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Redox Reactions

Redox Reaction in Electrochemical Cell

Redox reaction takes place in the electrochemical cell converted chemical energy in electrical energy by electron transfer electrolysis reactions. Therefore, the electrochemical or simply chemical cell is the device in which energy produced due to the indirect redox reaction of the polar molecules and converted electricity for mankind. Galvanic and Voltaic cells are examples of redox cells according to the name of the scientist, Luigi Galvani, and Alessandro Volta. The electrode potentials of redox reaction can define as the tendency of an element to lose or gain electron particles when contact with its own solution. When electrode losses electron particles called oxidation potential and gain electrons called reduction potential of the redox reaction and these two chemical reactions reverse to each other. Therefore, the standard electrode potentials calculated from the chemical equilibrium constant of these chemical reactions.

Redox electrode potential in electrochemical cells and chemical electrolysis reaction in the cell

Oxidation Reduction Electrode Potential

When oxidation (loss of electrons) reaction takes place on an electrode, the chemical potential of the electrode is called oxidation potential but reduction (gain of electrons) takes place on an electrode is called reduction potential. If the oxidation potential of an electrode = x volt, then its reduction potential = -x volt. For example, two half cells chemical reactions of ferric ion and ceric ion in the presence of dilute sulfuric acid show two electrode potentials in balance chemical equations of the redox reaction.

Ce+4 + Fe+2 ⇄ Ce+3 + Fe+3
Half cell reactions with electrode potentials
Ce+4 + e ⇄ Ce+3 (E0 = 1.44 volt)
Fe+2 ⇄ Fe+3 + e (E0 = -077 volt)

Electrode Potential in Chemical Cells

When the metal rod suspended in a solution of its ions having 1M concentration and 298K temperature, the electrode potential of the half cell defines as the standard electrode potential of the redox reaction. If gas molecules involved, the standard pressure (1 atm) should be used for this calculation. Standard electrode potentials of oxidation-reduction reactions represented, E0ox and E0red. Standard electrode potentials can be derived from chemical equilibrium constant and thermodynamics free energy changes by Nernst equation in the standard state of chemistry or physics.

ΔG0 = – RT lnk = -2.303 RT logk
where ΔG0 = standard free energy,
Again ΔG0 = -nFE0
where, E0 = standard electrode potential
∴ E0 = (RT/nF) lnk = (0.059/n) lnk

Normal hydrogen electrode, H2 gas, 1atm (pt) | H+ (a = 1), in which pure hydrogen atom in gas phases at unit pressure is kept in contact with hydrogen ion of unit activity uses as a standard electrode. The potential of the normal hydrogen electrode taken zero at all temperature and uses as the reference for calculating electrode potential.

Electrochemical Daniell Cell

Electrochemical equations of Daniell cell examples provide the quantitative basis of the redox reactions. Chemicals are arranged in this cell to produce electric potential energy out of spontaneous oxidation-reduction (redox) reactions. Daniell cell is an example of this type of electrochemical cell. Therefore, the electron charge transferred from the zinc atom to the copper atom in this cell reaction consists of two-electrode cathode and anode.

At the Zinc electrode
Zn (s) → Zn+2 (aq) +2e
At the copper electrode
Cu+2 (aq) + 2e → Cu (s)

Electrode, where oxidation (loss of electrons) occurs, called the anode. Conversely, the electrode where reduction (addition of electrons) occurs called the cathode in learning chemistry.

Examples of Redox Reactions in Chemistry

  1. Ceric sulfate is a very good oxidizing agent used in the redox titration process. The ionic half cell reaction of ceric sulfate, Ce+4 → Ce+3 + e. Ce+4 during the reaction exists as an anionic complex in the sulfuric acid solution. Hence the formal potential ceric ion in sulfuric acid solution = 1.42 V.
  2. Crystalline potassium permanganate widely used oxidant in the laboratory with high redox potential. Permanganate dissolve in benzene solution uses as a potential oxidant for organic hydrocarbon and also oxidizes hydrogen peroxide. Acidic permanganate solution very useful for redox titration because no special indicator needed to note the endpoint of the reaction. Therefore, the reaction in acid solution (pH solution) represented as,

2MnO4 + 8H+ + 5e ⇄ Mn+2 + 4H2O
E = +1.51 V

Electrolysis Reaction in Chemistry

An electrolytic cell is a device where a non-spontaneous chemical reaction (redox chemistry) takes place by an external source of electricity. The electrolysis process occurring in the electrolytic cells. The very useful application of electrolysis found in commercial electroplating and purification of the periodic table elements. For copper plating material molecule obtained from our environment first cleaned and dipped into the suspension of aqueous copper sulfate solution. Therefore, the electrolysis or redox chemical reactions take place at the two electrodes, Cu+2 + 2e → Cu (cathode), Cu → Cu+2 + 2e (anode)

Electrochemical Fuel cells

Fuel cells are readily electrochemical cells in which the chemical energy available from heat or fuel such as, methane, hydrogen, etc converted into electrical energy by oxidation in the suitable anode. At the cathodes of the cell, oxygen is dissolved to form OH ions.

Such cells provide power at comparatively low cost and low maintenance. The efficiency depends particularly on the surface of the electrodes, which serve as the catalyst for the attainment of equilibrium. The polarization due to the adoption and poisoning of the redox electrode reduces the cell performance considerably.  The density of the electrolyte and temperature have great influence on the e.m.f of the cell.