Question

It may be noted that lithium has the most negative \[{E^\theta }\]value (table 9.1). But its reaction with water is less vigorous than that of sodium which has the least negative \[{E^\theta }\] value among the alkali metals. This behaviour of lithium is attributed to its small size and very high hydration energy. Other metals of the group react explosively with water.

Answer 1

Hint: Lithium is a much lighter element that floats on water and due to its smaller size has higher atomization and hydration enthalpies.

Complete step-by-step answer:
Lithium is a very light element and its density is about half as compared to water. So, it first floats on the surface. Due to this the contact between lithium and water happens to a little extent. So, the reaction to form Lithium hydroxide and hydrogen gas proceeds less vigorously.

Other alkali metals have more density than water and they are fully immersed in water. Due to this the contact between other alkali metal and water happens to a significant extent. So, the reaction to form other alkali metal hydroxide and hydrogen gas proceeds vigorously.

\[{\text{Li + }}{{\text{H}}_2}{\text{O }} \to {\text{ LiOH + }}\dfrac{1}{2}{{\text{H}}_2}{\text{ slow reaction}}\]
\[{\text{Na + }}{{\text{H}}_2}{\text{O }} \to {\text{ NaOH + }}\dfrac{1}{2}{{\text{H}}_2}{\text{ fast reaction}}\]

Atomization refers to the conversion from solid to gaseous atom. The hydration enthalpy, is the energy required to change a gaseous atom to a hydrated aqueous state. Hydration means that the atom or ion is surrounded by several water molecules. The energy of atomization is very high for lithium as compared to other alkali metals. The hydration enthalpy is also higher than other alkali metals.

So, you cannot consider only the negative ionization energy as the only criteria or standard to decide on the vigour of reaction of alkali metals with water. You need to consider several other factors such as hydration enthalpy and the energy of atomization.

Firstly, energy of atomization and hydration need to be provided to change its solid state to gaseous and aqueous, respectively. Then ionization (i.e., knocking out electrons from the gaseous atom) takes place. This finally leads to the formation of Lithium hydroxide and Hydrogen gas.

So, the lower density of lithium (it being lighter than other elements), and high energies of atomization and hydration make the reaction of Lithium with water less vigorous than the reaction of other alkali metals with water. Other alkali metals have less positive atomization and hydration enthalpies.

Note: Do not rely on just ionization energy which requires gaseous atoms. Lithium being solid metal, requires to be changed into gaseous atom (atomization) and aqueous state (hydration) to react with water.