Thermodynamics of a Redox Reaction Practice Problems with Answers

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An 18-question problem with answers about the thermodynamics of a redox reaction in the electron transport chain.

1- Write a complete balanced redox reaction, wherein electron(s) are transferred between Coenzyme Q and lipoic/dihydrolipoic acid in a spontaneous forward direction. Indicate the states of all species on the reactant and product sides:  ubiquinone or ubiquinol; lipoic acid or dihydrolipoic acid.

2-For the forward reaction, which species is acting as the oxidizing agent?

  • ubiquinone
  • ubiquinol
  • lipoic acid
  • dihydrolipoic acid

3- For the forward reaction, which species is acting as the reducing agent?

  • ubiquinone
  • ubiquinol
  • lipoic acid
  • dihydrolipoic acid

4-

a- Write the half-reaction involving ubiquinone.

b- For the overall redox reaction (Question #1), would this half-reaction occur as an oxidation or a reduction?

c- What is the value (in volts) of the standard reduction potential for this half-reaction?

5-

a- Write the half-reaction involving lipoic acid.

b- For the overall redox reaction (Question #1), would this half-reaction occur as an oxidation or a reduction?

c- What is the value (in volts) of the standard reduction potential for this half-reaction?

6- Calculate ΔE’° for the overall redox reaction (Question #1).

7- Does the calculated ΔE’° value (Question #6) supports the fact that the overall redox reaction (Question #1) would be spontaneous in the forward direction?

8- a- Calculate ΔG’° for the overall redox reaction.

b- Under standard biochemical conditions, is this reaction endergonic or exergonic?

9- Calculate K’eq for the overall redox reaction. Assume 25°C.

10- Calculate the actual free energy change, ΔG, if this redox reaction occurred within our cells (body temperature of 37°C) and typical cellular concentrations were as follows:

[ubiquinone] = 1.2 mM

[ubiquinol] = 5.3 mM

[lipoic acid] = 7.9 mM

[dihydrolipoic acid] = 2.5 mM

11- Calculate the actual change in potential, ΔE, if this redox reaction occurred within our cells (see conditions in Question #10).

12-

a- Calculate the enthalpy change (ΔH) associated with this redox reaction, assuming the conditions outlined in Question #10 and an entropy change (ΔS) of +0.8 kJ/mol∙K.

b- Is this reaction enthalpically favored or opposed?

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