Alkali metals react with water vigorously to form hydroxides and dihydrogen. Which of the following alkali metals reacts with water least vigorously?
(a) Li (b) Na (c) K (d) Cs
Li has most negative standard reduction potential die 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, vaporisation and ionisation consume more energy. As a result reaction proceeds slowly.
Na or K have low melting points and molten metal spreads over water exposing a larger surface to water, making the reaction vigorous.