Examville.com - Electrochemical Cells

ELECTROCHEMICAL CELL OR GALVANIC CELL The cells, which are used to convert chemical energy into electrical energy, are called electrochemical or galvanic cells or voltaic cell. In these types of cells oxidation and reduction take place in separate containers called half-cells and the redox reaction is spontaneous. For example:

This arrangement contains two beakers, one of which contains 1 M solution of ZnSO4 and other 1M solution of CuSO4. Zinc rod is dipped in ZnSO4 solution and Copper rod is dipped in CuSO4 solution. These metallic rods are called electrodes. Further these electrodes are connected to the ammeter with the help of conducting wires through a key ammeter helps in giving the passage of electric current which flows in opposite direction to the flow of electrons. Both the beakers containing solutions are connected two each other with the help of a U–shaped tube called salt bridge. This salt bridge is filled with saturated solution of strong electrolyte like KCl, KNO3, NH4NO3, which does not under, go any change during chemical reaction. The saturated solution is mixed with agar gar or gelatin. The two openings of the U tube are plugged with some porous material such as glass wool or cotton. This U-tube is salt bridge. When electric current is allowed to pass through the cell then the flow of current is indicated by the ammeter.

The following observations are made: a) Zinc rod gradually loses its weight b) The concentration of the Zinc ions (Zn2+ aq) in the ZnSO4 solution increases c) Copper gets deposited on the electrode. d) The concentration of Cu2+ decrease in Cu SO4 (aq) solution e) There is a flow of electron in the external circuit from zinc rod to copper rod while the current flows from copper electrode to zinc electrode. It may be noted as a convention, the flow of electric current is taken opposite to the flow of electrons.

These observations can be explained as follow: Zinc is oxidized to Zn2+, which goes to the solution.

The zinc rod gradually loses its weight. The electrons released at the electrode move towards the other electrode through outer circuit. Here, these electrons are accepted by Cu2+ ions of CuSO4 solution, which get reduced to copper. So, the metal gets deposited on the copper electrode.

Zinc acts as anode because electrons are released i.e. oxidation occurs copper electrode behaves as a cathode because electrons are gained i.e. reduction occurs. The zinc rod is regarded as –ve terminal and copper electrode is regarded as +ve terminal. Electrons flow from negative terminal (anode) to positive terminal (cathode)

The containers in which oxidation and reduction half reactions are occurring are called half-cells. The container in which zinc rod is dipped into a ZnSO4 solution is oxidation half-cell and the other in which copper electrode is dipped into a CuSO4 solution is reduction half-cell.

Salt Bridge and its Function: It plays an important role in an electrochemical cell. It is basically a U shaped tube filed with concentrated inert electrolyte. The electrolyte added in the U tube should have following characteristics 1) 2) 3) 4)

The mobility of cation and anion of the electrolyte should be almost same. The ions of the electrolyte should not involve in electrochemical change. The ions don’t react chemically with the species of the cell. The electrolytes generally used are KCl, KNO3, and NH4NO3 etc. The saturated solution of these electrolytes is prepared in agar-agar jelly or gelatin. The jelly keeps the electrolyte in semi solid phase and thus prevents mixing.

Functions of Salt Bridge: 1) It completes the electrical circuit. The salt bridge connects the solution of two half-cells and their electrodes are connected by the means of a wire. Therefore salt will complete the circuit 2) It maintains the electrical neutrality of two half-cell solutions. This can be explained by taking an example of electro chemical cells without a salt bridge. The electrons released by the oxidation of Zn to Zn2+ ions will be accepted by Cu2+ ions of CuSO4. In the other half-cell and get reduced to copper.

3) The positively charged Zn2+ ions pass into the solution, which results into accumulation of extra positive charge in the solution around the anode. Similarly reduction of Cu2+ ions to Cu, there will be extra negative charge around cathode due to excess of SO42- ions. The accumulation of positive charge around zinc rod will prevent the further flow of electrons from the zinc rod. Similarly the accumulation of negative charge around copper electrode will prevent the flow of electrons to the copper ions. Thus the flow of electrons will occur only for a short period of time and after that cell will stop working. This accumulation of charges in the two half cells is prevented by using salt bridge because salt bridge provides the passage for the flow of charge in the internal circuit. When there is increase in the concentration of Zn2+ ions around anode then salt bridge provides Cl- ions to the anode half-cell and neutralizes the excess positive charge. Similarly the excess negative charge is neutralized by sufficient number of K+ ions migrated from the salt bridge to the half-cell. Thus, the salt bridge provides cations and anions to replace lost or produced in the two half-cells.

