396 Thermodynamics and Biology /2! : 5 



In words, Equation 27 says that equilibrium will favor the side of a 

 chemical equation which has the lowest total partial molal Gibbs' free 

 energy in the standard state ; that is, the bigger the difference in the free 

 energies of the standard states, the greater will be the tendency of the 

 equilibrium to favor one side of the reaction equation. In cases where 

 the value of AG is not known, it can be determined by a graph of 

 In A" plotted against l/(RT). Equation 27, as well as Equation 27', 

 states that this graph should be a straight line. A graph of this nature 

 is called an Arrhenius plot. It is discussed in more detail in the following 

 chapter. 



5. Reactions of Catalase 



In this section, the equilibrium theory embodied in Equation 27' is used 

 to amplify the discussion of the catalatic reaction of catalase presented 

 in Chapter 18. Equation 27' specifically is used to find the amount of 

 H 2 2 in equilibrium with air-saturated water in the absence of and in 

 the presence of the enzyme catalase. It is also used to find the "back" 

 rate for the production of intermediate complex and H 2 2 from cata- 

 lase, oxygen, and water. 



The over-all reaction being considered obeys the equation 



2H 2 2 £ 2H 2 + 2 (28) 



As standard states, one may choose 1 mole per liter for both hydrogen 

 peroxide and oxygen, and 55 moles per liter for water. The free energies 

 of these compounds, relative to gaseous molecular hydrogen and oxygen 

 at normal temperature and pressure, may be found in the International 

 Critical Tables. For this reaction 



AG = 2G° z0 + G° 2 -2G° 202 



Therefore, the following sum is needed 



2G& 2 o =2(-56,560) =- 113,120 kcal/mole 

 G 02 = + 3,904 



-2<% a o a = -2(-31,470) = + 62,940 



AG = - 46,300 



All these numbers are for 15°C. Because there is one more molecule on 

 the right than on the left in the preceding Equation 28, Equation 27' 

 becomes 



K _ e f2 v t — 2" y u-AGo/flr _=_ ^-i^ + 46.3oo/6oo _i_ io + 33 



