Question

One of the major intermediates in the anaerobic biological degradation of organic matter to methane is propionate (CH3CH2COO^-). Propionate conversion to methane can be described by the following reaction: 4CH3CH2COO^-(aq)+6H2O arrow 7CH4(g)+CO2(g)+4HCO3^-(aq) a) If deltaGF^0 for CH3CH2COO^- is -366 kJ/mol and assuming that reactants and products are at unit activity, is methane production from propionate thermodynamically favorable? b) It is known that methane is not formed directly from propionate. Rather, propionate is converted by bacteria to acetate (CH3COO^-) and hydrogen as follows: CH3CH2COO^- (aq) +2H2O arrow CH3COO^- (aq)+3H2(g)+ CO2(g) Show that this reaction is not thermodynamically favorable. In a well-operating anaerobic biological treatment system, propionate is efficiently converted to methane. How is this possible given that propionate conversion to acetate and hydrogen is not thermodynamically favorable?

Answer 1

From the standard books, if you take the values of standard free energy of formation for the given species and substitute in the equations shown below and calculate.

G^o_rxn = 7*G^o_CH4(g) + G^o_CO2(g) + 4*G^o_rxn - (4*G^o_{CH3CH2COO^-(aq)} + 6*G^o_H2O(l))

Then you will get a negative value, indicating the reaction as thermodynamically favorbale.

And G^o_rxn = G^o_CH3COO^-(aq) + 3*G^o_H2(g) + G^o_CO2(g) - (G^o_{CH3CH2COO^-(aq)} + 2*G^o_H2O(l))

You will get a positive value, indicates the reaction as not thermodynamically favorable.

Hence, In a well-operating anaerobic biological treatment system, propionate is efficiently converted to methane instead of converting to acetate.