234 SMITH'S INTERMEDIATE CHEMISTRY 



chemical change is determined by the frequency with which the 

 molecules of the interacting substances encounter one another. 

 The frequency of the encounters amongst a given set of molecules, 

 resulting in a definite chemical change, will in turn evidently 

 depend entirely upon the degree to which the molecules are con- 

 centrated in each other's neighborhood. Larger amounts of one 

 of the materials, for example, will not result in more rapid chemical 

 action, if the larger amount of material is also scattered through a 

 larger space. Chemical changes, therefore, are not accelerated by 

 increasing the mere quantity of any ingredient, but only by 

 increasing the concentration of its molecules. Thus, a large 

 amount of a 0.1 normal solution of hydrochloric acid with a piece 

 of zinc will generate hydrogen no faster than a smaller amount. 

 But substitution of a normal solution of hydrochloric acid, which 

 contains a higher concentration of hydrogen ions, will instantly 

 increase the speed of the action. In the second case, the number 

 of hydrogen ions reaching the zinc per second is greater, and 

 the displacement reaction Zn + 2H+ > Zn + + + H 2 f proceeds 

 more rapidly. So also, iron burns faster in oxygen (100 per cent) 

 than in air (20 per cent oxygen). 



With a reversible action the effect on the speed is the same, 

 excepting that the continued activity of the reverse action pre- 

 vents the direct one from reaching completion. 



Thus, if, in the action of hydrogen chloride upon oxygen, we 

 introduce into the same space an extra amount of oxygen, this 

 facilitates the formation of steam and chlorine by increasing the 

 possibilities of encounter between molecules of hydrogen chloride 

 and oxygen. At the same time it does not affect (cf. p. 230) the 

 number of encounters in a given time of steam and chlorine mole- 

 cules with one another which result in the reverse transformation. 

 The proportion of chlorine (and steam) formed, therefore, from a 

 given amount of hydrogen chloride will be greater, although the 

 total possible (by complete consumption of the materials) has not 

 been altered, since the quantity of one ingredient only has been 



