Action between Metals and Acids^ 837 



§ 6. The Problem for Experiment. — If the equation (i. a) 

 represents the condition of equilibrium between mercury and 

 an acid (assuming that the only reaction possible is of the 

 type 



Hg + HX = HgX + H) 



it is clear that when the substances come into contact a 

 certain amount of hydrogen must be displaced. Otherwise 

 the right-hand member of the equation can never become 

 equal to the left. 



If (as is likely in the case of Hg) the quantity y? is very 

 small, the quantity y — y , (being at constant temperature 



of the form h log c/c 01 where c is very small), may acquire 

 a considerable value, even when c is small, i. e. when only a 

 small quantity of hydrogen has been displaced. 



Thus equilibrium may be reached and displacement of 

 hydrogen cease (neglecting diffusion effects) before the 

 amount separated per unit surface has become perceptible. 



If, however, some means could be found of removing the 

 mercury salt as fast as it was formed the reaction would 

 continue, and thus the displacement of hydrogen might be 

 rendered evident. 



§ 7. Detrimental effect of Oxygen in the Surface-layer. — 

 The simplest way of obtaining an experimental answer to the 

 question whether the direct displacement of hydrogen by 

 mercury ever occurs is not immediately obvious. The purest 

 mercury in contact with the air will become coated with a 

 film of condensed oxygen — possibly a minute layer of oxide. 

 Hence, even if the acid with which it may be brought into 

 contact is free from dissolved oxygen, the interaction con- 

 templated in the equations above may be prevented. 



When an equivalent of hydrogen forsakes the acid solution 

 in the presence of oxygen, the loss of available energy can 

 be greater than before because, instead of separating as gas, 

 the hydrogen can now become part of a molecule of water. 

 Hence the amount of metal which must dissolve before equi- 

 librium is reached will much exceed that required to give 



y the value sufficient to satisfy (i. a) above. The equilibrium 



value of y in the present case may be written 



In which the quantity y " is much less than the corre- 

 sponding term y n h of equation (i. a). 



It is known from electrical measurements that the earlier 

 Phil Mag. S. 6. Vol. 17. No. 102. June 1909. 3 L 



