Sourabh Soni

 AIM Variation of Cell Potential With Change in Concentration of Electrolytes INTRODUCTION The EMF of a cell varies with the concentration of the two electrolyte solutions according to the following Nernst equation: where E = the electromotive force of the cell E° = the standard electromotive force of the cell GALVANIC CELL A galvanic cell is an important electrochemical cell. It is named after Luigi Galvani an Italian physicist. It is also called Voltaic cell, after an Italian physicist, Alessandro Volta. A galvanic cell generally consists of two different metal rods called electrodes. Each electrode is immersed in a solution containing its own ions and  these form a half cell. Each half cell is connected by a salt bridge, or separated by a porous membrane. The solutions in which the electrodes are immersed are called electrolytes. The chemical reaction that takes place in a galvanic cell is the redox reaction. One electrode acts as anode in which oxidation takes place and the other acts as the cathode in which reduction takes place.  The best example of a galvanic cell is the Daniell cell. DANIELL CELL The Daniell cell was invented by a British chemist, John Frederic Daniell. In the Daniell cell, copper and zinc electrodes are immersed in a solution of copper (II) sulfate (CuSO4 (aq)) and zinc (II) sulfate (ZnSO4 (aq)) respectively. The two half cells are connected through a salt bridge. Here zinc acts as anode and copper acts as cathode. At the anode, zinc undergoes oxidation to form zinc ions and electrons.  The zinc ions pass into the solution. If the two electrodes are connected using an external wire, the electrons produced by the oxidation of zinc travel through the wire and enter into the copper cathode, where they reduce the copper ions present in the solution and form copper atoms that are deposited on the cathode. The anodic reaction is represented as: The cathodic reaction is represented as: Total cell reaction is the sum of the two half cell reactions:   SALT BRIDGE The salt bridge is usually an inverted U-tube filled with a concentrated solution of an inert electrolyte. The inert electrolyte is neither involved in any chemical change, nor does it react with the solutions in the two half cells.  Generally salts like, KCl, KNO3, NH4NO3 are used as the electrolyte. . SIGNIFICANCE OF SALT BRIDGE Its main function is to prevent the potential difference that arise between the two solutions when they are in contact with each other. This potential difference is called the liquid junction potential. It completes the electrical circuit by connecting the electrolytes in the two half cells. It prevents the diffusion of solutions from one half cell to the other. It maintains the electrical neutrality of the solutions in the two half cells. REPRESENTATION OF AN ELECTROCHEMICAL CELL: The following rules are followed for representing an electrochemical cell. Anode is written on the left hand side and cathode on the right hand side. The anodic cell is represented by writing the metal first and then the metal ions present in the electrolyte. These two are separated by a vertical line or a semicolon. For example: The molar concentration or activity of the solution is written in brackets after the formula of the ion. For example: The cathodic cell is represented by writing the metal ion first of the electrolyte solution and then the metal. Both are separated by vertical line or semicolon. For example:   The molar concentration or activity of the solution is written in brackets after the formula of the ion. For example: Salt bridge is represented by two vertical lines. So Daniell cell is represented as: If the concentration of both the electrolytes is 1 M, then the cell notation is: ELECTRODE POTENTIAL When a metal electrode is dipped in a solution containing its metal ions, a potential difference is developed at the metal solution interface. This potential difference is called the electrode potential. For example, when a zinc rod is dipped in a solution containing Zn2+ ions, it oxidizes, and the Zn2+ ions pass from the zinc rod to the solution leaving excess of electrons at the zinc rod. Thus the zinc rod becomes negatively charged with respect to the solution and a potential difference is set up between the zinc rod and the solution. This potential difference is called the electrode potential of zinc. Similarly, when a copper rod is dipped in a solution containing Cu2+ions, the Cu2+ ions gain electrons from