EEE (Chemistry) Ist Year for 09

CHEMISTRY


INDEX


1) Electrochemistry & batteries


2) Corrosion & its control


3) Polymers


4) Water–Analysis-Treatment


5) Surface chemistry


6) Energy sources


7) Phase Rule


8) Material Chemistry







UNIT I-ELECTRO CHEMISTRY
The branch of science which deals with the relationship between electricity and chemistry (or) chemical energy.

               The substances are classified into four types depending on the property of flow of current through it.

1)      Conductor:  The substance which allows the flow of current  through it is called a conductor.

            Conductors divided into two

a)      Electronic conductors (or) metallic conductors.

b)       Electrolytic conductors into two fused salt and salt soil.

2)      Insulator (or) non-conductor:

                      The substance which does not allow the flow of current (or) electrons through it is called an insulator (or) non-conductor.

 EX:pure water, dry wood, rubber, plastic, plant origin material, cellulose, benzene, all non-metals except carbon.

3)      Semi conductors:

                        The substances which have the conductivity property intermediate between metals and non-metals (or) conductor and non-conductor is called semi-conductor. The property of conducting nature of semi-conductor can be improved by the addition of small amounts of impurities like electron-giving nature substances (phosphorus) (or) electron accepting nature substances like boron (or) aluminium and this addition of impurity is called doping.

                        The doping of conductor with phosphorus the conductor is called n-type semiconductor. The doping of the conductor with e^- acceptor impurities the conductor is called p-type. The property of doping has helped the invention of the devices like calculator , micro computer , pocket radios etc .

4)      Super Conductors: conductor which allows the flow of current with zero resistance is called super conductor

Ex: lead ,Cu, Barium, rere earth metal-oxides.

 Superconductors help us transfer of electrical power (or) energy from the place of generation to the place of utility. Superconductors are useful to create very high field electromagnet.

Resistance: The property of preventing the flow of current is called resistance, represented by ‘R’, measured in ohms Ω.

Ohm: It is defined as the resistance of the conductor to the flow of 1ampere current against 1volt potential difference.

     The property of resistance R is proportional to the length of the conductor ‘l’ and inversely proportional to area of cross section ‘a’.

                         R  α  l  ;  R α 1/a

                                R α l/a  →  R= ρ.l/a

Ρ is proportionality constant called specific resistance.

Specific resistance:      The resistance of a conductor of unit length and unit area of cross section is called specific resistance. Units for ρ is ohm.cm(or) ohm.m.

Conductance:    the reciprocal of resistance is called conductance or conductivity.

                        Conductivity G= 1/R.

            Specific conductance K= 1/ρ.

Units of K are Siemens/m.

 Conductivity:  It is defined as conductivity of all the ions present in 1cc of the substance.

Equivalent conductivity:  It is the conductance of all the ions formed by the dissociation of 1gm equivalent weight of the electrolyte dissolved in a certain volume V of the solvent at constant temperature. Denoted by Λ. Units Siemen.m².eq^-1.

         Λ =1000.K/C

Molar conductivity:     It is defined as the conductivity of all the ions formed by the dissociation of 1molecular wt. of the electrolyte dissolved in a certain volume V of the solvent at constant temperature. units mho.cm².mol^-1.

            µ =1000K/C

Relation between Λ & µ:          µ = z.Λ

The conductance of an ion depends on no. of ions, charge carried by the ion and velocity of the ion.      Λ_± = n_±.q_±.µ_±

Effect of dilution on conductivity and specific conductivity:

When you dilute a solution the no. of ions per cc of the solution decreases , hence the specific conductivity K decreases . Increase of dilution increases , the no. of ions or increases inter ionic distance which decreases attraction between the ions , as a result of this the conductivity of the solution increases.

Dilution of a solution increases ionization and decreases inter ionic attraction .The state at which complete ionization of the electrolyte and inter ionic attraction becomes zero, that state of dilution is called infinite dilution .After this state any addition of solvent doesn’t effect ionization or ionic attraction .

The equivalent conductivity at infinite dilution of an electrolyte is a constant and max. value and it is called equivalent conductivity at infinite dilution Λ∞. Λ∞ is the sum of ionic conductance.

                                    Λ∞ = Λ₊ + Λ _-

                      Λ₊ =n₊.q₊.µ_+.

                        µ₊ = Λ₊/K.   µionic mobility.

At infinite dilution no. of ions n is constant, charge q is constant ,so n.q is constant K. It is nothing but the charge carried by 1eq. of the ion, it is generally known as Faraday F.

                                    µ₊ = Λ₊/96500c.units cm/sec.

Conductivity cell:   In conductance measurement if the electrodes distance changes conductivity changes, area of cross section changes conductivity changes, if the salts from the glass surface of the vessel dissolves in water then conductivity changes . so, in conductivity measurements a specially designed pirex glass vessel in which the electrodes are coated  with platinum black , it is called conductivity cell is used.

     The ratio of electrode distance ‘l’ area of cross section ‘a’ is constant and it is called cell constant.

            Cell constant = l/a units cm^-1

            Conductance =(1/ρ)X (a/l)

                                 = k X a/l

                                 k= c X cell constant

conductivity cells generally used are of two types

1)      Conductivity cell shorter electrode distance used for low concentrated solutions or weak electrolytes.

