Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: SapereAude1490 on October 30, 2012, 08:29:16 AM
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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) :rarrow: M+(aq) + e-
by using Hess's law we can divide this process into three separate subprocesses:
(1) M(s) :rarrow: M(g)
(2) M(g) :rarrow: M+(g) + e-
(3) M+(g) + aq :rarrow: 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 ΔhydH
This ΔhydH 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 ΔhydH.
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 ;D)
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Think size and the distance between the cation and water dipoles. Coulomb forces are everything you need to explain the situation.
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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?
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You can have more than one layer of water molecules.
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That changes everything!!! :o
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?