ELECTROLYSIS AND IONIC THEORY

ELECTROLSIS

                                           

 Some of materials or substance such as metals allow electric current to pass through them. Molten salts and some solution also conduct electricity.

- Conductor is the substance which allow electric current to pass through it.

     Eg. Steel and metals

- Non Conductor (Insulator) is the substance which do not allow electric current to pass through. Eg. Wood, paper or rubber.

- Semiconductor or poor conductor is the substance which allow partially electric current have free electrons or mobile electrons. As movement of these free changed particles make it possible for material to conduct electricity.

    ELECTROLYTE AND NON ELECTROLYTES.

ELECTROLYTES

    Is the substance which desociate or decompose into free ions when they are in solution or molten state, thus allowing electric current to pass through.

Electrolyte always exist as solution of acid or base or salts.

       Eg.  NaCl > Na + + Cl-

NON-ELECTROLYTE

Is the solution or molten state which do not conduct electricity.

   Non-electrolyte do not dissociates into ions in solution or molten state, there fore they don't have free electrons to conduct electricity.

   

There are other molten materials such as copper ( Cu ) conduct current but they are not electrolyte.

      This is because conduction of electricity in metal is done by electrons and not ions, and also the metal do not decompose when electricity pass through them.

IONIC THEORY

  - Ionic theory say that ionic compounds desociate to form ions.

         Eg. MX > M+ + X-

Where;

            M represent metal (eg Na, Mg and so on)

            X-represent non mete (eg Cl, NO3-, and so on

- After desocition of ionic compounds ions are free to move and support passage of electricity through solution

- The power supply connected are connected and electrodes are charged

- cation move to cathode and anions move to pistively charged particle

Electrolysis

    Electrolysis is the process in which an electric current is passed through an electrolyte causing a chemical reaction to occur.

                                                      Action happen at anode;

                                                         At anode oxidation take place

                                                                X- > X + e

                                                          Action happen at cathode; 

                                                         

                                                           At cathode oxidation take place

                                                           M+ + e > M

- Electrode; 

   Is the component of the cell which make contact with the electrolyte.

- Cathode is Electrode attached to negative terminal

                  Or

Cathode is an Electrode through electrons enter the electrolyte

- Anode is an Electrode attached to positive terminal

                 Or

Anode is an Electrode through which electrons enter the electrolyte.

Strength of electrolyte depend on their dissociate when dissolved in solvent. If electrolyte dissociate partially when dissolved in solvent, then that electrolyte is weak electrolyte. And if electrolyte dissociate completely when dissolved in water then that electrolyte is strong electrolyte.

STRONG ELECTROLYTE

 Strong electrolyte is the electrolyte which is dissociate completely when dissolved in solved to release ions. And these electrolyte are high electric conductor due to their ability to dissociate completely in solvent.

     Example of strong electrolyte is sodium chloride. When sodium chloride dissolved in water dissociate completely to form free ions of sodium ions and chloride ions.

                              NaCl > Na+ + Cl-

Mineral acids such as sulphuric acid, hydrocloric acid and nitric acid are also strong electrolytes. They dissociate completely into ions when dissolved in water

   

                            H2SO4 > H2+ + SO4-

     

                            HCl > H+ + Cl-

WEAK ELECTROLYTE

     Weak electrolytes are electrolyte which are partially dissociate in solvent to give ions. They do not efficient conduct electricity due to releasing of few ions when dissolved in solvent.

      Example of weak electrolyte include a solution of ammonium chloride because ammonium chloride dissociate partially in solvent to form ammonium ions and chloride ions.

                                                 

Partially dissociate means that after formation of ions (ammonium ions and chloride ions) they recombine again to for ammonium chloride.

  Ather example of weak electrolyte include, organic acid such as ethanoic acid and

methanoic acid.

NB;

   Electrolyte can have the same concentration but vary in strength.

       Example;  6M hydrochloric acid is strong electrolyte

                         6M methanoic acid is weak electrolyte.

Also elctrolyte can be of similar strength but different concentration.

       Example; 4M hydrochloric acid is strong electrolyte

                        2M sulphuric acid is is strong electrolyte.

MECHANISM OF ELECTROLYSIS
   
   These acqueous or molten ionic compounds they have free movement of ions thus why they conduct electricity.
- These movement of ions to their respective Electrode called MIGRATION.
- when electric passing through acqueous ionic compounds the solution electrolysed cations move to negative electrode (cathode) and anions move to postive electrode (anode). In solid state ions do not migrate because ions are not free. Ions in asilid electrolyte are held firmly by electrostatic force forming a "giant" crystalline structure.

            ELECTRODE REACTIONS.

                         When voltage applied the cathode electrode become negatively and anode electrode become postively discharged. The opposite charge cause migrations of ions in opposite direction.

     Anions become negative due to extra electron they have and cation become postive due to deffisiance of electrons.

Anion on arrive on the surface of anode surrender the extra electrons to the cathode and become discharged.

