PHYSICAL CHEMISTRY 359 



Another reaction in a heterogeneous system which has 

 many points of interest is the catalytic formation of water 

 vapour over metalhc copper (Pease and Taylor, J.A.C.S., 

 1921, 43, 2179; and 1922, 44, 1637). Small concentrations 

 of oxygen markedly diminish the rate of reduction of copper 

 oxide by hydrogen, and the rate of combination of hydrogen 

 and oxygen over metallic copper. Between 130° and 200° C, 

 a mixture of hydrogen and oxygen reacts partially over a 

 copper surface to give water vapour, a portion of the oxygen 

 in the gas being removed by the copper to form oxide, and 

 this slowly combines with the hydrogen. Between the above 

 temperatures the rate of formation of oxide exceeds the rate 

 of reduction by the hydrogen, as long as oxygen is present in 

 the gas. If, however, the oxygen is suddenly cut off, and 

 pure hydrogen passed over the copper, an increased production 

 of water vapour immediately takes place, and sometimes 

 the velocity of reaction undergoes a sixfold increase. Oxygen 

 thus behaves as a negative catalj^st for the reaction, inhibiting 

 in some manner the combination of hydrogen with the adsorbed 

 oxygen. The explanation suggested by the authors is that 

 the reduction of oxide occurs only at the copper-copper oxide 

 interface. The presence of oxygen decreases the copper surface, 

 and, at the same time, the exposed copper-copper oxide inter- 

 face thus decreasing the area necessary for the reaction with 

 hydrogen. This explanation is similar to that given by Lang- 

 muir for the reduction of the adsorbed oxygen on a tungsten 

 or a platinum surface. Here it was suggested that the oxygen 

 atom is removed more readily by carbon monoxide or hydrogen, 

 when the collision occurs close to the place where the oxygen 

 atoms are attached to the metal. 



According to the present state of our knowledge of chemical 

 reactivity, only molecules possessing energy (either kinetic or 

 internal) above a certam minimum value are capable pf entering 

 into chemical combination. Thus the fundamental action of 

 a positive or negative catalyst is to modify the number of 

 active molecules in a chemical system at any time. This 

 process occurs in homogeneous systems and, more frequently, 

 on the surface of the third body, where the molecules are in a 

 specially active state. In many of these processes, it is sus- 

 pected that ionisation, more or less complete, is a necessary 

 condition for chemical reaction. Not only is this held for 

 reactions between inorganic substances, but also strong support 

 is given at the present time to the view that in organic chemistry 

 ionisation precedes chemical combination. The work of Kraus 

 and others on the conductivity of organic substances in non- 

 aqueous solvents certainly supports this view. It is thus a 

 matter of interest to examine those reactions in which it can 