Representation of an Electrochemical Cell: An electrochemical cell or galvanic cell consists of two electrodes – anode and cathode. The electrolyte solution having these electrodes is called half-cells. When two half-cells combine they form a cell. For representing electrochemical cells the following conventions are to be used

1) A galvanic cell is represented by writing the anode (where oxidation occurs) on the left hand side and cathode (where reduction occurs) on the right hand side.

2) The anode of the cell is represented by writing metal or solid phase first and then cation of the electrolyte, while cathode is represented by writing cation first and then metal in solid phase.

3) The metal and the cation are separated by a semi colon (;) or by a vertical line. The concentration of the electrolyte (cation) should also be mentioned within brackets after the cation.

4) The two half cells are separated by salt bridge which is indicated by two vertical lines

Representation of Some Common Cells by Cell Notation: (i)

(ii)

iii)

ELECTRODE POTENTIAL AND E.M.F OF A GALVANIC CELL Electric potential: The flow of electric current in an electrochemical cell shows that a potential difference exists between two electrodes. Consider a redox reaction occurring at these electrodes. When an electrode, for example, copper, is dipped in a solution of its ion, any of the following three possibilities can take place. a) The metal ions (Cu2+) may collide with the electrode and don’t undergo any change. b) Cu2+ ions may collide with electrode, gain electrons and get reduced to metal atoms. Cu2+ + 2e- → Cu (reduction)

c) Cu atoms on the electrode may loose electrons and get oxidized to Cu2+ Cu → Cu2+ + 2e- (oxidation)

Now, if the metal has relatively high tendency to get oxidized then its atoms will lose electrons and form Cu2+ ions, which go into the solution and get accumulated on the metal electrode. As a result the electrode acquires slightly negative charge with respect to the solution. Some of the Cu2+ ions from the solution take up some electrons and become copper atoms. After sometimes, equilibrium will be established.

Such equilibrium results in the separation of charges (negative on the electrode with respect to solution). Now, if the metal ions have relatively greater tendency to get reduced they will take up electrons from the electrode. This results in the development of positive charge on the electrode with respect to solution and also creates a separation of charges (positive on the electrode with respect to solution) Due to the separation of charges between the electrode and the solution, an electric potential is developed between metal electrode and its solution. This electrical potential difference which is set up between metal and its solution is called electrode potential. The electrode potential may be of two types Oxidation potential: The tendency of an electrode to loose electrons or to get oxidized is called its oxidation potential. e.g.

Reduction potential: The tendency of an electrode to gain electrons or to get reduced is called its reduction potential. e.g.

The electrode potential depends upon a) the nature of the metal and its ions b) concentration of ions in the solution c) temperature

E.M.F. or Cell Potential of a Cell Since electrochemical cells consist of two half cells, their electrodes in these halfcells have different reduction potentials. They have different tendency to loose or gain electrons. The electrode having high reduction potential will have the high tendency to gain electrons as compared to the other electrode with low reduction potential. In other words the electrode having low reduction potential has the higher tendency to loose electrons. As a result of this potential difference between the two electrodes there is a flow of electrons from the electrode with low reduction potential to the electrode with high reduction potential The difference between the electrodes potential of the two electrodes constituting electrochemical cells is called as electromotive force (E.M.F.) or cell potential of a cell. This is the driving force for the cell reaction and expressed in volts. Therefore the cell potential or e.m.f. arises from the difference in the tendencies of the two ions to get reduced. It is equal to the reduction potential for the substance that actually undergoes reduction minus the reduction potential of the substance that undergoes oxidation i.e.

As assigned, convention cathode is written on the right hand side and anode on the left hand side. Therefore e.m.f. of the cell may be written as

Thus, e.m.f. of a cell may be defined as the potential difference between two electrodes of the cell when either no or negligible current is allowed to flow in the circuit. Note: For calculating e.m.f. of a cell, the electrode potentials are always taken as reduction potentials (as a convention) for both the electrodes.

The e.m.f. of a cell is measured with the help of potentiometer. It depends upon the nature of the electrodes, temperature and concentration of the solution in the two half-cells.

Difference between e.m.f. and Potential Difference:

Difference between Electrochemical and Electrolytic Cell:

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