ON CHEMICAL REACTION RATES; ENZYMES 



195 



Since the specific rates are all different, the steady-state concentrations are 

 different; but if the process is in the steady-state condition, the concentra- 

 tions are constant. 



If the back reactions proceed at a measurable rate, the situation is more 

 complicated, but the principles are the same. 



When you hear the word "equilibrium" used, then think: Which is meant, 

 true equilibrium or steady-state? In the latter case, continuous processing 

 occurs; in the former no net reaction occurs. Figure 8-1 illustrates this 

 difference. 



source 

 (lake) 



tumbling stream 



Equilibrium Steady State 



Figure 8-1. Equilibrium and Steady State. 



ON CHEMICAL REACTION RATES; ENZYMES 



Concentration and Temperature 



The law of mass action has already been outlined under the discussion of 

 the approach of a system toward true equilibrium. The rate is always pro- 

 portional to some power of the concentration of reactants, and this index is 

 called the "order" of a reaction. 



There are really two orders obtainable from experiments, one with respect 

 to time, and the other with respect to concentration. These will have the same 

 value if the reaction is a simple one in which the slowest step is the first step, 

 the one which involves reactant concentrations explicitly. If some other step 

 than the primary one is rate-determining, or if products interfere with or 

 inhibit the reaction, the power, a, of the concentration, [A], which describes 

 best the over-all rate may be different from that which describes the initial 

 rate. 



Complicated cases are not considered here. Some of the simpler cases are 

 collected in Table 8-1, which shows the rate equation and the expression 

 and dimensions of the proportionality constant, k, called the specific rate con- 

 stant, when a = 0, 1/2, 1, and 2. In Table 8-2 are collected values of the 

 specific rate constant for some first and second-order reactions. 



