Re: Why is the electrochemical series different to the reactivity series?

There is no question that potassium is a much more reactive metal than lithium. Lithium, for 
example, reacts vigorously but steadily with water, while potassium will at least take fire, and 
probably explode if brought into contact with water. And yet the standard reduction potential for 
lithium indicates that it is a stronger reducing agent than potassium. What is going on?

The important point is that the reactivity series is based on a very general concept of reactivity in a 
wide variety of different situations. The electrochemical series is defined with respect to a very 
particular and special set of conditions -- dilute aqueous solutions at neutral or acidic pH values, 
or solids (notionally) in contact with water.

To understand the difference, we will stick with the example of lithium and potassium.
An isolated potassium atom will give up its electron much more readily than a lithium atom. The 
ionization potential is 520 kJ/mol for lithium, and 419 kJ/mol for potassium. But the lithium ion 
that can be formed will surround itself with very tightly bound water molecules, because it is a 
small ion, with a higher electrostatic attraction. Potassium is a larger ion, with much less attraction 
for the oxygen lone pairs of water, so it is much more indifferent to the presence of water. The 
difference in hydration energy between a lithium ion and a potassium ion is greater than the 100 
kJ/mol difference in ionization energy. These are not the only two factors -- sublimation energy, 
the energy necessary to turn an atom in a metal into an isolated gas phse atom, comes into it too. 
But the overall result is that a lithium reduction is 0.12 volt, or about 12 kJ/mol more favourable 
than a potassium reduction, if the product is to be an aqueous lithium or potassium salt.