[SapereAude1490]

Just the other day I was looking at the s-elements and found out that Li is the strongest reducing agent (- 3.05V). This is a little strange as with other elements (Na, K, Rb, Cs) the standard electrode potential decreases from - 2,71V to - 2,92V as you go down in the group.

Add to this the ionization energy of -5,39eV for Li and a steady decrease as you go down in the group and it's a bit of a mystery.

Now, there is an explanation in my textbook, and it goes like this: If we look at redox-potential as

M(s)  M⁺(aq) + e^-

by using Hess's law we can divide this process into three separate subprocesses:

(1) M(s)  M(g)

(2) M(g)  M⁺(g) + e^-

(3) M⁺(g) + aq  M⁺(aq)

(1) is just separation of atoms. Sublimation energy is about the same for all s-elements
(2) is ionization energy. You need to spend about 5,39 eV for Li to make it an ion.
(3) The Li⁺ is put in water and hydrated. Energy is released Δ_hydH

This Δ_hydH is more than enough to compensate for the (relatively) high 5,39 eV ionization energy, because, if I remember correctly from general chemistry, the smaller the ion the larger the Δ_hydH.

My question is this: Why? What happens on the molecular scale? I "get" all of thermodynamics here, but when I tried to find where does this energy come from, I can't find anything. I looked in Atkins' Physical Chemistry and inorganic and general chemistry textbooks. Nothing.

Any useful response will be rewarded by a mole snack. (or even two  )

[Borek]

Think size and the distance between the cation and water dipoles. Coulomb forces are everything you need to explain the situation.

[SapereAude1490]

OK.

Lithium atomic radius is 0,133 nm and ionic 0,068 nm. It's a very small ion. So... if the r is small so is the surface. However it has the same charge as Na⁺, K⁺, Rb⁺ and Cs⁺ so it has a big charge density(q/s), right?

It should then attract the polar H₂O strongly, because they can come closer. How many can actually fit around it? Less than Na⁺ and other bigger ions?

Also, do (1), (2) and (3) happen simultaneously? Do the H₂O molecules "kick"  the Li out from that chunk of metal?

[Borek]

You can have more than one layer of water molecules.

[SapereAude1490]

That changes everything!!! 

But seriously, I did not know that. Thank you.

That sounds a lot like analytical chemistry with primary layers and secondary layers.

What about (1), (2) and (3)? Do they happen at the same time?