Electrmotive Force (emf)
EMF stands for electromotive force. A galvanic cell consists of two electrodes each having its own potential. The difference of potential between these two electrodes of a cell causes a current to flow from electrode of higher potential to the electrode of lower potential is called emf of the cell.
EMF is expressed in volts. Greater the emf (i.e. potential difference between two electrode) greater the electricity flow and greater is the tendency of the cell redox to occur.
Potential of a cell assembled of two electrodes can be determined from the two individual electrode potentials using this formula-
ΔVcell = Ered,cathode − Ered,anode
or, ΔVcell = Ered,cathode + Eoxy,anode
Electrode Potential
Metal atoms have tendency to lose electrons and go into solution as metal ions. Electrode potential is a measure of the tendency of metal atoms to gain or loose electrons when in contact with a solution of its own ions. Electrode potential is the potential developed at the interface between metal and its salt solution; when a metal is dipped in its own salt solution.
When a metal strip M is immersed in a solution of its salt containing Mn+ ions, one of the processes as show below in the Figure can occur.
1. The dissolution process where atoms of metal electrode M may loose some electrons to the electrode and enter the solution as Mn+
M → Mn+ + ne (metal is oxidised)
The metal electrode gets negative charge and the solution gets extra positive charge.
2. The deposition process where metal cations Mn+ from the solution may come in contact with the metal strip, gain some electrons and get converted into metal atoms M, which get deposited on the surfance of metal strip. Seperation of charges take place and a potential is developed called electrode potential.
Mn+ + ne– → M (the ion is reduced)
The electrode reaction reaches an equilibrium as represented below-
There are two types of electrode potentials: Oxidation potential and reduction potential. Oxidation potential is the tendency of an electrode to lose electrons or get oxidized. Reduction potential is the tendency of an electrode to gain electrons or get reduced. Oxidation potential is the reverse of reduction potential. The electrode having a higher reduction potential has a higher tendency to gain electrons. So, it acts as a cathode. The electrode having a lower reduction potential acts as an anode.
Standard Electrode Potential
The potential of a half-reaction measured against the Standard Hydrogen Electrode under standard conditions is called the standard electrode potential for that half-reaction.
Standard conditions are-
Temperature = 298K
Pessure = 1atm
Concentration of the electrolyte = 1M.
Example-
Consider Zn and Hydrogen electrode
Zn ⇌ Zn2+ + 2e
H2 ⇌ 2H+ + 2e
When these two electrodes (two equilibria) are brought into electrical contact using an external wire and a salt bridge, the electrons will be pushed from the zinc equilibrium (electrode) to the hydrogen equilbrium (electrode) with a force of - 0.76V (the negative sign simply indicates the direction of flow - from zinc to hydrogen ions).
So, the standard electrode potential of Zn is −0.76 volts and the overall reaction is-
Zn + 2H+ → Zn2+ + H2
Uses of Standard Electrode Potentials
Uses of standard electrode potentials are given below –
1. It is used to measure relative strengths of various oxidants and reductants.
2. It is used to calculate standard cell potential.
3. It is used to predict possible reactions.
4. Prediction of equilibrium in the reaction.
Dry Cell
It is a compact form of Leclanche cell. It consists of a zinc container having a moist paste of ammonium chloride and zinc chloride. The container (Zn) acts as anode. The container is lined with porous paper which separates zinc from paste but allows the ions to pass through it. A graphite rod is placed in the center and it acts as cathode. It is surrounded by MnO2 and carbon. The electrode reactions are complex. These may be written as follows-
At Anode: Zn(s) → Zn+2 + 2e−2
At Cathode: MnO2 + NH4+ + e−2 → MnO(OH) + NH3
Ammonia is not given out as gas but it combines with Zn+2 ions forming the complex ion [Zn(NH3)4]+2. EMF of dry cell is 1.25 to 1.50 volts.
Zn container consumed and holes start appearing in it during use. These holes are responsible for the leakage of the battery. Dry cell does not have a long life because, ammonium chloride is acidic and it corrodes the zinc container even when the cell is not in use. An extra cover for the zinc container is provided in leakage proof dry cell.
Glvanic Cell or Voltaic Cell
An electrochemical cell that converts the chemical energy of spontaneous redox reactions into electrical energy is known as a galvanic cell or a voltaic cell.
