BONDS AND ENERGY EXCHANGE 35 



Fig. 15. A sketch of the water molecule and its electrically equivalent 

 dipole. 



ing nucleic acid solutions; this effect is, however, primarily associated 

 with increase of the repulsion due to the negatively charged phosphate 

 groups. 



A second aspect derives from the fact that the water molecules are 

 themselves dipoles; their positive and negative charges are permanently 

 separated from each other, so that the molecule as a whole is electrically 

 neutral, but still can effect electrical interactions. Thus water can act to 

 split molecules bonded appreciably by electric forces. As an example, 

 salt (sodium chloride) is found to be entirely separated into constituent 

 ions when salt is dissolved in water. 



Some investigations have yielded results suggesting that, in water, the 

 hydrogen bond strength may be less than lOOOcal/mol. In any event, 

 the strengths of ionic and hydrogen bonds, due to electric forces, are 

 subject to important modifications when the molecules are dissolved in 

 water. 



BONDS AND ENERGY EXCHANGE 



Given the existence of the various chemical bonds, it remains to ex- 

 plain the energy exchanges involved in chemical reactions. In order to 

 stick two atoms together in, say, a covalent bonding, we have to bring 

 them close together. As long as they are many atomic diameters apart, 

 there is no force acting between the atoms, but as they approach each 

 other, the outer electrons of each exert a repulsive force on the other, so 

 that we have to do work to push them still closer together. It is not until 

 we have pushed them so close that an electron from one atom experiences 

 the attractive force of the nucleus of the other atom that the electron 

 swap which makes the two atoms exert a net attraction on each other can 

 take place. It turns out that the net attraction is not very great, but as 

 long as it exists, it holds the atoms together. If we now supply the 

 energy to move the electron back where it was in the first place, the 

 repulsion between the atoms is enough to force them apart and give them 

 an appreciable velocity. This velocity corresponds to an energy of 

 motion, which is the form in which we get back the energy we initially 

 expended in forcing the atoms together. Thus, as long as the atoms are 

 bound to each other, they have this latent energy. In this fashion, com- 



