DEFINATION: - The device used to convert chemical energy into electrical energy is called an electrochemical cell.
EXPLANATION: -When a metal plate ( electrode ) is immersed into an electrolyte, positive or negative ions from the electrolyte move towards it and a net e.m.f. is developed between it and the electrolyte. When two electrodes of different metals are immersed into the electrolyte, a net e.m.f. is generated between them. The electrode at higher electric potential is called the positive electrode and the one having lower potential is called the negative electrode. Steady electric current flows on completing the electric circuit by connecting the two electrodes with a conducting wire. Such a device is called an electrochemical cell. Electrochemical cells are of two types:
1 ) Primary cells: The cells that get discharged only and cannot be recharged after use are called the primary cells.
2 ) Secondary cells: The cells which can be recharged after use and restored to original condition are called the secondary cells.
TYPES OF CELLS: - There are many types of cells in which the most important types are as under
Voltaic Cell: Italian scientist Volta prepared such a cell for the first time. Hence it is known as voltaic cell in his memory. Dilute H2SO4 is used as an electrolyte. Positive charge deposits on copper electrode and negative charge on zinc electrode. So copper electrode is at a higher potential than the zinc electrode.
On connecting the electrodes with a conducting wire, current ( conventional ) starts flowing from copper to zinc. Inside the cell, flow of current is due to motion of positive ions towards the copper electrode and negative ions towards the zinc electrode. Volta observed that the e.m.f. developed depends on the type of metals used and the electrolyte and not on the size of electrodes. He also found that by connecting them in series a large p.d. can be obtained. Such a connection of cells is called a Voltaic battery or Voltaic pile.
Daniel Cell: In this cell, solution of CuSO4 is filled in a copper container. Copper wall of the cell acts as positive terminal. A porous porcelain pot containing zinc electrode and dilute solution of H2SO4 is kept inside the copper container as shown in the figure. The
porous pot allows ions to pass through but does not allow the solution to get mixed up.Zn2+ ions enter CuSO4 solution through the porous pot and combine with SO4 ( - 2 ion) to form
ZnSO4 whereas 2e - remain on the Zn electrode. Cu2+ gain two electrons from the copper wall and reduce to Cu which deposits on the walls of the copper container. Zn electrode acts as anode and copper container as cathode. An emf of 1.1 V is obtained in this cell. Hydrogen produced due to electrolysis forms a layer of hydrogen gas on the copper container which increases the internal resistance of the cell and finally the cell stops working. This phenomenon, known as polarization, reduces the efficiency of the cell. To avoid it, substances which oxidize hydrogen are added in Leclanche and dry cells. ( Note: The electrode where oxidation occurs is called anode and where reduction occurs is called cathode. )
Leclanche Cell: This cell consists of a glass container filled with
NH4Cl as an electrolyte. Zn electrode and a porous
pot containing carbon electrode are partially
immersed in it. The porous pot contains MnO2 and
charcoal powder. Charcoal powder is used for better
electrical conduction as MnO2 is a poor conductor.
Following reactions occur:
Zn = Zn+2(ion) + 2e- ( oxidation at Zn electrode )
2 NH4 +1(ion) + 2e- = 2 NH3 + H2 ( reduction at carbon electrode )
Zn+2 (ions) combine with Cl-1(ion) forming ZnCl2 while going towards carbon electrode. H2 combines with MnO2 forming Mn2O3 and water thus preventing it from collecting around the carbon electrode and reducing its efficiency. This process is called depolarizing action. However, this action being slow, H2 collects around the carbon electrode reducing efficiency
and voltage of the cell. If the cell is switched off, H2 gets removed and the cell can be used again. Thus the cell is useful for short duration use. Emf of this cell is 1.5 V.
Dry Cell: Dry cell is a type of portable Lechlanche cell. The figure
shows sectional view of the dry cell. Zn pot acting as a negative terminal is anode. It contains electrolyte NH4Cl in the form of a paste. MnO2 mixed with charcoal is packed around central carbon rod which is a positive terminal (cathode). MnO2 acts as a depolarizer to which charcoal is added for better electrical conductivity. The whole arrangement is sealed at the top with a sealant. The brass cap on top of carbon rod has a hole to allow H2 gas produced to escape. Outer surface of Zn pot except the bottom is provided with plastic coating.
Standard Cell: Normally, the electrodes used in cells corrode with
time and cannot provide a constant emf for a long time. But some cells, known as standard cells, can provide a constant emf for a very long time. One such cell, known as Weston cell, is shown in the figure. In this cell, mercury ( Hg ), which acts as cathode, kept in contact with a paste of mercurous sulphate ( Hg2SO4 ) ( which acts as a depolarizing agent ) is filled in one part of a glass tube of H-shape. The other part of this tube is filled with cadmium mixed with mercury and it acts as anode. Concentrated solution of CdSO4 is used as an electrolyte. Bottom of both the parts of this tube is sealed with platinum wires and thus connecting terminals are prepared, so that the cell can be connected in external circuits. At 20° C, the emf of this cell is 1.0183 V and it remains constant over a wide range of temperatures.
Standard Cell: Lead Storage Cell ( accumulator ): Unlike primary cell, the secondary cell can be recharged and reused. Lead storage cell is the most widely used secondary cell in practice. The positive terminal of a lead storage cell is of PbO2 and the negative terminal is of Pb. The dilute solution of H2SO4 is the electrolyte. In fully charged condition, the specific gravity of the electrolyte is 1.285 which can be measured by a device called hydrometer. The emf of the cell is 2.1 V. During discharge, PbSO4 deposits on both the electrodes. In fully discharged state, the specific gravity falls to 1.15.
