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.

CHEMICAL KINETICS

CHEMICAL KINETICS

Chemical kinetics is the study of speed of rate of chemical reaction and mechanisms through which reactions take place

• Rate of chemical reaction is the speed at which chemical reaction occurred.
• In the chemical reaction reactants are changed to products and the proportion of both reactants and products change with time.
• The amount (concentration) of reactants is maximum at the biggining but as the time progress the amount of reactants decreases while that of product increases.

The rate of chemical reaction can be obtained by measuring the amount of reactants used or amount of products formed at a given time.

                                                                           

                             Rate of chemical reaction = amount of reactants used /time interval

                                                                                         Or

                            Rate of chemical reaction = amount of products formed / time interval

           

              The SI unit of time can be expressed as second (s), minutes, day, weak, months or years

COLLISSION THEORY 

Collision theory explain that for chemical reaction to take place the particles of reactants must collide. According to collision theory reacting particles must approach each other and collision must take place.

   Not all collision cause reaction, for the reaction to occur the particles must collide with sufficient energy. Collision with sufficient energy called effective collision

ACTIVATION ENERGY

Although reacting particles continue collide with one another, only collision possess certain minimum amount of energy result in chemical reaction. This energy called ACTIVATION ENERGY and particles which possess activation energy called activated particles.

NB:

     Activation energy is the minimum amount of energy required by reacting particles for the chemical reaction to occur.

Activation energy of reactants can be reduced by additional of catalyst.


                                                       

FACTORS AFFECTING RATE OF CHEMICAL REACTION

Rate of chemical reaction can be affected by the following factors

1. Temperature
2. Concentration 
3. Catalyst
4. Surface area
5. Pressure

1. TEMPERATURE

Temperature is directly proportional to the rate of chemical reaction, temperature increases the kinetic energy of collide particles hence effective collision of colliding particles. The effective collisions increase rate of rate of chemical reaction. There fore lower the temperature decrease rate of chemical reaction

               NB:
                       For exothermic reaction temperature is inversely proportion to the rate of chemical reaction.

2. CONCENTRATION
      When concentration of reactants increases the rate of chemical reaction also increases. High concentration increases the chance for particles to collide and increases the chance for reaction to occur. There for concentration is direct proportional to the rate of chemical reaction, and visevecer is true.

                                         

3. CATALYST
       Catalyst is any substance that can alter rate of chemical reaction but it's self remain unchanged at the end of chemical reactio. Catalyst has the funtion of lowering the activation energy of reacting particles, there fore lowering activation energy increases the rate of chemical reaction.

                                       

4. SURFACE AREA.
     As surface area to volume ratio increase the rate of chemical reaction also increase. If reacting particles are solid the only molecules which are in contact they will conduct reaction, but the rest they will not participate in reaction.
     For powdered reacting substance the surface area to volume ratio is large and all particles will participate in chemical reaction hence rate of chemical reaction will be increased.

                                   

  5. PRESSURE.
         High pressure make the reacting particles to come closer and increase the chance to collide, due to this reason as temperature increase also rate of chemical reaction increase, and lower pressure also rate of reaction also lowered.

REVERSIBLE AND IRREVERSIBLE REACTIONS

REVERSIBLE REACTIONS;
      Reversible reaction is the reaction which can go in either direction. In reversible reactions none of reactants is completely used up and so the reaction does not going into completion. Means that reactants form products and at the same time products form reactants, as shown in the diagram below;

                                         

       Example of reversible reaction is formation of ammonium chloride and then dissociates into ammonia and chloride gase. Ammonia is an alkaline gas while hydrogen chloride gas is acidic. Ammonia is less dense than hydrogen chloride and difuse faster. There fore turns the wet red litmus paper into blue. Hydrogen chloride gas diffuses more slowly and turns the blue litmus paper red much later.

 The two gases recombine on cooling to form solid ammonia chloride. Heating of amonnia chloride is reversible chemical reactions.

IRRIVERSIBLE REACTIONS
    Irriversble reaction take place in only one direction and go on completion. The irreverble reactions proceed until at least one of the reactant used up completely. In these reactions products can not react back to form a reactants.

