rm{d}}^{2 + }} + 2{{\rm{e}}^ – } \to {\rm{Cd}}\)	\( – 0.403\)
\({\rm{Ni}}\)	\({\rm{N}}{{\rm{i}}^{2 + }} + 2{{\rm{e}}^ – } \to {\rm{Ni}}\)	\( – 0.25\)
\({\rm{Sn}}\)	\({\rm{S}}{{\rm{n}}^{2 + }} + 2{{\rm{e}}^ – } \to {\rm{Sn}}\)	\( – 0.14\)
\({{\rm{H}}_{\rm{2}}}\)	\(2{{\rm{H}}^ + } + 2{{\rm{e}}^ – } \to {{\rm{H}}_2}\)	\(0.00\)
\({\rm{MgCu}}\)	\({\rm{C}}{{\rm{u}}^{2 + }} + 2{{\rm{e}}^ – } \to {\rm{Cu}}\)	\( + 0.337\)
\({{\rm{l}}_{\rm{2}}}\)	\({{\rm{I}}_2} + 2{{\rm{e}}^ – } \to 2{{\rm{I}}^ – }\)	\( + 0.535\)
\({\rm{Ag}}\)	\({\rm{A}}{{\rm{g}}^ + } + {{\rm{e}}^ – } \to {\rm{Ag}}\)	\( + 0.799\)
\({\rm{Hg}}\)	\({\rm{H}}{{\rm{g}}^{2 + }} + 2{{\rm{e}}^ – } \to {\rm{Hg}}\)	\( + 0.885\)
\({\rm{B}}{{\rm{r}}_{\rm{2}}}\)	\({\rm{B}}{{\rm{r}}_2} + 2{{\rm{e}}^ – } \to 2{\rm{B}}{{\rm{r}}^ – }\)	\( + 1.08\)
\({\rm{C}}{{\rm{l}}_{\rm{2}}}\)	\({\rm{C}}{{\rm{l}}_2} + 2{{\rm{e}}^ – } \to 2{\rm{C}}{{\rm{l}}^ – }\)	\( + 1.36\)
\({\rm{Au}}\)	\({\rm{A}}{{\rm{u}}^{3 + }} + 3{{\rm{e}}^ – } \to {\rm{Au}}\)	\( + 1.50\)
\({{\rm{F}}_{\rm{2}}}\)	\({{\rm{F}}_2} + 2{{\rm{e}}^ – } \to 2\;{{\rm{F}}^ – }\)	\( + 2.87\)

Characteristics of Electrochemical Series

The substances that are more powerful reducing agents than hydrogen are positioned above hydrogen in the series and have standard reduction potentials that are negative.
All of the chemicals in the series below hydrogen that have positive reduction potentials are weaker reducing agents than hydrogen. In the series, substances that are more strong oxidisers than the \({{\rm{H}}^{\rm{ + }}}\) ion are mentioned after hydrogen. The metals at the top (with high negative standard reduction potentials) have a proclivity for losing electrons. These are metals that are in use. The nonmetals at the bottom (with high positive standard reduction potentials) have a proclivity for accepting electrons. These are nonmetals that are active.

Applications of Electrochemical Series

Reactivity of Metals

The activity of a metal is determined by its tendency to lose electrons or form cations. The magnitude of the standard reduction potential influences this tendency. The metal with a high negative (or smaller positive) standard reduction potential rapidly loses an electron or electrons and becomes a cation. Chemical activity is a term used to describe a metal like this.

Electropositive Character of Metals

The tendency to lose electrons or electrons also influences the electropositive character. The electropositive feature of metals decreases from top to bottom in the electrochemical series, similar to reactivity. To determine if a given metal will displace another from its salt solution, use the following formula: A metal higher in the series will evict a metal lower in the series from its solution, i.e., a metal with low standard reduction potential will evict a metal from its salt’s solution with a greater standard reduction potential value. A metal higher in the series has a greater tendency to provide electrons to the cations of the metal to be precipitated.

Displacement of Hydrogen from Dilute Acids by Metals:

Dilute acids are reduced by a metal that may donate electrons to \({{\rm{H}}^{\rm{ + }}}\) ions in the presence of dilute acids. A metal with a negative reduction potential has the ability to lose an electron or electrons. As a result, the metals at the top of the electrochemical series readily hydrogen from dilute acids, but the tendency to liberate hydrogen gas from dilute acids reduces as the series progresses. \({\rm{Cu}},{\rm{ Hg}},{\rm{ Au}},{\rm{ Pt}}\), and other metals in the electrochemical series below hydrogen do not produce hydrogen from dilute acids.

Displacement of Hydrogen from Water

The metals above iron have the ability to liberate hydrogen from water. In the electrochemical series, the tendency diminishes from top to bottom. Coldwater liberates hydrogen for alkali and alkaline earth metals, but hot water or steam liberates hydrogen for \({\rm{Mg}},{\rm{ Zn}}\), and \({\rm{Fe}}\).

