Question

The correct order of reducing character of alkali metals is

• A. $Rb < K < Na < Li$
• B. $Li < Na < K < Rb$
• C. $Na < K < Rb < Li$
• D. $Rb < Na < K < Li$

Answer 1

Answer: (C)

$Na < K < Rb < Li$

Answer 2

The alkali metals are strong reducing agents, lithium the most and sodium the least powerful. The standard electrode potential $\left(E^{o}\right)$ which measures the reducing power represents the overall change
$M(s) \rightarrow M(g)$ Sublimation enthalpy
$M(g) \rightarrow M^{+}(g)+e^{-}$ ionization enthalpy
$M^{+}(g)+H_{2} O \rightarrow M^{+}(a q)$ hydration enthalpy
With the small size of its ion, lithium has the highest hydration enthalpy which accounts for its high negative $E^{\circ}$ value and its high reducing power.
Hence, the order of reducing character of alkali metals is $Na < K < Rb < Li$