Explain the cause of low reactivity of lithium inspite of being strong reducing agent in acqueous solution.
All the elements of Group 1 react vigorously with water to give metal hydroxides and liberate hydrogen gas, but this reactivity increases down the group.
Lithium's density is only about half that of water, so it floats on the surface and ultimately disappears, giving off hydrogen gas and forming a colourless solution of lithium hydroxide. But unlike the rest of the s-block elements, this reaction is not vigorous.
2Li(s)+2H2O(l)→2LiOH(aq)+H2(g)
Using Hess’s Law, we can split it into several theoretical steps with known enthalpy changes.
- Atomisation energy:
Li(s)→Li(g); ∆H=+ve
- Ionisation energy:
Li(g)→Li+(g)+e−; ∆H=+ve
- Hydration enthalpy:
Li+(g)→Li+(aq); ∆H=-ve
The reasons for the slow and non-explosive reaction of Li with water are:
- As we move down the group, as atomic size increases, atomisation energy and ionisation enthalpy decrease and as such, their values are quite high for Li or we can say that the energy to be supplied for the reaction to take place (activation energy) is quite high. Even though, the value of hydration enthalpy (-ve) is also higher for Li than other elements of the group, the reaction releases heat very slowly, so activation energy has to be supplied first and is then later recovered. More activation energy means slower reactions and less vigorous reactions.
- The reaction generates heat slowly, and Li has a high melting point, too high for it to melt.
As far as being less reactive than the Group 2 elements of the s-block is concerned, it follows the same logic. Be and Mg would have been exceptions as reactivity with water decreases across a period but they have a protective layer of oxide on their surfaces, so they react only at very high temperatures and are largely inert towards water.
All the elements of Group 1 react vigorously with water to give metal hydroxides and liberate hydrogen gas, but this reactivity increases down the group.
Lithium's density is only about half that of water, so it floats on the surface and ultimately disappears, giving off hydrogen gas and forming a colourless solution of lithium hydroxide. But unlike the rest of the s-block elements, this reaction is not vigorous.
2Li(s)+2H2O(l)→2LiOH(aq)+H2(g)
Using Hess’s Law, we can split it into several theoretical steps with known enthalpy changes.
- Atomisation energy:
Li(s)→Li(g); ∆H=+ve
- Ionisation energy:
Li(g)→Li+(g)+e−; ∆H=+ve
- Hydration enthalpy:
Li+(g)→Li+(aq); ∆H=-ve
The reasons for the slow and non-explosive reaction of Li with water are:
- As we move down the group, as atomic size increases, atomisation energy and ionisation enthalpy decrease and as such, their values are quite high for Li or we can say that the energy to be supplied for the reaction to take place (activation energy) is quite high. Even though, the value of hydration enthalpy (-ve) is also higher for Li than other elements of the group, the reaction releases heat very slowly, so activation energy has to be supplied first and is then later recovered. More activation energy means slower reactions and less vigorous reactions.
- The reaction generates heat slowly, and Li has a high melting point, too high for it to melt.
As far as being less reactive than the Group 2 elements of the s-block is concerned, it follows the same logic. Be and Mg would have been exceptions as reactivity with water decreases across a period but they have a protective layer of oxide on their surfaces, so they react only at very high temperatures and are largely inert towards water.
