ElectroChemistry

 

Electrolytic and Non-electrolytic conductors

Electrochemistry deals with the interactions of electrical energy with chemical species. It is broadly divided into two categories, namely (i) production of chemical change by electrical energy (phenomenon of electrolysis) and (ii) conversion of chemical energy into electrical energy.

Redox Reactions:  All electrochemical reactions involve transfer of electrons and are, therefore, oxidation-reduction (redox) reactions.

Conductors: Substances which allow the passage of electric current through then are called electrical conductors or simply conductors.

Insulators: Substances which do not allow the flow of electric current through them are termed insulators.

Electrical conductors are of two types:

(i)     Metallic or electronic conductors:

Conductors which transfer electric current by transfer of electrons, without transfer of any matter, are known as metallic or electronic conductors.

Ex:  Metals such as copper, silver, aluminum, etc., non-metals like carbon.

(ii)   Electrolytic conductors:

In which the flow of electric current is accompanied by chemical decomposition are known as electrolytic conductors.

Ex: Aqueous solutions of acids, bases and salts

Non-electrolytes: The substances whose aqueous solutions do not conduct electric current are called non-electrolytes.

Ex: Solutions of cane sugar, Glycerin, alcohol, etc.,

 Anode: The electrode through which the current enters the electrolytic solution is called the anode (positive electrode).

Cathode: The electrode through which the current leaves the electrolytic solution is known as cathode (negative electrode).

Cations: The ions which carry positive charge and move towards cathode.

Anions: The ions carrying negative charge which move towards anode.

Oxidation: Removal of electrons is termed oxidation  which occurs at anode.

Reduction: Addition of electrons is called reduction  that takes place at cathode.

Distinction between metallic and electrolytic conduction

Metallic conduction	Electrolytic conduction
1.   Electric current flows by movement of electrons.	1.   Electric current flows by movement of ions.
2.   No chemical change occurs.	2.   Ions are oxidized or reduced at the electrodes.
3.   It does not involve the transfer of any matter.	3.   It involves transfer of matter in the form of ions.
4.   Ohm's law is followed.	4.   Ohm's low is followed.
5.   Resistance increases with increase of temperature.	5.   Resistance decreases with increase of temperature.
6.   Faraday law is not followed.	6.   Faraday law is followed.

  Electrolyte:

The process of chemical decomposition of an electrolyte by passage of electric current through its solution is called electrolytes.

Electrolysis:

Chemical change (oxidation and reduction) occurring at electrodes when electric current is passed though electrolytic solution is called electrolysis.

For example, when electric current is passed through a solution of hydrochloric acid, the H⁺ ions move towards cathode and CI^- ions move towards anode.

Electrolytic Cell: The device in which electrolysis (chemical reaction involving oxidation and reduction) is carried out by using electricity or in which conversation of electrical energy into chemical change is done is known as electrolytic cell.

Faraday’s Laws of Electrolysis

(i)     Faraday's First Law
 When an electric current is passed through an electrolyte, the amount of substance deposited is proportional to the quantity of electric charge passed through the electrolyte.

(ii)         Faraday's Second Law

 When the same quantity of charge is passed through different electrolytes, then the masses of different substances deposited at the respective electrodes will be in the ratio of their equivalent masses.

Arrhenius Theory of Electrolytic dissociation

(i)     An electrolyte, when dissolved in water, breaks up into two types of charged particles, one carrying a positive charge and the other a negative charge. These charged particles are called ions. Positively charged ions are termed cations and negatively charged as anions.

AB --> A⁺  + B^-

(ii)    The process of splitting of the molecules into ions of an electrolyte is called ionization. The fraction of the total number of molecules present in solution as ions is known as degree of ionization or degree of dissociation. It is denoted by
α= (Number of molecules dissociated into ions)/(Total number of molecules)
(iii)    Ions present in solution constantly re-unite to form neutral molecules and, thus, there is a state of dynamic equilibrium between the ionized the ionized and non-ionised molecules, i.e.,
                        AB <-->  A⁺ + B^-
Applying the law of mass action to above equilibrium
[A⁺ ][B^- ] /[AB] =K
K is known as ionization constant. The electrolytes having high  value of K are termed strong electrolytes and those having low value of K as weak electrolytes