Important Points of the Galvanic Cell
Oxidation occurs at Anode (−ve Electrode)
Reduction occurs at Cathode (+ve Electrode)
Current flows from Cathode to Anode
Electrone flows from Anode to Cathode
Salt bridge(Contains electrolytes) complete the circuit
Electromotive Force (emf) of the Cell is 1.1 Volt
Oxidation Half Cell Reaction- (at Anode)
Zn(s) → Zn+2 + 2e
Reduction Half Cell Reaction- (at Cathode)
Cu+2 + 2e → Cu(s)
Complete Cell Reaction-
Zn(s) + Cu+2 → Zn+2 + Cu(s)
Cell Representation-
Zn(s)|ZnSO4(aq) || CuSO4|Cu(s)
Salt Bridge
A salt bridge is a inverted U tube filled with a concentrated solution of an inert electrolyte like KCl or NH4NO3 which does not take part in the cell reaction. The electrolyte is taken in the form of solution and mixed with agar-agar. The mixture is heated and filled in the U tube when hot. On cooling it sets into a jelly like mass and does not flow out, during its use.
Salt bridge has two functions.
1. It completes the inner circuit. It acts as a contact between the two half cells without any mixing of electrolytes.
2. It prevents accumulation of charges in two half cells and maintains electrical neutrality.
Cations and anions of the salt bridge move into two half cells and neutralise the excess charge. The anions move into oxidation half cell and neutralise the excess charge. The cations move into the reduction half cell and neutralise the charge. In a Daniell cell a salt bridge is replaced by a porous pot, to make the cell more handy to use.
Fuel Cells
Production of electricity by thermal plants is not a very efficient method and it produces pollution also. In thermal plants, chemical energy is produced by the combustion of fossil fuels such as coal, gas, oil etc. The heat produced is used for converting water into high pressure steam. It is then used to run a turbine to produce electricity. The efficiency of thermal plants is only 40%. But the galvanic cell directly converts chemical energy into electrical energy and is highly efficient. It is now possible to make such cells in which reactants are fed continuously to the electrodes and the products are removed from the electrolytic compartment. Galvanic cells which convert the energy of combustion of fuels such as hydrogen, methane and methanol etc. directly to electrical energy are called fuel cells.
A successfull cell is hydrogen-oxygen fuel cell.
Hydrogen-oxygen fuel cell contains two porous carbon electrodes. These electrodes are impregnated with suitable catalysts such as Ag, Pt, CoO etc. A concentrated solution of KOH or NaOH acts as electrolyte. This solution is placed between two carbon electrodes. One carbon electrode acts as cathode. The other carbon electrode acts as anode. H₂ gas is bubbled through the anode (-). O2 gas is bubbled through the cathode (+). During the use, following reactions occur at the electrodes.
At cathode:
O2(g) + 2H2O (l) + 4e → 4OH (Reduction)
At anode:
H2(g) + 2OH¯ → 2H2O(l) + 2e (Oxidation)
Complete cell reaction:
2H2(g) + O2(g) → 2H2O(l)
Water Cathode of porous carbon containing suitable catalysts Anode of porous carbon containing suitable catalysts H2 Concentrated aqueous KOH/NaOH
H2 and O2 are passed under pressure. The cell runs continuously as long as H2 and O2 is supplied. The efficiency of the cell is 60-70 percent. This cell has been used in Apollo space programme. Water vapours produced were condensed and added to the drinking water supply for the astronauts. The potential of the cell is about 0.9 V. For increasing the efficiency of fuel cells new electrode materials, better catalysts and electrolytes are being produced. They are being good in automobiles on experimental basis.
Advantage of Fuel Cells
1. Fuel cells do not cause pollution problem.
2. Eletrical energy may be obtained continuously by the continuous supply of the fuel.
3. In comparison to conventional batteries, less weight of fuel is required in fuel cells to produce a given quantity of electricity.
Disadvantage of Fuel Cells
1. The catalyst used Pt, Pd and Ag are costly.
2. There is a problem in handling the gaseous fuels at high pressure.
3. The electrolytes used NaOH and KOH are corrosive.
4. It is difficult to handle gaseous fuel, liquid electrolyte and solid catalyst together in a fuel cell.
Effect of Dilution on Conductivity
The specific conductance depends on the number of ions present per centimeter cube of the solution. Though degree of dissociation increases on dilution but the number of ions per centimeter cube decreases. So, the specific conductance decreases on dilution.
The equibvalent conductance is the product of specific conductance and volume of the solution containing one gm-equivalent of the electrolyte.
Λ = κ . V
As the decreasing κ value is more than compensated by the increasing V value, hence, Λ increases. on dilution.
Remember:
☛ The conductivity of solution increases on dilution.
☛ The specific conductivity decreases on dilution.
☛ The equivalent and molar conductivities increase with dilution.
