What is true?Kinetics or thermodynamics.
please design a exprement for show it.
Dear Ehsan,
It is both. For more information and illustration of equations and figures please use the following links:
http://www.engr.uconn.edu/~jmfent/CHEG320_electrode%20kinetics%20lectures.pdf
http://web.stanford.edu/~kaleeg/chem32/kinT/
https://www.google.ps/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwif7cGzopTLAhXkO5oKHXr7Am4QIAgiMAE&url=http%3A%2F%2Fwebcache.googleusercontent.com%2Fsearch%3Fq%3Dcache%3AEuR51hypnVgJ%3Awww.engr.uconn.edu%2F~jmfent%2FCHEG%252520320_Electrode%252520Kinetics_extended%252520notes.pdf%2B%26cd%3D2%26hl%3Den%26ct%3Dclnk%26gl%3Dps&usg=AFQjCNG-zAm67aItGGXDQatpUT-e7BFVkw&sig2=7REmRW7g1YBvvQFwLmTc8w
https://www.google.ps/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwif7cGzopTLAhXkO5oKHXr7Am4QIAgcMAA&url=http%3A%2F%2Fwebcache.googleusercontent.com%2Fsearch%3Fq%3Dcache%3AZEl5n-7Zg00J%3Awww.springer.com%2Fcda%2Fcontent%2Fdocument%2Fcda_downloaddocument%2F9783642029141-c1.pdf%253FSGWID%253D0-0-45-838014-p173918406%2B%26cd%3D1%26hl%3Den%26ct%3Dclnk%26gl%3Dps&usg=AFQjCNEu-chFxZmJ7oR45wGQWjCq5q7s_g&sig2=uF5488VFRKgUAmA8aUeKTg
Hoping this will be helpful,
Rafik
Thank you for your answer.
If electrode potential is low, rate of corrosion is great???
The electrochemical cell potential is a thermodynamic parameter. The over potential is a driving force (kinetic) parameter.
Dear Ehsan,
As I mentioned in the first link depicted in my first answer:
https://www.researchgate.net/deref/http%3A%2F%2Fwww.engr.uconn.edu%2F~jmfent%2FCHEG320_electrode%2520kinetics%2520lectures.pdf
Requirements of kinetic theories
•Consider the general reversible electrode reaction
O + ne = R (7)
with kf and kb as the forward and backward
reaction rate constants
•This is a typical redox reaction
Note that O and R stand for Oxidized state and
Reduced state respectively
• For this reaction, the equilibrium state is
governed by the Nernst equation - See
section on thermo:
E
eq
= E° - RT / nF [ln (CR * / CO*)] (8)
• Relates the equilibrium potential of the
electrode (Eeq ) to the concentration of the
reactants and products (O and R)
Any electrode kinetics theory must reduce
to the Nernst equation when equilibrium
conditions are placed upon it
-1st
requirement of an electrode kinetic
theory
E vs. i
• Electrode kinetic theory - must provide
information about the dependence of current
on electrode potential
• When a current is passed, some
electrochemical change occurs at the
electrode
Overvoltage
• Due to irreversibility - sustaining a given
current requires that a penalty be paid in
terms of electrode potential – penalty called
overvoltage
η = Eeq - E (9)
Eeq - expected (equilibrium) electrode potential
E - actual electrode potential
Tafel equation
•Experimental observation: At low currents, the
current is exponentially related to the
overpotential
η= a + b log i (10)
•a and b are constants , i is the current density
•Called the Tafel equation
•Any successful electrode kinetic theory must also
explain Tafel behaviour –
2nd requirement of an
electrode kinetic theory
Recap
1. Any electrode kinetics theory must reduce
to the Nernst equation when equilibrium
conditions are placed upon it
2. Any electrode kinetic theory must also
explain Tafel behaviour
3. Rate equations must reduce to the Nernst
and Tafel equations when the appropriate
conditions are introduced.
Kinetics of Electrode Reactions
Rates of forward and backward reaction
O + ne = R (7)
•For the above reaction, the rate of the forward reaction is given
by:
R
f = k
f
C
O (0, t) = i
c /nF
(11)
Where C
O (0, t) is the surface concentration of O
•Similarly the rate of the backward reaction is given by:
R
b = k
b
C
R (0, t) = i
a /nF
(12)
Recall that reaction rate and current are interlinked –
also
recall that reduction occurs at the cathode and oxidation at
the anode
Net reaction rate
•The net reaction rate (and hence the net
current) is given by
Rnet = R
f – R
b = i/ nF = [i
c
– i
a]/ nF
= [k
f
C
O (0, t) -
k
b
C
R (0, t)] (13)
Potential dependence of kf and kb
• Both kf and kb are potential dependent
functions
•The forward reaction (a reduction) is an
electron accepting process -
the rate
increases as the electrode potential becomes
more negative
•This is because the electrode can give away
electrons more easily
• The opposite trend is seen for the backward
(oxidation) reaction - the rate increases as
the electrode potential becomes more
negative
This potential dependence needs to be
qualified to obtain a true picture of
electrochemical kinetics
Potential dependence of kf and k
This text indicates the requirements of kinetic theories. To view the full equations, please use the link depicted above.
Rafik
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