ELECTROCHEMISTRY, REDOX PROCESSES, OXIDATION, OXIDATION HALF – EQUITION

 ELECTROCHEMISTRY

             It is the branch of chemistry which deal with the redox chemical changes occurs during different chemical reactions.

 REDOX PROCESSES:

             Process in which reduction and oxidation takes place are called redox process. Actually it is derived from two words “Red” from reduction and “ox” – from oxidation.

 OXIDATION:

             Addition of oxygen or

            Removal of hydrogen or

            Removal of electrons is called oxidation. Overall it is increase in  oxidation number.

 REDUCTION:

             Removal of oxygen or

            Addition of hydrogen or

            Addition of electrons is called reduction. Overall it is decrease in oxidation number.

 OXIDATION HALF – EQUITION:

             Half equation which shows only lose of electrons is called reduction half equation e.g;

             Cu²⁺ + 2e^- Cu

            I₂ + 2e^-  2I^-

 REDOX REACTION:

             Redox reaction is written by combining oxidation half reaction and reduction  half reaction number of electrons should be balanced in both half equations

 OXIDIZING AGENT (OXIDANT):

            Species which gains electrons is called oxidisng agent e.g;

 

REDUCING AGENT (REDUCTANT):

             Species which loses electrons is called reducing agent e.g;

     

OXIDATION NUMBER (OXIDATION STATE):

            The apparent charge on an atom of an element in a molecule or an ion is called oxidation number or oxidation state of that element.

RULES TO DETERMINE OXIDATION NUMBER:

(1)       Element in free state have zero oxidation state e.g;

(2)       In case of neutral molecules, the algebraic sum of all positive and negative oxidation numbers in zero e.g;

           

 

(3)       Oxidation number of an ion consisting of a single element is the same as the charge on the ion

 

(4)       In case of ions having more than one elements, the algebraic sum of all the oxidation numbers

is equal to the charge on the ion e.g;

SO  

 

(5)       Hydrogen has +1 oxidation number in all its compounds except metal hydrides where it is -1 e.g;

(6)       Oxygen in all its compounds has -2 oxidation number except in periods and superoxides where it is -1 and -1/2 respectively e.g;

            Normal oxides; (+2)

(7)       The rule for assigning the oxidation numbers is based on the difference of electro negativity. More electro negativity atom has negative oxidation state while less electronegative atom have positive oxidation state e.g; (+x) + (-1x2) oxygen has +2 oxidation state in OF_2.

(8)       Oxidation number of an element may be different in different compounds e.g;

            Iron (II) chloride,

(9)       Oxidation number of an element may be a positive, negative or zero value e.g;         

(10)     Oxidation number of an element may be frictional e.g;

            Oxidation number of Fe in Fe₃O₄ is + 2.67

ELECTROLYSIS:

            The process in which due to a d c supply of electric charge through the electrolyte redox chemical changes takes place the electrodes is called electrolysis.

INDUSTRIAL APPLICATION OF ELECTROLYSIS IN DIFFERENT PROCESS:

            Electrolysis have large application in industrial  processes e.g;

(i)        The electrolysis of brine using a diaphragm cell to obtain chlorine, hydrogen and sodium hydroxide.           

 

(ii)       The extraction of aluminum from molten aluminum oxide/cryolite.

(iii)      The electrolytic purification of copper.

 

THE ELECTROLYSIS OF BRINE USING A DIAPHRAGM CELL:

            Brine is a concentrated aqueous sodium chloride. Ions present in the electrolyte are;

           

CATHODE	ANODE
Na+ H+	Cl- OH-

            At the anode cell Cl ions are oxidized in performance to OH ions because they are present in large concentration.

            At the cathode H ions reduced. As the concentration of Na ions is large but their resution potential is very small as compared to H ions.

            In the cell aqueous sodium hydroxide left which is industrially also important unfortunately, chlorine reacts with aqueous sodium hydroxide ro form sodium chloride and sodium chlorate (I), NaOCL. Cl₂ + 2NAOH            NaCLO + NaCL + H₂O.

 

            This reaction reduce the yield of chlorine gas and sodium hydroxide and cost of production will be greater. To prevent this reaction from occurring a diaphragm cell is used which physically keeps the chlorine and the aqueous sodium  hydroxide separate. In the diaphragm cell.

(1)       A porous asbestos diaphragm separate the mole compartment  from the cathode compartment.