Re: Li, K, Ca, Na are only elements in activity series that replace H in H2O?

Rich,

     In regards to displacement reactions with water to produce hydrogen 
gas and the activity series, I believe the answer lies in the periodic 
table and electrochemistry.  Historically, the IA metals are most 
reactive.  Compared to IA metal ions, IIA metal ions have higher ionic 
charge-to-size ratios.  Because of this, their energy of hydration is 
about 5 times higher than the cooresponding IA ion in the same row or 
period.  So Mg, for instance, will have a lot of covalent character and 
will tend to complex with water instead of ionically bonding with it.  The 
trend of ionization potential as well as standard electrode potentials 
mimic the energy of hydration trends, especially down a family or group...
     Electrochemically, any metal with a negative standard reduction 
electrode potential (more negative than -.8 volts which is where water 
reduces) will have a tendency to reduce water to hydrogen gas.  Note that 
Li, Na, K, and Mg all fall into this catagory.  Magnesium should not, but 
recognize that it DOES react with water with energy (THE WATER MUST BE 
BOILING).  The above data helps explain the trend of how the IIA metals 
interact with water: Beryllium does not react (weakest reducing agent), 
whereas magnesium will react in boiling water, and the rest will react 
with ease (in cold water), especially barium which is the strongest 
reducing agent of the group.