Reducing Power of Metals

The tendency to lose an electron or electrons is the basis for reducing nature. The higher negative the reduction potential, the more likely it is that an electron or electrons will be lost. As a result, in the electrochemical series, decreasing nature diminishes from top to bottom. As the typical reduction potential becomes more and more negative, the reducing agent’s power grows.

Sodium is a more powerful reducing agent than zinc, while zinc is more powerful than iron. Alkali and alkaline earth metals are effective reducers.

Products of Electrolysis

When there are two or more types of positive and negative ions in a solution, certain ions are discharged ord at the electrodes in preference to others during electrolysis. In such a competition, the ion with the higher standard reduction potential (stronger oxidising agent) is discharged first at the cathode.

Calculation of Standard emf \({{\rm{E}}^{\rm{O}}}\) of Electrochemical Cell

The total of the standard reduction potentials of the two half cells (reduction half cell and oxidation half cell) is the cell’s standard emf.

\({\rm{E}}_{{\rm{cell}}}^{\rm{O}}{\rm{ = E}}_{{\rm{red}}}^{\rm{O}}{\rm{ + E}}_{{\rm{OX}}}^{\rm{O}}\)

As is typical, the standard oxidation potential is always expressed in terms of reduction potential.

As a result, the conventional oxidation and reduction potentials are the same.

Therefore,

\({\rm{E}}_{{\rm{cell}}}^{\rm{O}}{\rm{ = }}\) ( standard reduction potential of reduction half cell) – ( standard reduction potential of oxidation half cell)

As oxidation takes place at the anode and reduction takes place at the cathode. Hence,

\({\rm{E}}_{{\rm{cell}}}^{\rm{O}}{\rm{ = E}}_{{\rm{cathode}}}^{\rm{O}}{\rm{ – E}}_{{\rm{anode}}}^{\rm{O}}\)

Summary

1. The electrochemical, electromotive, or activity series of the elements is formed when the electrodes (metals and nonmetals) in contact with their ions are ordered on the basis of the values of their standard reduction potentials or standard oxidation potentials.
2. The standard state potential of a cell is its potential at standard state circumstances, which are approximated at \(1\) mole per litre \(\left( {1{\rm{ M}}} \right)\) concentrations and \(1\) atmosphere pressure at \({25^{\rm{o}}}{\rm{C}}\).
3. The substances that are more powerful reducing agents than hydrogen are positioned above hydrogen in the series and have standard reduction potentials that are negative.
4. The metal with a high negative (or smaller positive) standard reduction potential rapidly loses an electron or electrons and becomes a cation. Chemical activity is a term used to describe a metal like this.
5. A metal with a negative reduction potential has the ability to lose an electron or electrons. As a result, the metals at the top of the electrochemical series readily hydrogen from dilute acids.
6. When there are two or more types of positive and negative ions in a solution, certain ions are discharged ord at the electrodes in preference to others during electrolysis

FAQs on Activity Series of Metals

Q.1. What is the activity series of metals?
Ans: The electrochemical, electromotive, or activity series of the elements is formed when the electrodes (metals and nonmetals) in contact with their ions are ordered on the basis of the values of their standard reduction potentials or standard oxidation potentials. A set of standard electrode potentials has been constructed by measuring the potentials of various electrodes vs the standard hydrogen electrode \(\left( {{\rm{SHE}}} \right)\).

Q.2. How can the reactivity of metals be determined by activity series?
Ans: The activity of a metal is determined by its tendency to lose electrons or form cations. The magnitude of the standard reduction potential influences this tendency.
The metal with a high negative (or smaller positive) standard reduction potential rapidly loses an electron or electrons and becomes a cation. Chemical activity is a term used to describe a metal like this.

Q. 3. How are the products of electrolysis determined with activity series?
Ans: When there are two or more types of positive and negative ions in a solution, certain ions are discharged ord at the electrodes in preference to others during electrolysis. In such a competition, the ion with the higher standard reduction potential (stronger oxidising agent) is discharged first at the cathode.

Q.4. Which type of metals can displace hydrogen from the dilute acids?
Ans: Dilute acids are reduced by a metal that may donate electrons to \({{\rm{H}}^{\rm{ + }}}\) ions in the presence of dilute acids. A metal with a negative reduction potential has the ability to lose an electron or electrons. As a result, the metals at the top of the electrochemical series readily hydrogen from dilute acids, but the tendency to liberate hydrogen gas from dilute acids reduces as the series progresses. \({\rm{Cu}},{\rm{ Hg}},{\rm{ Au}},{\rm{ Pt}}\), and other metals in the electrochemical series below hydrogen do not produce hydrogen from dilute acids.

Q.5. Which type of metals can displace hydrogen from water?
Ans: The metals above iron have the ability to liberate hydrogen from water. In the electrochemical series, the tendency diminishes from top to bottom. Coldwater liberates hydrogen for alkali and alkaline earth metals, but hot water or steam liberates hydrogen for \({\rm{Mg}},{\rm{ Zn}}\), and \({\rm{Fe}}\).

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