Re: How did scientists find out an accurate energy value of the hydrogen bond?

I don't know if you realize it but there is no single value for the amount 
of energy in the intermolecular hydrogen bond. It varies from molecule to 
molecule as it depends on 
1.	 the distance between the donor and acceptor atom. 
2.	 the distance between the (calculated) hydrogen position, and the 
acceptor atom. 
3.	 and the angle from donor atom over the hydrogen to the acceptor 
atom. 

However, you can get a rough estimate by first carrying out the following 
heat of vaporization experiment.

1. Use 2 nested styrofoam cups and 100.0 ml of water. 
2. Measure the mass of the water and calorimeter to 0.01 g.
3. Stir the water for about 30 seconds; then measure the initial temp of 
the water to 0.1 C.
4. Carefully, raise the calorimeter under a steam generator so that steam 
bubbles into the bottom of the calorimeter. Keep stirring.
5. When the temperature hits about 40 remove the calorimeter from the 
generator. Keep stirring and record the highest temp. attained.
6. Measure and record the mass of the calorimeter, which will include the 
mass of the condensed steam. 
7. Calculate the mass of the condensed steam by subtracting.
8. The molar heat of vaporization can be calculated as follows: the heat 
lost by the steam while condensing plus the heat lost by the condensed 
steam while cooling equals the heat gained by the cold water in the 
calorimeter.

This will yield a value of about 41kJ/mole. If the attractions were due 
only to London Forces, you would expect a value of about 8 kJ/mole because 
methane has such a value, and it has the same number of electrons as 
water. So the difference of 33 kJ/mole is due to hydrogen bonding in water.

Enrico Uva