Q40. In the two half reactions:
Acetaldehyde + 2 H+ + 2 e− → Ethanol ΔE° = −0.16 V
NADH + H+ → NAD+ + 2 H+ + 2 e− ΔE° = −0.32 V
(F = 23,063 cal/V)
The ΔG° for the coupled reaction will be
Calculation of ΔG° for Coupled Redox Reaction (NADH–Acetaldehyde System)
Understanding the ΔG° calculation for a coupled redox reaction is very important in
biochemistry and electrochemistry. By using standard reduction potentials (E°), we can determine whether a reaction
is spontaneous or non-spontaneous. In this problem, we calculate the Gibbs free energy change for the reaction
between NADH oxidation and acetaldehyde reduction.
Given Data
Half Reactions:
Acetaldehyde + 2H+ + 2e− → Ethanol
E° = −0.16 V
NAD+ + 2H+ + 2e− → NADH
E° = −0.32 V
Faraday constant (F) = 23,063 cal V−1 mol−1
Number of electrons (n) = 2
Step 1: Identify Oxidation and Reduction
Reduction (Cathode)
Acetaldehyde → Ethanol
E°red = −0.16 V
Oxidation (Anode)
NADH → NAD+ (reverse reaction)
E°ox = +0.32 V
Step 2: Calculate Cell Potential
E°cell = E°red + E°ox
E°cell = (−0.16) + (0.32) = +0.16 V
Step 3: Calculate ΔG°
Formula:
ΔG° = −nFE°cell
ΔG° = −(2)(23,063)(0.16)
ΔG° = −7,380 cal ≈ −7,400 cal
Correct Answer
(B) −7,400 cal
Explanation of All Options
- (A) +7,400 cal ❌ → Positive ΔG° indicates non-spontaneous reaction.
- (B) −7,400 cal ✅ → Correct value and spontaneous reaction.
- (C) −22,200 cal ❌ → Incorrect magnitude; calculation mismatch.
- (D) +22,200 cal ❌ → Wrong sign and incorrect magnitude.
Important Concepts & Formulas
E°cell = E°cathode − E°anode
ΔG° = −nFE°
- Positive E° → spontaneous reaction
- Negative ΔG° → spontaneous reaction
- NADH acts as an electron donor
Final Conclusion
The coupled reaction between NADH oxidation and acetaldehyde reduction produces a
positive cell potential (+0.16 V) and negative ΔG° (−7,400 cal). Therefore, the reaction
is thermodynamically favorable and spontaneous.
Final Answer: (B) −7,400 cal