Conductance, Specific conductance and Molar conductance

Electrolytic Conductance
 The conductance is the property of the conductor (metallic as well as electrolytic) which facilitates the flow of electricity through it. It is equal to the reciprocal of resistance i.e.,
                Conductance = 1/Resistance = 1/R                            ..... (i)
It is expressed on the unit called reciprocal ohm (ohm^-1 or mho) or Siemens
The reciprocal of specific resistance is termed the specific conductance or it is the conductance of one centimeter cube of a conductor.
       or Specific conductance = Conductance × cell constant
Equivalent conductance:
Equivalent  conductance is defined as the conductance of all the ions produced by one gram equivalent of an electrolyte in a given solution.
                        A = k ×  1000/N
                The unit of equivalent conductance is ohm^-1 cm^-2 equi^-1.
Molar conductance
The molar conductance is defined as the conductance of all the ions produced by ionization of 1 g mole of an electrolyte when present in V ml of solution. It is denoted by .
           Molar conductance     μ = k ×V
                It units are ohm^-1 cm² mol^-1.
                Equivalent conductance =  (Molar conductance)/z

Kohlrausch law

 "At infinite dilution, when dissociation is complete, each ion makes a definite contribution towards equivalent conductance of the electrolyte irrespective of the nature of the ion with which it Is associated.

·         The value of equivalent conductance at infinite dilution for any electrolyte is the sum of contribution of its constituent ions", i.e., anions and cations.

/\_∞ = λ_a + λ_c

·         Degree of dissociation

a =(Equivalent conductance at a given concentration)/(Equivalent
                                                                 conductance at infinite dilution

·         Determination of solubility of sparingly soluble salts

Conductometric Titrations:
Method of volumetric analysis based on the change in the conductance with changing concentrations of  the solution by the addition of titrant.
It’s preferred over volumetric titration because

·         Give more accurate end-point

·         Can be done even for color solutions where suitable indicator is not available

·         Can be used for dilute solutions

Cell Constant:
Ratio of Specific conductance to observed conductance.
Transport Number:

The definition of transport number  is that current carried by ion ‘x’ divided by the sum of the current of all the ions in solution, which is also called the transference number of ion x.

Electrochemical Cell

Electrolytic cell

It is a device in which electrolysis (chemical reaction involving oxidation and reduction) is carried out by using electricity or in which conversion of electrical energy into chemical energy is done

Galvanic or voltaic cell

It is a device in which a redox reaction is used to convert chemical energy into electrical energy. The chemical reaction responsible for production of electricity takes place in two separate compartments. Each compartment consists of a suitable electrolyte solution and a metallic conductor. The metallic conductor acts as an electrode. The compartments containing the electrode and the solution of the electrolyte are called half-cells. When the two compartments are connected by a salt bridge and electrodes are joined by a wire through galvanometer the electricity begins to flow. This is the simple form of voltaic cell

DANIELL CELL

 It is designed to make use of the spontaneous redox reaction between zinc and cupric ions to produce an electric current (Fig.12.7). It consists of two half-cells. The half-cells on the left contains a zinc metal electrode dipped in ZnSO₄ solution.

The half-cell on the right consists of copper metal electrode in a solution CuSO₄. The half-cells are joined by a salt bridge that prevents the mechanical mixing of the solution.
When the zinc and copper electrodes are joined by wire, the following observations are made:

·         There is a flow of electric current through the external circuit.

·         The zinc rod loses its mass while the copper rod gains in mass.

·         The concentration of ZnSO4 solution increases while the concentration of copper sulphate solution decreases.

·         The solutions in both the compartments remain electrically neutral

ELECTROLYTIC CELL                       VOLTAIC OR GALVANIC CELL
        (e.m.f. is applied to cell)                (e.m.f. is generated by cell)

	Electrolytic cell Anode         Cathode	Voltaic or Galvanic cell Anode             Cathode
Sign Electron flow Half-reaction	+                   - Out                in Oxidation      reduction	-                         + Out                    in Oxidation         reduction

SALT BRIDGE AND ITS SIGNIFICANCE

Salt bridge is usually an inverted U-tube filled with con­centrated solution of inert electrolytes. An inert electrolyte is one whose ions are neither involved in any electrochemical change nor do they react chemically with the electrolytes in the two half-cells.

