Question

Which of the following statements about standard reduction potential and spontaneity are true?

Answer 1

Answer:

Statements A and C are true.      

Explanation:

Let's evaluate the given statements:

A) A reaction with negative Gibbs standard free energy (ΔG) will result in a reaction thermodynamically spontaneous, under standard conditions, by the Nernst equation:  

$\Delta G = - nFE_{cell}$        (1)

where n: is the number of electrons transferred in the cell reaction, F: is the Faraday constant and E: is the cell potential

For an electrochemical reaction to be spontaneous, that is to say, in a galvanic cell, the change in Gibbs free energy must be negative. From equation (1), we have that a negative ΔG will result in a spontaneous electrochemical reaction.  

Hence, statement A is correct, and D is incorrect.

C) A coupled redox reaction with a positive standard reduction potential is thermodynamically spontaneous:  

On equation (1), for the Gibbs free energy to be negative and, therefore, the reaction to be thermodynamically spontaneous, the standard reduction potential must be positive.    

Hence, statement C is correct, and B is incorrect.    

I hope it helps you!  

Answer 2

Answer:

A and C are true , B and D are false

Explanation:

For A)

from the first law of thermodynamics (in differential form)

dU= δQ - δW = δQ - PdV

from the second law

dS ≥ δQ/T

then

dU ≤ T*dS - p*dV

dU - T*dS + p*dV ≤ 0

from the definition of Gibbs free energy

G=H - TS = U+ PV - TS  → dG= dU + p*dV + V*dp - T*dS - S*dT

dG -  V*dp +  S*dT = dU - T*dS + p*dV ≤ 0

dG ≤ V*dp -  S*dT

in equilibrium, pressure and temperature remains constant ( dp=0 and dT=0). Thus

dG ≤ 0

ΔG ≤ 0

therefore the gibbs free energy should decrease in an spontaneous process → A reaction with a negative Gibbs standard free energy is thermodynamically spontaneous under standard conditions

For B) Since the standard reduction potential is related with the Gibbs standard free energy through:

ΔG⁰=-n*F*E⁰

then, when ΔG⁰ is negative , E⁰ is positive and therefore a coupled redox reaction with a positive standard reduction potential is thermodynamically spontaneous.