                                     

    Examples of irriversble reaction include a metals such as Na, Mg, K and so on when reacts with dillute acid to form salt and hydrogen gas

                        Mg + 2HCl  > MgCl2 + H2


 Asses yourself;

1) Explain the meaning of each of the following items
         
     a) reversible reaction

     b) activation energy

    c) catalyst

   d) irreversible reactions


2) Explain the following factors on how affect rate of chemical reaction.

      a) pressure

      b) catalyst

      c) temperature

     d ) Concentration

     e) surface area ( nature of reactants)


EXOTHERMIC AND ENDOTHERMIC REACTIONS

      During chamical reaction there change in energy which takes place, those energy changes can be either endothemic or exothermic

ENDOTHERMIC REACTIONS
 
     Endothermic reaction occur when the sum of heat content of products is  less than sum of heat content of products, then reaction will be endothemic reaction. Consider the example below

             P + Q > R+ S

Then if the sum of heat (energy ) content of P and Q (reactants)  are less than that of R and S (products) the extra energy or heat required by products side is absorbed from the surrounding. Such reaction is endothemic reaction.

There fore
           Endothermic reactions are the reactions which absorb energy ( heat) from the surrounding during chemical reaction proceeding.
                                               

       Example of endothermic reaction are;
   
          1. Dissolving ammonia chloride in water
          2. Mixing water and poassium chloride
          3. Photosynthesis


NB;
   Reversible reaction pocess both endothermic and exothermic reaction, when the forward reaction is exothemic then backward reaction will be endothemic and viceveser.


Exercise.

     1. With aid of energy level diagram explain endothemic reactions.

    2. Give reason why in endothemic reaction aborb energy from the sarounding?

   3. Metion five examples of endothermic reaction.

EXOTHERMIC REACTION

      In exothermic reaction the sum of heat content of reactants are large than the sum of heat content of products. Thus during product formation the excess amount of energy (heat) from the reactants released out from the the system to the surroundings, this reaction is exothermic reaction.

Consider the example below;
   
                A + B > D + C

 from the illustration above if the sum of heat content of A and B (reactants ) are large than thsum of heat content of D and C ( products) then the reaction will be exothermic reaction.
                                 
                                 

      There fore;
                Exothermic reaction is the chemical reaction which release heat (energy) to the surrounding when proceeding.

    Examples of exothemic reaction include;
     
          1). Combustion reactions
          2).mixing of water and strong acids such as concentrated sulphuric acid.


CHEMICAL EQUILIBRIUM
    There some reversible reactions which are forward and reverse reactions occur at the same time. At the beginning amount of reactants are at maximum, and amount of products are zero, as the reaction proceed the amount of reactants decrease and that of product increase untill, the equal amount of reactants and products obtained. At this condition the reaction said to be at equilibrium.

There fore ;
      Chemical equilibrium is the state at which the forward and backward reaction are proceeding at the same rate.
   Forward and backwards reaction proceed until equilibrium is mantained, is not static but it it is dynamic, means reactants form products (forward reaction) at the same rate as products react to form reactants (backward reaction) this process occur at equilibrium state and thus why equilibrium is dynamic and not static.

                     

FACTORS AFFECTING THE POSITION OF EQUILIBRIUM

French chemist Henri Louis Le Chatelier came up with a generalization which is commonly known as Le chatelier's principle,,, which states that;

"If stress is applied to a system at equilibrium, the system readjusts, if possible to reduce the effective of the stress."

  The principle means that if there is any change in the chemical equilibrium, position of equilibrium will change to overcome or counteract the change.

The following are factors which affect position of chemical equilibrium.

         1) Temperature

         2) concentration

        3) pressure

        4) catalyst   

FACTORS AFFECTING CHEMICAL EQUILBRIUM

     The following are factors which affect chemical equilibrium

           1) temperature

           2) pressure

           3) concentration

1. Temperature

        Effect on temperature on chemical equilibrium depends on whether chemical reaction is endothemic or exothermic.

    In endothemic reaction;
       Increase in temperature leads the chemical equilibrium to shift from backward to forward direction. This is because increasing temperature in endothemic reaction fever forward direction (more products formation).

    In exothermic reaction;
        Increase in temperature leads the chemical equilibrium to shift from forward direction to backward direction (shift from right to left). Dencrease in temperature leads the chemical equilibrium to shift from backward to forward.

2. Concentration.

     When the concentration reactants increased the direction proceeds in the forward direction to decrease the concentration of reactants untill equilibrium is mantained.

     When the concentration of products is increase, the reaction proceeds in the reverse direction ( backwards direction) to decrease the concentration of reactants untill equilibrium is mantained.

 3. Pressure

        Pressure affects the equilibrium of reversible reactions involving gase.
Increase in pressure cause the chemical equilibrium to shift so as to produce substance occupying less volume or moles.

                                           

 There fore addition of pressure in a system will cause the chemical equilibrium to shift more on the side where there is low volume or moles.