Electrochemical Cells PDF

8/25/2024 Electrochemistry What are Electrochemical Cell? A device which converts chemical energy into electrical energy and vice versa consists of two electrodes immersed in one or more suitable electrolytes 1 ...

8/25/2024 Electrochemistry What are Electrochemical Cell? A device which converts chemical energy into electrical energy and vice versa consists of two electrodes immersed in one or more suitable electrolytes 1  8/25/2024 Electrodes and Electrolytes Anode = at which oxidation occurs Cathode = at which reduction occurs Electrolytes - medium that facilitates the flow of ions between electrodes Oxidation and Reduction Reactions Oxidation and reduction reactions occur in separate containers which are not in direct contact with each other 2  8/25/2024 Galvanic Cell vs Electrolytic Cell Galvanic Cell An electrochemical cell that produces electricity as a result of chemical reactions spontaneous reaction occurring inside the cell produces electricity Electrolytic Cell An electrochemical cell in which electrical energy brings about chemical reaction non-spontaneous driven by an external source of current A Simple Galvanic Cell (Daniel cell) contains 1.0 M solution of ZnSO4 and the other contains 1.0 M solution of CuSO4 zinc rod is dipped in the ZnSO4 solution and copper rod is dipped in the CuSO4 solution 3  8/25/2024 Reactions at the Anode In a galvanic cell, Oxidation occurs at the anode (negative terminal) The half cell where oxidation occurs is called the oxidation half cell Reactions at the Cathode In a galvanic cell, Reduction occurs at the cathode (positive terminal) The half cell where reduction occurs is called the reduction half cell 4  8/25/2024 Electron Flow in a Galvanic Cell the flow of the electrons occurs from the negative terminal (anode) to the positive terminal (cathode) the flow of electric current is taken opposite to the flow of electrons Functions of Salt Bridge Completes the circuit and permits ions to pass between the two half cells Prevent intermixing of the solution of both the half cells Prevents liquid-to-liquid junction potential which is harmful for the cell 5  8/25/2024 Representation of an Electrochemical Cell The anode is always written on the left and the cathode is always written on the righthand side. The anode of the cell is represented by writing the metal or solid phase first and then the electrolyte (or cation of the electrolyte) while the cathode is represented by writing the electrolyte first (or cation) and then, the metal or solid phase. The metal and the cation are separated either by a vertical line (/) or by a semicolon (;) The salt bridge which separates the two half cells is indicated by a double line (//). Electrode Potential and EMF of a Galvanic Cell 6  8/25/2024 The electrode potential depends upon Nature of metal and its ions Concentration of the ions in the solution Temperature EMF or Cell Potential of a Cell difference of electrode potential between the two electrodes constituting an electrochemical cell Expressed in volts (V) 7  8/25/2024 Electrochemical Series arrangement of elements in the order of increasing reduction potential values Also known as activity series Applications of Electrochemical Series 1. Relative strength of the oxidizing and reducing agents 2. Calculation of cell potential or EMF of the cell 3. Predicting spontaneity or feasibility of a reaction 4. To predict whether a metal will react with acids to give H2 gas 5. Replacement tendency 8  8/25/2024 Example 1. Calculate the standard EMF of a cell containing Sn2+|Sn and Br2 | Br- electrodes [Eo (Sn2+|Sn) = -0.14 V Eo(Br2|Br-) = 1.08 V] 2. Predict whether the reaction 2Ag(s) + Zn2+(aq) 2Ag+(aq)+Zn(s) is feasible or not Example 3. Using the electrochemical series, predict whether Zinc and Silver would react with dil. H2SO4 or not 4. An iron wire is immersed in a solution containing ZnSO4 and NiSO4. The concentration of each salt is 1M. Predict, giving reasons, which of the following reaction is likely to proceed. i. Iron reduces Zn2+ ions ii. Iron reduces Ni2+ ions 9  8/25/2024 Nernst Equation Electrode Potential electrode potential of an electrode depends upon the concentration of the electrolyte solution and the temperature The electrode potential is termed as the standard electrode potential when  Concentration of electrolyte is 1 M  Temperature = 298 K  Pressure of gaseous component is 1 atm or 1 bar (1.00 × 105 Pa)  Solid components are in their standard states 10  8/25/2024 Nernst Equation 0.0591 [𝑃𝑟𝑜𝑑𝑢𝑐𝑡] 𝐸=𝐸 − log 𝑛 [𝑅𝑒𝑎𝑐𝑡𝑎𝑛𝑡] For a cell reaction, 0.0591 [𝐶] [𝐷] 𝐸=𝐸 − log 𝑛 [𝐴] [𝐵] where: n = number of electrons E = electrode potential 𝐸 = standard electrode potential Application of Nernst Equation 11  8/25/2024 1. Calculation of cell potential of the cell Example: Calculate the potential for each half cell and the total EMF at 25 °C for the cell represented schematically as Pb / Pb2+ (0.001 N) // Cl– (0.1 N) / Pt, Cl2 (1 atm) Example: Write the Nernst equation and calculate the EMF of the following cell at 298 K. Mg(s) / Mg2+ (0.001 M) // Cu2+ (0.0001 M) / Cu(s) 12  8/25/2024 2. Calculation of Equilibrium Constant At equilibrium, the components in the two cell compartments have the same free energy; that is, ∆𝐺 = 0 for the cell reaction at the equilibrium concentrations. The cell no longer has the ability to do work 𝑎𝐴 + 𝑏𝐵 ⇌ 𝑐𝐶 + 𝑑𝐷 [ ] [ ]. 𝐾 = ; 𝐸 =𝐸 − log 𝐾 [ ] [ ] At 𝐸 = 0, log 𝐾 = 𝑎𝑡 𝑇 = 298𝐾. Example: Calculate the equilibrium constant for the reaction between silver nitrate and metallic zinc. The standard electrode potential of the cell is 1.56 V. 13  8/25/2024 3. To find the concentration of one ionic species in a cell if the concentration of the other species is known Example: Find the concentration of Cd2+ ions in the given electrochemical cell 0.0591 [𝑃𝑟𝑜𝑑𝑢𝑐𝑡] 𝐸=𝐸 − log 𝑛 [𝑅𝑒𝑎𝑐𝑡𝑎𝑛𝑡] 4. To find the pH of a solution Example: An electrochemical cell is represented as follows: 𝑃𝑡 𝑠 𝐻 𝑔, 1 𝑎𝑡𝑚 |𝐻 (𝑎𝑞) |𝐴𝑔 𝑎𝑞, 1.00 𝑀 |𝐴𝑔(𝑠) Given 𝐸 = 0.9 𝑉 𝑎𝑡 25℃; 𝐸° / = 0.80𝑉 Find the pH of the solution 0.0591 [𝐻 ] 𝐸 = 𝐸° − log 𝑛 [𝐴𝑔 ] 14