chem 223 exam 3 part 5 - cell potential intro

coloumb's law

PE = qQ/r

cell potential

potential energy that can come out of redox reaction

W el formula

W el = -nF E cell

W el = ΔGrxn, therefore

ΔG° = -nF E cell

ΔG° = -nF E cell schematics

n = number of electrons in balanced reaction

F = Faraday constant = 96485 c/mol e-

E°cell > 0

E°cell < 0

spontaneous ; won't reduce H+

nonspontaneous ; will reduce H+

for the value of standard cell potential

when you either reverse the half reaction or multiply it to balance the electrons, DO NOT change the value of the standard cell potential

just normal cell potentials can be reversed though

deriving ΔG formula

-nF E cell =

and -nF E cell = ΔG° + RTlnQ

thus, ΔG = ΔG° +RTlnQ

ΔG vs ΔG° formulas

ΔG = ΔG° +RTlnQ

ΔG° = -RTlnK

ΔG formula for when you're not given ΔG°

ΔG = RTln(Q/K)

E cell is E°cell when

concentrations of ALL ions are 1 M

[O] = [R] = [O'] = [R'] = 1M

and Q = 1

Nernst equation (E cell)

Ecell = E°cell - (RT/nF) lnQ

thus,

E°cell = (RT/nF) ln K

Q = reaction quotient ([products]/[reactants])

R = 8.314

Ecell =

E°cell =

ΔGrxn

ΔG°rxn

At equilibrium, what is Ecell?

Ecell = 0

thus, to find Ecell at equilibrium, must use

E°cell = (RT/nF) ln K

nernst equation when temp is standard (298K or 25 degrees)

Ecell = E°cell - (0.0591/n) lnQ

formula when going from E°cell to ΔG°rxn

ΔG°rxn = -nF E°cell

formula when going from E°cell to K

E°cell = (RT/nF) ln K

formula when going from K to ΔG°rxn

ΔG°rxn = -RTlnK

what must always be true for spontaneous?

K > 1

between Zn and Zn2+, which is a better oxidizing agent?

Zn2+, since it as the ability to acquire electrons

Zn is neutral, so no desire to acquire electrons

steps for cell potential problems

1.) make sure electrons for both half reactions are not on the same side, otherwise it can't be canceled out

--can reverse direction of reaction

--ALWAYS reverse half reaction that would MAKE THE RESULTING ADDITION ANSWER POSITIVE

2.) add together

--resulting answer should be positive since it's voltaic cell and voltaic cells are spontaneous (E°cell > 0)

for answers of electrolytic cells

resulting added together cell potential can be positive, 0, or negative

ΔGrxn equation with ΔG°rxn

ΔGrxn = ΔG°rxn + RTlnQ