Catalysifi or the Speeding-up of Chemical Reactions 35 



A good mechanical analogy is found in the case of a weight 

 moving down an inclined plane. If the angle is gradually 

 increased the plane is ultimately sufficiently tilted for the weight 

 to start off and move slowly down the plane. Such motion may 

 be regarded as I'epresenting a chemical reaction normally proceeding 

 with small velocity. If the plane is now smeared with oil — the 

 tilt remaining unaltered — it is found that the weight moves with 

 a much greater velocity than in the preceding case. So we have 

 the analogy to the increase in the rate of a chemical reaction by 

 the addition of traces of a foreign substance. In the case of the 

 weight moving down the plane it is evident that the oil cannot 

 affect the actual force causing the weight to slide down the 

 plane. It does, however, alter the resistance to the motion by 

 decreasing the friction and so the velocity of the weight is 

 increased, 



,, , ., Force 

 since V elocity = z- — -. — 



resistance 



In the same way a catalyst cannot be regarded as increasing the 

 actual chemical affinities in a reaction but merely as acting in 

 some way to decrease the resistance. 



Many examples of catalytic action are known, but at this 

 stage we may refer to two common cases. Potassium chlorate 

 on being heated decomposes and gives off oxygen. If a little 

 manganese dioxide is added to another quantity of the chlorate 

 it is found that on heating, the oxygen is liberated much more 

 rapidly and at a much lower temperature than in the first 

 experiment. The manganese dioxide, moreover, may be recovered 

 unchanged at the end of the reaction. Again, if a solution 

 containing oxalic acid is heated with some nitric acid, little 

 decomposition occurs. If a trace of a manganous salt is added 

 the reaction proceeds briskly and the oxalic acid is broken down 

 into carbonic acid gas. In these examples we see two criteria 

 for catalysis, namely, (l) a catalyst can assist the course of a 

 chemical reaction and itself remain unchanged and (2) a small 