                                            X- > X + e-

For divalent electrons.       X2- > X + 2e-

     The process of lossing electrons called oxidation,  there fore the reaction taking place in anode is oxidation reation.

When cation arrive on the surface of cathode, they receive electrons from cathode and became discharged

                 

                                         M+ + e- > M

For divalent electrons. M2+ + 2e- > M

  

The process of gaining electrons is called reduction, there fore the reaction that take place take at cthode is reduction reaction.

                 NB; for oxidation half reactions, electrons appear on the right hand side of an equa

                         tion.

                       for reduction half reactions, electrons appear on the left hand side of an equa

                         tion.   

PREFERENTIAL DISCHARGE OF IONS DURING ELECTROLYSIS
                                                   

    A molten or acqueos solution contains four (4) ions, two are H+ and OH- these are coming from water and another two ions from elctrolyte

    But only two of this are discharged. The discharge depend on three factors which are;

                  1) Ease of discharge of ions 

                  2) The concentration of ions 

                  3) the nature of electrodes.

1. Ease discharge of ions.

         Metals and hydrogen are anion, the ease of discharge of ions determined by the position of the element in the electrochemical series.

Electro chemical series is an arrangement of elements depending on their ease in gaining electrons.
     
                             

               K+.           least readily
                              discharge           NO3-
               Ca2+.                                 SO4-
               Na+.                                     Cl-
               Mg2+.                                  Br-
               Al3+.                                     I-
               Zn2+.                                  OH-
               Fe2+
               Pb2+
               H+
               Cu2+        most readily discharge
               Hg+ 
               Ag+it
   Anions of the element  high in the electrochemical series are not readily discharged because their ions are very stable.
 For examplecopper sulphate electrolyte
       Ions present in the solution are Cu2+, SO4+, H+ and OH-. Cu2+ and H+ migrate to cathode but copper will be discharged instead of H+ because copper is below hydrogen in electrochemical series.

   And at anode, the hydroxide are discharged instead of sulphate ions because hydroxide is below sulphate in electrochemical series.

2. Concentration of ions.
      The ions with high concentration in electrolyte tend to be favoured in the discharge.

3. Nature of electrode.
        Nature of electrode especially cathode also determine which catode to be discharged first.
          Fo example when sodium chloride solution electrolysed using Mercury cathode, when Mercury cathode used, sodium ions are discharged to form sodium metel which is readily in dissolves in the Mercury to form sodium amalgam.

Laws of electrolysis
     Michael Faraday is the first scintist to investigate the relation between the amount of products formed at electrodes and the electric current passing through the electrolyte.

     He observed that the amount of products formed at electrode is direct proportional to the quantity of electric passing through electrolytes.

    From his observation summarized what is commonly known as Faraday's laws of electrolysis.

        FARADAY'S FIRST LAW OF ELECTTOLYSIS

Faraday's observed that the quantity of a substance produced at an Electrode during electrolysis depends on three factors, which are;

     - the quantity of electricity passing through the electrolyte per unit time.

    - the amount of time taken by an electric current to pass through electrolyte.

    - the charge on an ions.

The first farady's law of electrolysis state that;

     "The mass of substance produced or dissolved at an Electrode during electrolysis is proportional to the quantity of electricity transferred at electrode".

                                                       

   

     NB;
           Current is measured in an Empire (A).

           Time measured in second (S).

          Electric charge measured in coulombs (C)

When a current passing through a electrolyte at a given time, then electric charge that passes is;
 
                   Q = It

Where; Q = is quantity of electric passed

              I = is current passed through electrolyte

              t = is time taken by current to pass through electrolyte.

If m is mass deposited, then Faraday' first law of electrolysis can be expressed as;

            m = ZIt

     Where Z is constant known as electrochemical equivalent.

                 -    From the formula, m= ZIt

                 -        Z = m/It

                 -     And        It = Q, .          then   Z = m/Q

                 -    SI Unit of Q is C
         
                 -    there fore unit of Z is g/C .

-- The electrochemical equivalent (Z) of substance is the mass of the substance discharged when 1 coulombs of electricity passing through an electrolyte.

FARADAY'S COSTANT.

    Is the amount of electric charge carried by one mole of electrons.

The charge on one mole of electrons is approximately to 96 500C. This is the Faraday's constant.

FARADAY'S SECOND LAW OF ELECTTOLYSIS.

FARADAY'S second law state that;

 " When the same quantity of electrolysis is passed through solution of different electrolytes, the mass of substance libareted or deposited at the electrodes is directly proportional the chemical equivalent of the substance".

 This means,  m1 = E1...........(i)

             And,   m2 = E2.............(ii)

Then devide eqn i by eqn ii

              m1/m2 = E1/E2

Where m1 and m2 are the respective masses of substance liberated or deposited on electrodes.

           E1 and E2 are chemical equivalent of the two substance respectively.

NOTE;

           The chemical equivalent of a substance is a relative atomic mass of substance divided by the number of electrons required to oxidize or reduces each unit of the substance.

                                Chemical equivalent = R.A.M/number of charges on an ion.