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

Options

A. `Li`
B. `Na`
C. `K`
D. `Cs`

Answer

A

Explanation

`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.