Question

Energetics: fermentation. In fermentation reactions, "good" electron acceptors (like O_2 and NO_3^-) are unavailable. In ethanol fermentation pathways, the electron-accepting half-reaction is written as reaction 0-5 (with CO_2 as the apparent electron acceptor) in the half-reaction handout given in class. a) Write the two half-reactions and the net reaction for glucose fermentation to ethanol. This is essentially the net reaction for ethanol formation form corn. b) What is the net Delta G degree' for this reaction, in kJ/e- eq and in kJ/mol glucose? c) If this fermentation is done in batch, and ethanol is allowed to accumulate, would you expect the Delta G value to increase (become more +) or decrease with time? Why?

Answer 1

Alcohol fermentation is a simple fermentation process in which a reduced product is formed such as ethanol. All the electrons starting at glucose must end in ethanol. The electron acceptor should be ethanol and electron donor should be glucose. The two half reactions of ethanol fermentation are as follows:

1/6 CO₂ + H⁺ + e ^- ----------------------> 1/12 CH₃CH₂OH + 1/4 H₂O

1/24 C₆H₁₂O₆ + 1/4 H₂O --------------> 1/4 CO₂ + H⁺ + e^-

The overall balaced chemical equation is :

1/24 C₆H₁₂O_{6 -------------------------------->} 1/12 CH₃CH₂OH+ 1/12 CO₂

b. The main reaction is,

Glucose--------------> 2 ethanol + 2 carbon dioxide

CH₃CH₂OH = - 174.9

C₆H₁₂O_{6 --} 1273.3

CO₂₌ -394.4

∆G0 = ∆G0 (products) - ∆G0 (reactants)

= [ (2 X -174.0 ) +( 2 X -394.4)] - [-1273.3]

= [-348 + (-788.8)] + 1273.3

= 136.5 KJ/mol