Significance of salt bridge:

·         It connects the solutions of two half-cells and completes the cell circuit.

·         It prevents transference or diffusion of the solutions from one half-cell to the other.

·         It keeps the solutions in two half-cells electrically neutral.

The Daniell cell can be represented as:

                       Zn|Zn²⁺||Cu²⁺|Cu
                          Anode     Salt bridge    Cathode
                        Oxidation half-cell         Reduction half-cell
ELECTRODE POTENTIAL
When a metal is placed in a solution of its ions, the metal acquires either a positive or negative charge with respect to the solution. On account of this, a definite potential difference is developed between the metal and the solution. This potential difference is called electrode potential.
(i)                  Oxidation potential:
When electrode is negatively charged with respect to solution, i.e., it acts as anode. Oxidation occurs.
      M --> Mⁿ⁺ + ne^-

(ii)                 Reduction potential:
When electrode is positively charged with respect to solution, i.e., it acts as cathode. Reduction occurs.
      Mⁿ⁺ + ne^- --> M
The emf of the cell is equal to the sum of potentials on the two electrodes.
Emf of  the cell = E_Anode + E_Cathode
STANDARD ELECTRODE POTENTIAL

The potential difference developed between metal electrode and the solution of its ions of unit molarity (1M) at 25°C (298 K) is called standard electrode potential.

Calomel Electrode

Since a standard hydrogen electrode is difficult to prepare and maintain, it is usually replaced by other reference electrodes, which are known as secondary reference electrodes. These are convenient to handle and are prepared easily. Two important secondary reference electrodes are described here.

(i) Calomel electrode: It consists of mercury at the bot­tom over which a paste of mercury-mercurous chloride is placed. A solution of potassium chloride is then placed over the paste. A platinum wire sealed in a glass tube helps in making the electrical contact. The electrode is connected with the help of the side tube on the left through a salt bridge.
The potential of the calomel electrode depends upon the concentration of the potassium chloride solution.

If potassium chloride solution is saturated, the electrode is known as saturated calomel electrode (SCE).
If KCl solution is  1 N then the electrode is known as normal calomel electrode (NCE). The electrode reaction when the electrode acts as cathode is:
             1/2 Hg₂Cl₂ + e^- <---> Hg + Cl^-

 

Quinhydrone Electrode

Quinhydrone electrode is one of several oxidation-reduction electrodes

 

                                                                                              

·         Redox electrode at which the reversible reaction occurs. For determining the pH value of this half cell, it’s combined with other reference electrode, usually saturated calomel electrode.

                                                                                              

Glass Electrode (Ion Selective Electrode)

Allows to determine the concentration of a specific ion in aqueous (& in rare cases non-aqueous) solutions.

·         The important ion selective electrode is Glass membrane electrode

·         Useful in finding water hardness etc..\

Nernst Equation

        The electrode potential and the emf of the cell depend upon the nature of the electrode, temperature and the activities (concentrations) of the ions in solution.
Potential at zinc electrode (Anode)
        E_ox = E_ox^o - 0.0591/n  log₁₀ [Zn³⁺]
 Potential at copper electrode (Cathode)
        E_red = E_red^o - 0.0591/n  log₁₀ [Cu²⁺]
PRIMARY VOLTAIC CELL (THE DRY CELL)

 In this cell, once the chemicals have been consumed, further reaction is not possible. It cannot be regenerated by reversing the current flow through the cell using an external direct current source of electrical energy. The most common example of this type is dry cell.

The container of the dry cell is made of zinc which also serves as one of the electrodes. The other electrode is a carbon rod in the centre of the cell. The zinc container is lined with a porous paper. A moist mixture of ammonium chloride, man­ganese dioxide, zinc chloride and a porous inert filler occupy the space between the paper lined zinc container and the carbon rod. The cell is sealed with a material like wax.

