Electrochemistry Flashcards
electrochemical cells
contained systems in which redox rxns occur
3 types of electrochem cells
1) galvanic
2) electrolytic cells
3) concentration cells
all three types of the electrochemical cells contain…
electrodes where oxidation and reduction take place
oxidation electrode called anode
reduction electrode called cathode
emf positive
ΔG < 0 spontaneous cell can release energy
emf negative
ΔG > 0 nonspontaneous cell will absorb energy
for all electrochemical cells, electrons move…
from anode to cathode
for all electrochemical cells, current (I) runs…
from cathode to anode
galvanic cells
house spontaneous rxns
non-rechargeable batteries
cell release energy into environment
emf Ecell is positive
emf and free energy change…
must always have opposite signs
electrodes of galvanic cells
- half- cells
- connected by conductive material usually copper wire
- surrounded by electrolyte soln w cations and anions
- connected by salt bridge that consists of an inert salt (charge gradient)
cell diagram
Zn(s)|Zn2+(1M) || Cu2+(1M)|Cu(s)
an-anode soln-cathode soln-cath
electrolytic cells
- houses nonspontaneous rxns
- reduction at cathode, ox at anode
- require energy input to proceed
- driven by external voltage source (chemical compounds decomposed)
- isolate elements
Faraday constant
10^5
electrodeposition equation
mol M = It/nF
M: moles of metal ion at a particular electrode
I: current
t: time
n: # of e- equivalents
F: Faraday constant
concentration cell
- 2 half cells for the spontaneous redox rxns
- electrodes chemically identical
- concentration gradient results in potential difference between the two compartments and drives the movement of electrons in the direction that results in equilibrium of the ion-gradient
- current will stop when the concentrations of ionic species in the half-cells are equal
- V/emf is 0 when concentrations are equal
- Voltage calculated using Nernst equation
rechargeable cell…
can function as both a galvanic and electrolytic cell
lead-acid battery
- as a voltaic cell, when fully charged, 2 half cells: Pb anode and PbO2 cathode and connected by 4M H2SO4 conductive material
when fully discharged consists of 2 PbSO4 electroplated lead electrodes with a dilute concentration of H2SO4
- dilute acid electrolyte when discharging
- when charging the lead-acid cell is part of an electrolytic circuit
- low energy-to-weight ratio
nickel-cadmium batteries
- rechargeable battery
- 2 half cells solid cadmium anode and nickel(III) oxide-hydroxide cathode connected by conductive material typically KOH
- Ni(OH)2 and Cd(OH)2 products
- surge currents: periods of large current early in the discharge cycle
- Ni-Cd half rxns tend to provide higher surge current
in galvanic cells electrons move from…
neg (low EP) to pos (high EP)
anode of electrolytic cell is…
positive
cathode ALWAYS attracts ______ regardless of charge
cations
oxidation ALWAYS takes place ______
at the anode
reductions ALWAYS takes place _______
at the cathode
anode ALWAYS attracts ________
anions
a positive E°red indicates…
spontaneous reduction
why is the ΔG of the electrolytic cells positive?
anode has pos reduction potential and cathode has negative, so an external voltage source must force the appropriate rxns in the appropriate places
standard electrochemical force of a cell
E°cell = E°red,cath - E°red,an
standard change in free energy from standard emf
ΔG° = -nFE°cell
Nernst eq
Ecell = E°cell - (0.0592/n • logQ)
or (RT/nF • lnQ)
rxn quotient
Q = [C]^c•[D]^d / [A]^a•[B]^b
standard change in free energy from eq constant
ΔG° = -RTlnKeq
free energy change nonstandard
ΔG = ΔG° + RTlnQ
in galvanic cells, species with the more ________ is reduced whereas in electrolytic the opposite is true.
more positive reduction potential