the copper rod leaving positive charge on the copper rod. As a result a potential difference is set up between the copper rod and the solution and is called the electrode potential of copper. In an electrochemical cell, the anode has a negative potential and cathode has a positive potential.The potential of each individual half cell cannot be measured. We can measure only the difference between the potential of the two half cells.The potential of a half cell can be measured by connecting it with Standard Hydrogen Electrode (SHE). The standard electrode potential of a SHE is assumed to be zero.The electrode potential at standard conditions such as 25°C temperature, 1 atm pressure, 1 M concentration of electrolyte, is called the standard electrode potential. It is denoted by the symbol E0. The electrodes are arranged in the increasing order of their standard reduction potential and are called Electrochemical series. ELECTROMOTIVE FORCE (EMF) OR CELL POTENTIAL The potential difference between the two electrodes in a galvanic cell is called a cell potential or emf of the cell. It is measured in volts.emf of the cell = Potential of the half cell on the right hand side (Cathode) - Potential of the half cell on the left hand side (Anode) For example, emf of Daniell cell, EXPERIMENT:  To study the variation of cell potential in Zn | Zn2+ || Cu2+ | Cu with change in concentration of electrolytes (CuSO4 or ZnSO4) at room temperature. Apparatus Required 100 ml beakers, 250 ml beaker, measuring flask, 100 ml measuring cylinder, connecting lead, voltmeter, U tubes, cotton,Copper strip, zinc strip, copper sulphate, zinc sulphate, potassium chloride. TO PERFORM EXPERIMENT (a) Preparation of solutions copper sulphate 1. Weigh 29.969 g of copper sulphate on a watch glass. Transfer it to a 250 ml beaker and dissolve it in water (sufficiently less than 100 ml). Transfer the solution into 100 ml measuring flask and make its volume to 100 ml. It is 1.00 M CuSO4 solution and store it. Mark it A. 2. Prepare 1.00 ml copper sulphate solution each of 0.5 M, 0.25 M, 0.125 M and 0.0625 M strength by dilution as follows. To prepare 0.5 M CuSO4 solution, take 50 ml of 1.0 M CuSO4 solution prepared in step 1 in another measuring flask with the help of measuring cylinder make its volume to 1.00 ml. Mark it as B. Similarly, you can prepare the solution of 0.25 M, 0.125 M and 0.0625 M. (b) Preparation of zinc sulphate solution Prepare 100 ml 1.0 M zinc sulphate solution by dissolving 28.756 g zinc sulphate in water. (c) Preparation of salt bridges (i) Take a U tube of glass of about 10 cm length. (ii) Dissolve about 2.0 g of agar agar completely in about 25 ml of water taken in a borosil beaker by heating it at a low flame. (iii) Dissolve about 12 g of solid potassium chloride (KCl) completely in the agar agar solution in water by stirring. (iv) Fill the U tube with it and allow it to remain in an upright position till agar agar gets set. (d) Setting up of simple electrochemical cells and measuring its EMF Assemble the electrochemical cells with different combinations of concentrations. Zn (s) | Zn2+ (aq) || Cu2+ (aq) | Cu (s) Use separate salt bridges for different readings. (i) Take 30 ml of 1.0 M copper sulphate solution (from flask A) in a 100 ml beaker and 30 ml of 1.0 M zinc sulphate solution in another 100 ml beaker. (ii) Immerse the metal strips in solutions containing the same metal ions. (iii) Connect both the metal strips to each other through a voltmeter with the help of connecting lead. (iv) Connect both the solutions through salt bridge. (v) Note the voltmeter reading that gives the E.M.F of the cell. (vi) Repeat the same procedure with copper sulphate solutions of different concentrations using 0.1 M solution of zinc sulphate. CONCLUSION EMF of the cell increases with decrease in concentration of the electrolyte around the anode and increase in concentration of the electrolyte around cathode. PRECAUTIONS 1. Electrodes should be cleaned by sand paper. 2.There should be no air bubble in the salt bridge. 3. Use separate salt bridge for every cell. 4.Each voltmeter reading should be taken after stirring the solution. 5.The concentration of copper sulphate and zinc sulphate should neither be too low nor too high. 6.Connect the copper strip to the positive terminal and the zinc strip to the negative terminal of the voltmeter. 7.The two half cells should be connected using a salt bridge. 8.Note the reading only when the pointer becomes stable.