Atom Ratio- H/M 



Figure 2. Typical pressure-composition diagram for 

 system. 



il-hydrogen 



Several factors determine the rates of hydrogen -metal reaction. 

 First of all, the nature of the system itself is a strong determinant of 

 the reaction rate. Certain alloys react almost as fast as the hydrogen 

 can be supplied, whereas others react very slowly. Second, the clean- 

 liness of the metal surface determines the rate of reaction. An oxide 

 fila, for example, will often slow down the reaction rate at first. 

 Third, the smaller the particle size the faster will be the reaction 

 rate. This is not too surprising since the larger the surface area of a 

 given amount of solid, the higher should be the diffusion raLe. Finally, 

 the rate of reaction can be increased by the addition of a small amounc 

 of catalyst. This will be dealt with in more detail later. 



Hydrides having high dissociation pressures are less stable than 

 those with lower dissociation pressures. For most hydrides, the loga- 

 rithm of the dissociation pressure decreases in a linear manner with the 

 reciprocal temperature, viz, 



LoginP 



10 K H 



-I * 



B 



12 



