Question

Which alkali metal is the strongest reducing agent in aqueous solution?

Answer 1

Hint: As we know that alkali metals are Group \[1\] elements of the periodic table. These include Lithium, Sodium, Potassium, Rubidium, and cesium. They are called alkali because on reaction with water forms alkalis and releases hydrogen. Because of their high reactivity they are available in nature in the form of salt like \[{\text{NaCl}}\], \[{\text{KCl}}\] etc. Their electronic configuration is \[{\text{n}}{{\text{s}}^{\text{1}}}\] that means the one electron in valence shell can be removed easily because of its weak bond and they can positive cations and achieve the state of noble gas as \[{\text{ns}}\].

Complete step by step answer:
To answer the above question we should understand some terms:
Redox reactions: these involve gain and loss of electrons in which one atom is oxidised and other is reduced. Their full form is “oxidation-Reduction reactions”.
We must need to know that the reducing agents are the substances which lose electrons and get oxidized. e.g., sodium, lithium, formic acid. These are called electron donors.
Oxidizing agents: are the substances which accept electrons and get reduced itself. E.g. Oxygen, halogens. These are termed as electron acceptors.
\[{{\text{A}}_{{{\text{e}}^{\text{ - }}}}}^{{{\text{e}}^{\text{ - }}}}\xrightarrow[{{\text{loses electron}}}]{{{\text{oxidation}}}}{\text{A}}\]

\[{\text{B}}\xrightarrow[{{\text{gains electron}}}]{{{\text{reduction}}}}{{\text{B}}_{{{\text{e}}^{\text{ - }}}}}^{{{\text{e}}^{\text{ - }}}}\]
As per this question, our answer is Lithium (\[{\text{Li}}\]) is the alkali metal which shows the strongest reducing property. The increasing order of reducing property among alkali metals is:
\[{\text{Li < Na < K < Rb < Cs}}\]\[\xrightarrow{{{\text{Li < Na < K < Rb < Cs}}}}\]
We need to remember that the lithium is strongest because lithium in solid state undergoes sublimation to form lithium gas .In this step energy is supplied so it is positive $\left( + \right)$.
This lithium gas molecule undergoes ionization and forms lithium ion. Here also energy is supplied so the reaction is positive $\left( + \right)$ .
Because of the small size of Lithium ion it gets surrounded by water molecules and absorbs them. This results in release of hydration enthalpy. This energy is negative which is very high and thus it gives electrons easily. The hydration enthalpy is high because of \[\dfrac{{{\text{charge}}}}{{{\text{size}}}}\] ratio.
Following reaction explains
\[{\text{L}}{{\text{i}}_{{\text{(s)}}}}\xrightarrow[{{{\Delta H + ve}}}]{{{\text{sublimation energy}}}}{\text{L}}{{\text{i}}_{{\text{(g)}}}}\]
\[{\text{L}}{{\text{i}}_{{\text{(g)}}}}\xrightarrow[{{{\Delta H + ve}}}]{{{\text{ionisation energy}}}}{\text{L}}{{\text{i}}^{\text{ + }}}{\text{ + }}{{\text{e}}^{\text{ - }}}\]
\[{\text{L}}{{\text{i}}^{\text{ + }}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{O}}\xrightarrow[{{{\Delta H - ve}}}]{{{\text{hydration energy}}}}{\text{L}}{{\text{i}}^{\text{ + }}}_{{\text{(aq)}}}{\text{ + }}{{\text{e}}^{\text{ - }}}\]
Let’s learn little about all three energy used or released in above process:
Sublimation enthalpy (\[{{\Delta }}{{\text{H}}_{{\text{subl}}}}\]): heat required to convert \[1\] mole of substance from solid state to gaseous state and in this energy is absorbed.
Ionization enthalpy: It is the amount of energy needed to remove an electron from the outer shell.
Hydration enthalpy: It is the amount of energy released and a new bond is formed.

Note:
We must remember that the reducing agents are also known as reductant and oxidizing agents are known as oxidant. Ionisation energy of cesium among all alkali metals is less. We should remember that ionization energy is positive because it requires energy to lose electrons. Both ionization and sublimation are endothermic processes because heat is supplied. Smaller the ion more is the hydration enthalpy. Hydration enthalpy is always negative and is an exothermic reaction.