Redox reaction and Electrode Potential
Redox reaction is the electron transfer reactions between the one type of electrode to another electrode by chemicals. Thus the tendency of an element to loss or gain electron calculated by standard chemical potential changes. Electrochemical cell or cells equations, ceric sulfate solution, permanganate ion in the acidic solution are examples of where direct or indirect redox reaction takes place. Hence the standard electrode potentials equations help to predict the reactions possibly or not.
The standard electrode potentials can be calculated by the equilibrium constant of these chemical reactions.
Redox Reaction in Acidic Solution
Let us take the simple equations of a redox reaction, oxidation of ferric ion by ceric ion, or reduction of ceric ions by ferric ion in the presence of dilute H2SO4.
Ce+4 + Fe+2 ⇄ Ce+3 + Fe+3
The individual half cell reactions
Ce+4 + e ⇄ Ce+3 (Reduction)
Fe+2 ⇄ Fe+3 + e (Oxidation)
Redox Reactions in Standard Electrode Potential
We can derive chemical equilibrium constant in terms of thermodynamics free energy changes of the reactions. If we apply the Nernst equation for the above equation
ΔG0 = – RT lnK = – nFE0
where ΔG0 = standard free energy,
E0 = electrode potential.
In the above case value of n=1. Thus the standard electrode potential equations of redox chemicals and equilibrium constant can be calculated from the above equation.
Electrochemical Daniell Cell
Electrochemical equations of Daniell cell examples provide the quantitative basis of the redox reactions. When this cell proceeds electric potential energy form by spontaneous oxidation and reduction reactions. Daniell cell is an example of this type of electrochemical cells.
Therefore electrons are transferred from the zinc atom to cooper atom in this cell reaction consists of two-electrode cathode and anode.
At the Zinc electrode
Zn(s) → Zn+2 +2e
At the copper electrode
Cu+2(aq) + 2e → Cu(s)
The type of electrode where electrons are lost is called anode but where electrons are gained is called the anode.
Examples of Electrochemical Cells Reaction
When an indirect redox reaction takes place, it produced chemical energy. Thus electrochemical cells converted chemical energy into electrical energy.
Galvanic cell or Voltaic cells are examples of electrochemical cells where chemical energy converted into electrical energy by chemical changes.
Ceric Sulfate in the Redox Reaction
Ceric sulfate is a very good oxidizing agent used in redox reactions for the titration process.
The half rection
Ce+4 → Ce+3 + e
Ce+4 during the reaction exists as an anionic complex in the H2SO4 solution. Hence the formal potential in H2SO4 solution = 1.42 V.
Permanganate Ion in Redox solution
Permanganate solution is a powerful oxidizing agent in the redox process.
Thus the reaction in acid solution represented as
2MnO4– + 8H+ + 5e ⇄ Mn+2 + 4H2O
E = +1.51 V
It is very useful for redox titration because of no special indicator needed for noted the endpoint of the reaction.