CHAP. 32] THE ORIGIN OF LIFK 849 



B. The Equilibria of Carbon Compounds 



The partial pressure of carbon dioxide in the atmosphere is kept low by two 

 biiflFer systems. The first system, which is rapid, is the absorption of carbon 

 dioxide in the sea to form COs-^, HCO3- and H2CO3; the second, which is slow, 

 is the reaction of carbon dioxide with silicates, e.g. 



CaSi03 + C02 = CaCOa + SiOa AV = IQ-s. 



This reaction, which is the basic cause of the low CO2 content in the atmosphere, 

 is known as the Urey eqnilibrinm. The equilibrium partial pressure of CO2 is 

 10-8 atm, while the observed partial pressure at sea-level of 3.3 x 10-4 atm is 

 somewhat higher. 



The equilibrium constant at 25° in the presence of liquid water for the re- 

 action 



CO2 + 4H2 = CH4-f2H20(i) 



is 8 X 1022, Assuming equilibrium was attained, and using Pco^= 10"^ atm and 

 ■^Ha = 1 -5 X 10-3 atm, we find that Pch^ = 4 x 10^ atm. It is evident then that the 

 partial pressure of CO2 would be less than 10"^ atm and hmestone (CaCOs) 

 would not form if equilibrium conditions are assumed. 



Complete thermodynamic equilibrium could not exist in a reducing atmo- 

 sphere because of the dependence of the equilibrium proportions of compounds 

 on pressure and hence on altitude. It is more likely that the steady-state 

 concentrations of CO2 and CH4 would not be determined by the equilibrium at 

 sea-level, but rather by the equilibrium at higher altitude where the ultra- 

 violet light would provide the necessary activation energy to bring about rapid 

 equilibrium. Under these conditions water would be a gas, and the equihbrium 

 constant is lO^o, so 



A'25. = 1020 ^ PcH4^H,2 = (Xch,A'h,o2/A'cO,Xh,4)^-2, 



where the A's are the mole fractions and P is the total pressure. If the surface 

 partial pressures were Pch4=1, PcOg = 3.3 x 10-4 (the present value), 

 Ph.2=1.5x 10-3, i\^Q x'^ would be equal to these partial pressures. We shall 

 use Xh2O=10-^, which is the present value for H2O above the tropopause. 

 Equilibrium will be established under these conditions when P = 2.5xl0-^ 

 atm, which is the present atmospheric pressure at about 180 km. It is reasonable 

 to assume that equilibrium was established at some high altitude, and, there- 

 fore, that carbon dioxide and hydrogen could both be present at small partial 

 pressures and methane present at a moderate partial pressure in a reducing 

 atmosphere where the pressure of hydrogen is 1.5 x 10-^ atm. 



Carbon monoxide should not have been an important constituent of the 

 atmosphere as can be seen from the following reactions, 



COa-fHs = CO + H2O (1), ^25° = 3.2x10-4 



Pco/Pco, = 3.2 X 10-4Ph, 



