[Explained] Why are Alkali Metals Strong Reducing Agents?

Alkali metals which are also known as group 1 metals are good reducing agents. Lithium is most powerful reducing agent among the alkali metals.

Why are Alkali Metals Strong Reducing Agents?
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 Alkali metals are strong reducing agents. This is due to their values of ionization energies. The low values of ionization energies help them to lose electrons easily. This is why all alkali metals act as strong reducing agent. The reducing character of alkali metals decreases in going from Li to Cs.


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Physical and Chemical Properties of Alkali Metals

 The group 1 or Group 1 A of the periodic table consists if six elements including hydrogen. These elements are 

  • Hydrogen
  • Lithium 
  • Sodium 
  • Potassium 
  • Rubidium 
  • Cesium 
  • Francium 

They are collectively known as alkali metals.

Element Symbol Atomic Number Electron Configuration Brief Representation of Electron Configuration
Lithium Li 3 1s2 2s1 [He] 2s1
Sodium Na 11 1s2 2s2 2p6 3s1 [Ne] 3s1
Potassium K 19 1s2 2s2 2p6 3s2 3p6 3d10 4s1 [Ar] 4s1
Rubidium Rb 37 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1 [Kr] 5s1
Cesium Cs 55 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1 [Xe] 6s1
Francium Fr 87 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 7s1 [Rn] 7s1

One question that might pop up is that lithium being the first element of the alkali metals has the highest ionization energy but is still the the strongest reducing agent. The next paragraph explains this.

Lithium is the first element of group 1. It has the highest value of ionization energy. Therefore,  it should have been the weakest reducing agent. However, lithium is the strongest reducing agent as indicated by its high oxidation potential value. The anomalous behavior can be explained as follows.

The oxidation potential of a metal is the sum of the energies involved in the following processes.

 $$M(s) \xrightarrow {Sublimation} M (g)$$

  $$M(s) \xrightarrow {Sublimation} M (g)^2 + {e^-}$$

 $$M^2 (g) \xrightarrow {Hydration} M^+ (aq)$$

Among the alkali metal cations, Li+ undergoes hydration to the maximum extent due to its smallest state. The hydration energy of Li+ is maximum and therefore a large amount of energy is released during its hydration. 


Why are Alkali Metals Strong Reducing Agents?
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The large amount of energy released during hydration compensates the energy needed to remove the electron from Li atom (I.E.) This facilitates the release of electron which makes lithium strongest reducing agents that hydrogen. Therefore, they react with compounds containing acidic hydrogen atoms such as alcohols, acetylene etc. to liberate hydrogen.

 2Li + 2C2H5OH  ⟶ 2C2H5OLi + H2

2Na + 2C2H5OH   2C2H5ONa + H2

2Na + H ー C=C ー H  NaC = CNa + H2

 Now you might think what is this hydration energy that causes lithium to be the strongest reducing agent.


Hydration Energy

The alkali metal cations have a strong tendency to get hydrated.

    M+ (g) + aq (excess water) ⟶  M+ (aq)

The process of hydration is exothermic and the energy involved is called hydration energy. The hydration energy of alkali metal cations decreases in going from Li+  to Cs+.

The hydration energy depends upon the charge-radius ratio(q/r). Since the radius of alkali metal cations increases in going from Li+ to Cs+, the hydration energy decreases in going down the group. 

Hydration energy is a measure of the tendency of an ion to undergo hydration. This is why the tendency of alkali metal cations to undergo hydration in going from Li+  to Cs+. Li+ ions are most heavily hydrated in aqueous solutions.

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Ionization enthalpy or ionization energy

Ionization energy also affects why alkali metals are such powerful reducing agents. The low values of ionization energies of the group 1 elements causes them to lose their only valence electron and thus act as good reducing agents.

The ionization energy values follows the following trends:

Alkali metals possess very low values of ionization energy. The ionization energy of an alkali metal atom is lowest in the period.

The alkali metal atoms possess electronic configuration of the type [Noble Gas] ns1.

The noble gas core shields the valence s-electron from the nucleus. Therefore in alkali metals the valence electron is loosely held by the nucleus and can be removed easily by supplying a small amount of energy. This is why alkali metals possess quite low ionization energies.

The ionization enthalpy of alkali metals decrease progressively in going from Li to Cs.

In going from Li to Cs. the distance of the valence electron from the nucleus increases progressively due to the addition of a new shell at each succeeding element. The increase in the number of shells causes an increase in the screening effect which consequently decreases the effective nuclear charge experienced by the valence electron. 

This facilitates an easier removal of the valence electron. This is why the ionization energies of alkali metals decrease on moving down the group. 

The second ionization energies of the alkali metals are very high

When and electron is removed from an alkali metal atom, the cation formed has a stable noble gas configuration. For example. 

Li+ = 1s2

Na+= 1s2 2s2 2p6

The noble gas configuration is a very stable configuration. The removal of an electron from such as configuration is very difficult and requires a large amount of energy. This why the second ionization energies of alkali metals are very high.

Alkali Metal ion Electron Configuration
Li+ 1s2
Na+ 1s2 2s2 2p6
K+ 1s2 2s2 2p6 3s2 3p6


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