ELECTROCHEMISTRY. During electrolysis positive ions (cations) move to negatively charged electrode (catode) and negative ions (anions) to positively charged electrode (anode) For the case of NaCl we have: cathodic reduction: Na + + electron = Na and anodic oxidation:
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During electrolysis positive ions (cations) move to negatively charged
electrode (catode) and negative ions (anions) to positively charged
electrode (anode)
For the case of NaCl we have:
cathodic reduction:
Na + + electron = Na
and
anodic oxidation:
Cl   electron = Cl
Electrochemical cell is composed of two halfcells, realized e.g. as
metal electrode immersed in the solution of its salt.
The halfcells are conductively connected, e.g. by salt bridge.
Each halfcell contains oxidized and reduced component, which
create a redox couple
+ e.g. as

p .... osmotic pressure
P .... solvatation pressure
P > p negative electrode charge
P < p positive electrode charge
Hydrogen electrode e.g. as
Precious metals such as platinum or palladium absorb vigorously
hydrogen. A solid solution is formed, analogical to the metal alloys.
Here we have hydrogen present in its atomic form, not as a two atom
molecule. Thus, in this state hydrogen has properties of a metal.
If we saturate a platinum electrode coated with platinum black by a
stream of hydrogen and immerse this electrode to the solution,
protons will be released into the solution due to the solvatation
pressure until they balance the proton osmotic pressure. This leads
to the generation of a potential, dependent on hydrogen partial
pressure.
Standard hydrogen electrode
is realized under conditions of [ H+] = 1 (i.e. pH = 0) and hydrogen
pressure 1 atm. By convention its potential = 0
By comparison of the potential of a halfcell, realized as a metal
electrode immersed in 1 N solution of its salt, with standard hydrogen
electrode we obtain electrochemical series.
Some examples:
Electrode Potential (Volt)
Li/Li+  3, 02
K/K+  2, 92
Na/Na+  2, 71
Zn/Zn2+  0, 76
Fe/Fe2+  0, 43
Fe/Fe3+  0, 04
H/H+ 0, 00
Cu/Cu2+ + 0, 34
Cu/Cu+ + 0, 51
Ag/Ag+ + 0,80
Au/Au+ + 1, 50
Metals, placed above hydrogen in this table, have a tendency to form
positive cations and with distance from hydrogen, their
electropositivity increases.
More electropositive metal displaces less electropositive metal from
the solution.
Potential of a metal electrode dissolving metal cations into solution
is given by
Nernst equation:
E =  RT/nF . ln c
where R ...universal gas constant
n ... number of electrons representing the difference between
the metal and its ion
c ... concentration of the ions in solution
We can express the amount of energy released in electrochemical
process as:
DG =  nFE
Under standard conditions (concentration 1 M, pressure 1 atm) we
get:
DG0 =  nFE0
where E0 is the standard potential of the cell
Standard potential of the cell can be calculated as a sum of standard
potentials of electrodes:
E0 = E0(anode) + E0(catode)
Concentration cell electrochemical
E = E0 – RT/2F ln (c2/c1)
We can use Nernst equation for the calculation of a potential
generated in redox reactions in the living cell. The reaction:
reductant + oxidant = oxidized reductant + reduced oxidant
can be simplified:
electron donor = electron acceptor + electron
then DE0 = E0(acceptor)  E0(donor)
as DG0 =  nF DE0
we get:
E = E0 + RT/nF ln( [acceptor]/[donor] )
Membrane potential potential