As the cell operates, the zinc is oxidized to Zn²⁺
 Zn  --->   Zn²⁺ + 2e^-     (Anode reaction)
The electrons are utilized at carbon rod (cathode) as the ammonium ions are reduced.
 2NH₄⁺+2e^- --> 2NH₃ + H₂    (Cathode reaction)
The cell reaction is
Zn+ 2 NH₄⁺ --->   Zn²⁺ + 2NH₃ + H₂
Hydrogen is oxidized by MnO₂ in the cell.
2MnO₂ + H₂ ---> 2MnO(OH)
Ammonia produced at cathode combines with zinc ions to form complex ion.
Zn²⁺ + 4NH₃ ---> [Zn(NH₃)₄]₂₊
E_cell is 1.6 volt
Alkaline Dry Cell

Alkaline dry cell is similar to ordinary dry cell. It contains potassium hydroxide. The reaction in alkaline dry cell are:

Zn + 2OH^- ---> Zn(OH)₂ + 2e^-                    (Anode reaction)
 2MnO₂ + 2H₂O + 2e^- ---> 2MnO(OH) + 2OH^-            (Cathode reaction)
 Zn + 2MnO₂ + 2H₂O ---> Zn(OH)₂ + 2MnO(OH)        (Overall)
 E_cell is 1.5 volt

SECODARY VOLTAIC CELL (LEAD STORAGE BATTERY)

The cell in which original reactants are regenerated by passing direct current from external source, i.e., it is re-charged, is called secondary cell. Lead storage battery is the example of this type.

It consists of a group of lead plates bearing compressed spongy lead, alternating with a group of lead plates bearing leaf dioxide, PbO₂. These plates are immersed in a solution of about 30% H2SO4. When the cell discharge; it operates as a voltaic cell. The spongy lead is oxidized to Pb2+ ions and lead plates acquire a negative charge.

                        Pb --> Pb²⁺ + 2e^-                       (Anode reaction)

Pb²⁺ ions combine with sulphate ions to form insoluble lead sulphate, PbSO₄, which begins to coat lead electrode.

                        Pb²⁺ + SO₄^2- ---> PbSO₄            (Precipitation)

        The electrons are utilized at PbO₂ electrode.

                PbO₂ + 4H⁺ + 2e^- ---> Pb²⁺ 2H₂O                (Cathode reaction)

                 Pb²⁺ + SO₄^2- ---> PbSO₄            (Precipitation)

        Overall cell reaction is:

                Pb + PbO₂ + 4H⁺ + 2 SO₄^2- ---> 2PbSO₄ + 2H₂O

            Ecell is 2.041 volt.

When a potential slightly greater than the potential of battery is applied, the battery can be re-charged.

                    2PbSO₄ + 2H₂O --->  Pb + PbO₂ + 2H₂SO₄

CONCENTRATIONS CELLS

If two plates of the same metal are dipped separately into two solutions of the same electrolyte and are connected with a salt bridge, the whole arrangement is found to act as a galvanic cell. In general, there are two types of concentration cells:

(i)                   Electrode concentration cells:

In these cells, the potential difference is developed between two like electrodes at different concentrations dipped in the same solution of the electrolyte. For example, two hydrogen electrodes at different has pressure in the same solution of hydrogen ions constitute a cell of this type.

              (Pt,H₂ (Pressure p₁))/Anode |H⁺ | (H₂ (Pressure p₂)Pt)/Cathode

If p₁, p₂ oxidation occurs at L.H.S. electrode and reduction occurs at R.H.S. electrode.

           E_cell = 0.0591/2 log(p₁/p₂)  at 25^o C

In the amalgam cells, two amalgams of the same metal at two different concentrations are interested in the same electrolyte solution.

(ii)                  Electrolyte concentration cells:

In these cells, electrodes are identical but these are immersed in solutions of the same electrolyte of different concentrations. The source of electrical energy in the cell is the tendency of the electrolyte to diffuse from a solution of higher concentration to that of lower concentration. With the expiry of time, the two concentrations tend to become equal. Thus, at the start the emf of the cell is maximum and it gradually falls to zero. Such a cell is repre­sented in the following manner:

(C₂ is greater than C₁).

                  M|Mn⁺(C₁)||Mⁿ⁺(C₂)|M

Fuel Cells

          -are energy conversion devices, convert the free energy change of a chemical reaction directly into electricity (electrochemical energy conversion) and not as heat in a chemical reaction.

Advantages:

·         Noise and thermal pollution low

·         Maintenance cost is low

·         Energy conversion is very high