2)      Conductivity cell of long electrode distance used for high concentrated solutions or strong electrolytes.

Conductivity water: 

                                    In conductivity measurement use of water is alsom important as salts in water differsthe conductivity of the same solutions also differs. So in conductivity measurements waters of constant specific conductivity at 25^oc must be used and this water is called conductivity water and this water is prepared by repeated distillation of distilled water mixed with a small amount of alkaline potassium permanganate in a pyrex glass distillation head .

                        The conductivity water prepared must be stored in well stoppered pyrax glass bottles only other wise co₂ from air dissolves in water forming carbonic acids changing conductivity and silicates from glass dissolves in water changing conductivity.

Prob:In a conductivity cell distance between the two electrodes is 10cm and area of cross section  a=25sq.cm , the conductance of a solution taken in conductivity cell is 0.2ohm^-1cm^-1

Sol:- l=10cm, a=25sqcm

Cell constant =l/a= 10/25=0.4cm^-1

Specific conductance =conduct X cell const

            = 0.2 X 0.4= 0.08 mho cm^-1

Experimental determination of specific conductance of  a substance:-

                                                (Fig 1)

Part I:

                        0.1N kcl solution whose specific conductance ‘k’ is known accurately is taken in the conductivity cell and is connected with the electrical circuit as shown in fig. current is passed through the conductivity cell and the jockey J moved along the wire to detect the null point ’J’. then according to the weatstones bridge principle

                                    x/l₁= R/l₂

from this ‘x’ the resistance of kcl sol can be cal as R.l₁/l₂

then cell constant =k.x

knowing the value of k and x the cell constant can be determined.

Part II:

            The conductivity cell used in the Ist part is cleaned and the given electrolyte sol is taken this conductivity cell is connected in the electric circuit while the current is passing through the sol detect the null point ‘J’ by pressing  jockey on meter bridge wire “AB”. Now the resistance of the sol “x¹” can be calculated using the wheatstones bridge principle

                                    x¹/l^’ ₁ = R¹/ l₂¹

now cell constant = k¹.x¹

The specific conductance “k” of the given electrolyte solution is calc by

K¹= specific conductance of electrolyte + sp.conductance of  water

Kohlrawsch law of independent migration of ions:-

It is observed that the conductance of cl ion at infinite dilution is the same whether it is present  in kcl or Nacl or Mgcl₂ or Cacl₂ similarly the conductance of k⁺ ion at infinite dil is the same whether it is present in kcl or kno₃ or k₂so₄ . this is because at infinite dil an ion moves freely without the influence of other ion at infinite dilution. Basing on this observation kohlrausch has started a law as the eq conductivity of an electrolyte at infinite dilution is is the sum of ionic conductances of the ions of the electrolyte at const temp.

                        Λ_∞= Λ₊+Λ_-

                              Or

            Each ion contributes a cons conductance to the eq. conductance of the electrolytes at infinite dilution at const temp.

For Ex, Λ_∞ of kcl is 149.85

Application of kohlrausch law:-

1)      Calc of Λ_∞ of weak electrolyte like ch₃cooh , Nh₄oh.

Infinite dil stage of acetic acid is not practically reached. So it cant be measured experimentally but kohlrausch law helps to calc Λ_∞  value of Ch₃cooh as shown below

From the Λ_∞ values of Nacl , Hcl and Ch₃cooNa we can write

Λ_nacl= Λ_Na⁺+ Λ_cl^- …………………..(1)

Λ_hcl= Λ_H⁺+ Λ_cl^-…………………….(2)

Λ_ch3coona= Λ_Ch3coo^- + Λ_na⁺…………(3)

Now eq(3)- (1)

            Λ_ch3coo^—Λ_/cl­^- + Λ_H⁺+ Λ_cl^- = Λ_ch3cooh

2)      Kohlrausch law is useful to calc the degree of dissociation of electrolytes. At experimental conditions the conductance of a sol is proportional to the ions formed in the solution from the electrolyte. So the conductance  of electrolyte Λ α ∞

Similarly Λ_∞ α 1

Thus knowing the values of Λ experimentally and knowing the value of Λ_∞ using kohlrausach law the degree of dissociation  α of the electrolyte can be calculated.

3)      Calculating the solubility of the salt :

The solubility of sparingly soluble salts like PbSo₄, BaSo₄, AgCl, etc are very small and it cant be accurately determined through trimetric method in such case the solubilities can be determined using conductivity measurement. The saturated solution of sparingly soluble salt can be considered as the salt solution at infinite dilution using kohlrausch law, Λ_∞ of the salt solution can be obtained. The experimental sol is taken in the conductivity cell and its resistance is measured using the formulae cell constant = k.R substituting the cell constant value and the resistance of the sol you can calc specific conductance of the solution.

                                    Λ_∞ = 1000k/C

In this formulae ‘Λ’ is substituted and ‘k’ is sub and ‘c’ value is calculated

C=conc of the salt in solution

  = the solubility of the salt in solution in salt

Knowing the solubility the solubility product k_sp also can be calculated as shown below

                                    AB= A⁺+B^-

Ksp= [A⁺][B^-]=sXs=s^2.

4)      Calculation of ionic product of water:

           K_kw=-10 g kw

Pure water contains very less number of H₂o molecules ionized so pure water can be considered as infinite dilution condition and its equ. Conductivity can be calculated using kohlrausch law