Question

Why is second ionisation enthalpy of alka;i metals exteremely high ?

Answer 1

The first ionization enthalpy of alkali metals is the energy that is needed to take one electron from the neutral atom. When it is done the ion with the charge +1 has noble gas configuration, that makes the ion very stable. That's why much more energy is needed to remove the second electron from the cation with the noble gas configuration.