Electrochemical Potential and Rate of redox

[Chocolatebear89]

---

Members do not see this ad.

I believe the higher the electrochemical potential, the faster the rate of reduction/oxidation occurs. I was wondering if an increase in the concentration of reactants will increase the rate of reduction/oxidation since it does in other rate kinetics (Sn2/E2).

 

[Rabolisk]

No, not necessarily. Higher potential means lower deltaG, which has to do with equilibrium, not kinetics. An increase in the concentration of reactants will probably increase the rate of most redox reactions, but it is highly dependent on the exact mechanism. Rate is experimentally determined, and then the mechanism is deduced.

 

[Chocolatebear89]

52. What is the effect of increasing the concentration of reactants in a voltaic cell?
A. The voltage increases, while the spontaneity of the reaction remains the same.
B. The spontaneity of the reaction increases, but the voltage remains the same.
C. Both the voltage and the spontaneity of the reac- tion increase.
D. The reaction rate increases, but the voltage and spontaneity of the reaction are unchanged.

 

[Prateek]

52. What is the effect of increasing the concentration of reactants in a voltaic cell?
A. The voltage increases, while the spontaneity of the reaction remains the same.
B. The spontaneity of the reaction increases, but the voltage remains the same.
C. Both the voltage and the spontaneity of the reac- tion increase.
D. The reaction rate increases, but the voltage and spontaneity of the reaction are unchanged.

E = E0 - (RT/nF) ln (products/reactants)

If you increase the concentration of the reactants, then 'E' will increase, meaning voltage will increase

G = -nFE

If E increases, then G will be more negative.

So would the answer be C?

 

[Chocolatebear89]

Yes the answer is C, but isn't spontaneity related to Gibb's free energy and equilibrium? I thought concentration doesn't affect those.

 

[Prateek]

Yes the answer is C, but isn't spontaneity related to Gibb's free energy and equilibrium? I thought concentration doesn't affect those.

Yea but one form of the Gibbs free energy equation is

G = -n * F * E

where E is affected by concentration

 

[Rabolisk]

Delta G knot is independent of concentration, but not dG. Same thing with E. Equilibrium constant is independent of concentration, obviously, but Q is not.

 

[mcgill2012]

wait... why does increasing the concentration of reactants increase the EMF?

 

[Rabolisk]

E = Eknot - (RT/nf)lnQ

Q decreases as concentration of reactants increases. At Q < 1, lnQ is negative. The right term then becomes negative, which means E > Eknot.

 

[sangria1986]

E = Eknot - (RT/nf)lnQ

Q decreases as concentration of reactants increases. At Q < 1, lnQ is negative. The right term then becomes negative, which means E > Eknot.

is this necessary for mcat?

 

[Rabolisk]

Yes. It's a really basic concept, really. Being able to go from Gibbs Free Energy to EMF to equilibrium constants is the sort of thing that the MCAT will test, conceptually or mathematically.

 

[mcgill2012]

does this explanation also make sense?
increasing concentration of reactants shifts the rxn to the right, by le chateliers principle, increasing the thermodynamic favorability of the forward reaction and therefore making deltaG more negative. The greater the magnitude of the negative delta G, the greater the magnitude of the positive EMF, according to DeltaG = -nFE. Thus, EMF increases by increasing reactants.