Q.Explain why can alkali and alkaline earth metals not be obtained by chemical reduction methods?

Step 1: Rank their reducing power

Alkali and alkaline earth metals have very low ionisation enthalpies and are among the most electropositive elements known — this makes the free metals themselves extremely powerful reducing agents, stronger than virtually any other chemical reducing agent (carbon, hydrogen, more reactive metals, etc.).

Step 2: Consequence for chemical reduction

To reduce M$^{n+}$ to M chemically, one would need a reducing agent even stronger than the alkali/alkaline earth metal itself — but no ordinary chemical species is powerful enough, so chemical reduction methods (e.g. reduction by carbon or hydrogen, as used for many other metals) simply fail.

Step 3: Why aqueous electrolysis also fails

Electrolysis of an aqueous solution of their salts would preferentially liberate H$_2$ at the cathode (since H$^+$/H$_2$O is reduced more easily than the highly electropositive M$^+$/M$^{2+}$), and the metal, if formed, would instantly react with water anyway.

Step 4: The actual method …

Reason from their extremely low ionisation enthalpy: since the metals are the strongest available reducing agents, no chemical reagent can out-reduce them, and aqueous electrolysis fails too …

• Suggesting a strong chemical reducing agent (like carbon or aluminium) could work — none is powerful enough to reduce these highly electropositive metals. …