EXPLANATION: -When a metal plate ( electrode ) is immersed into an electrolyte, positive or negative ions from the electrolyte move towards it and a net e.m.f. is developed between it and the electrolyte. When two electrodes of different metals are immersed into the electrolyte, a net e.m.f. is generated between them. The electrode at higher electric potential is called the positive electrode and the one having lower potential is called the negative electrode. Steady electric current flows on completing the electric circuit by connecting the two electrodes with a conducting wire. Such a device is called an electrochemical cell. Electrochemical cells are of two types:
1 ) Primary cells: The cells that get discharged only and cannot be recharged after use are called the primary cells.
2 ) Secondary cells: The cells which can be recharged after use and restored to original condition are called the secondary cells.
TYPES OF CELLS: - There are many types of cells in which the most important types are as under
Voltaic Cell: Italian scientist Volta prepared such a cell for the first time. Hence it is known as voltaic cell in his memory. Dilute H2SO4 is used as an electrolyte. Positive charge deposits on copper electrode and negative charge on zinc electrode. So copper electrode is at a higher potential than the zinc electrode.
On connecting the electrodes with a conducting wire, current ( conventional ) starts flowing from copper to zinc. Inside the cell, flow of current is due to motion of positive ions towards the copper electrode and negative ions towards the zinc electrode. Volta observed that the e.m.f. developed depends on the type of metals used and the electrolyte and not on the size of electrodes. He also found that by connecting them in series a large p.d. can be obtained. Such a connection of cells is called a Voltaic battery or Voltaic pile.
Daniel Cell: In this cell, solution of CuSO4 is filled in a copper container. Copper wall of the cell acts as positive terminal. A porous porcelain pot containing zinc electrode and dilute solution of H2SO4 is kept inside the copper container as shown in the figure. Theporous pot allows ions to pass through but does not allow the solution to get mixed up.Zn2+ ions enter CuSO4 solution through the porous pot and combine with SO4 ( - 2 ion) to form
ZnSO4 whereas 2e - remain on the Zn electrode. Cu2+ gain two electrons from the copper wall and reduce to Cu which deposits on the walls of the copper container. Zn electrode acts as anode and copper container as cathode. An emf of 1.1 V is obtained in this cell. Hydrogen produced due to electrolysis forms a layer of hydrogen gas on the copper container which increases the internal resistance of the cell and finally the cell stops working. This phenomenon, known as polarization, reduces the efficiency of the cell. To avoid it, substances which oxidize hydrogen are added in Leclanche and dry cells. ( Note: The electrode where oxidation occurs is called anode and where reduction occurs is called cathode. )
Leclanche Cell: This cell consists of a glass container filled withNH4Cl as an electrolyte. Zn electrode and a porous
pot containing carbon electrode are partially
immersed in it. The porous pot contains MnO2 and
charcoal powder. Charcoal powder is used for better
electrical conduction as MnO2 is a poor conductor.
Following reactions occur:
Zn = Zn+2(ion) + 2e- ( oxidation at Zn electrode )
2 NH4 +1(ion) + 2e- = 2 NH3 + H2 ( reduction at carbon electrode )
Zn+2 (ions) combine with Cl-1(ion) forming ZnCl2 while going towards carbon electrode. H2 combines with MnO2 forming Mn2O3 and water thus preventing it from collecting around the carbon electrode and reducing its efficiency. This process is called depolarizing action. However, this action being slow, H2 collects around the carbon electrode reducing efficiency
and voltage of the cell. If the cell is switched off, H2 gets removed and the cell can be used again. Thus the cell is useful for short duration use. Emf of this cell is 1.5 V.
Dry Cell: Dry cell is a type of portable Lechlanche cell. The figureshows sectional view of the dry cell. Zn pot acting as a negative terminal is anode. It contains electrolyte NH4Cl in the form of a paste. MnO2 mixed with charcoal is packed around central carbon rod which is a positive terminal (cathode). MnO2 acts as a depolarizer to which charcoal is added for better electrical conductivity. The whole arrangement is sealed at the top with a sealant. The brass cap on top of carbon rod has a hole to allow H2 gas produced to escape. Outer surface of Zn pot except the bottom is provided with plastic coating.
Standard Cell: Normally, the electrodes used in cells corrode with
time and cannot provide a constant emf for a long time. But some cells, known as standard cells, can provide a constant emf for a very long time. One such cell, known as Weston cell, is shown in the figure. In this cell, mercury ( Hg ), which acts as cathode, kept in contact with a paste of mercurous sulphate ( Hg2SO4 ) ( which acts as a depolarizing agent ) is filled in one part of a glass tube of H-shape. The other part of this tube is filled with cadmium mixed with mercury and it acts as anode. Concentrated solution of CdSO4 is used as an electrolyte. Bottom of both the parts of this tube is sealed with platinum wires and thus connecting terminals are prepared, so that the cell can be connected in external circuits. At 20° C, the emf of this cell is 1.0183 V and it remains constant over a wide range of temperatures.Standard Cell: Lead Storage Cell ( accumulator ): Unlike primary cell, the secondary cell can be recharged and reused. Lead storage cell is the most widely used secondary cell in practice. The positive terminal of a lead storage cell is of PbO2 and the negative terminal is of Pb. The dilute solution of H2SO4 is the electrolyte. In fully charged condition, the specific gravity of the electrolyte is 1.285 which can be measured by a device called hydrometer. The emf of the cell is 2.1 V. During discharge, PbSO4 deposits on both the electrodes. In fully discharged state, the specific gravity falls to 1.15.
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