ON CHEMICAL REACTION RATES; ENZYMES 



197 



to these rules of thumb, of course: for example, certain free radical recom- 

 binations have no temperature coefficient of rate; and by contrast the rate of 

 inactivation of enzymes by heat, and of the denaturation of proteins, can in- 

 crease by 1000 times over a 10-degree rise! The last column of Table 8-3 

 illustrates this point quantitatively. 



'Different processes of the same general type may have different activation energies Therefore both A* 

 and the ratio of rates are given as a range of values. Units of E* : kcal/mole. 



In general this dependence upon temperature is understandable in terms 

 of the postulates of the kinetic theory of matter. Molecules are presumed to 

 be in a state of continuous motion and have a heat content (H) which de- 

 pends upon the number of (degrees of freedom of) rotations, vibrations, etc. 

 It is axiomatic that in such a case of random motion not all molecules will 

 contain exactly the same kinetic energy at any one instant. In fact, it is in- 

 herent in the kinetic postulates that the energy distribution must be of the 

 form shown in Figure 8-2. 



The average heat energy, Q^ v , per mole of material is 1/2 RT (300 cal) 

 for each translational degree of freedom, RT (600 cal) for each vibrational 

 degree of freedom, and 1/2 RT [or each rotational degree of freedom. For a 

 diatomic gas at 27°, then, with one degree of vibrational freedom, two of ro- 

 tational, and three of translational, the average heat energy, Q^ v , is 2100 cal 

 per mole of gas. 



In any collision of reactant molecules which is to result in reaction, a mini- 

 mum or threshold energy must be involved in the collision, or else the mole- 

 cules will simply bounce off each other. Let this threshold energy be E*. A 

 few molecules will have the excess energy sufficient to react; not every col- 



