Question

A bacterium can use H_2 as electron donor and either C_2Cl_6(aq) or SO_4(aq) as electron acceptor, according to the following reactions (equations not balanced): C_2Cl_6(aq) + H_2(g) = C_2Cl_4(aq) + H^+_(aq) + Cl^-_Which electron acceptor yields more free energy per mole of H2? For each reaction calculate the Delta_r G per mole of_H2 consumed under these conditions: 25 degree C, 1 atm, pH 7.0, and 1 mM for all aqueous species except H^+_(aq).

Answer 1

1)

the balanced equation is

C2Cl6 + H2 ---> C2Cl4 + 2H+ + 2Cl-

we know that

dGo rxn = dGo products - dGo reactants

so

dGo rxn = dGo C2Cl4 + ( 2 x dGo H+ ) + ( 2 x dGo Cl-) - dGo H2 - dGo C2Cl6

dGo rxn = 28.7 + ( 2 x 0) + ( 2 x -131.3) - 0 + 51.5

dGo rxn = -182.4

so

dGo per moles of H2 consumed = -182.4 kJ

2)

the balanced equation is

S042- + 4H2 + 2H+ --> H2S + 4H20

dGo rxn = dGo H2S + ( 4 x dGo H20) - dGo S042- - ( 4 x dGo H2) + ( 2 x dGo H+)

dGo rxn = -27.8 + ( 4 x -237.1) + 744.8 - ( 4 x 0) + ( 2 x 0)

dGo rxn = -231.4 kJ / 4 mol H2

dGo rxn = -57.85 / mol H2 consumed

so

C2Cl6 yields more free energy per mole of H2