Alkali metals react with water vigorously to form hydroxides and dihydrogen. Which of the following alkali metals reacts with water least 

vigorously ?

Li has most negative standard reduction potential due to very high enthalpy of hydration. Thus, reaction of Li with water will be most exothermic, but surprisingly Li reacts with water gently, whereas Na and K vigorously.

The explanation is in kinetics and not in thermodynamics of the reaction. No doubt maximum energy is evolved with Li but its fusion, vaporization and ionization consume more energy. As a result reaction proceeds slowly.

Na or K has low melting points and molten metal spreads over water exposing a larger surface to water, making the reaction vigorous.