Decomposition potential PDF

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decomposition potential in electrochemistry...

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Chemistry Department

CHM4283 Assignment topic Decomposition Potential Instructor: Mam Fatima Jabbar

Group Members Usman Ghani-62 Sajjad Ur Rehman-52 Faseehullah-70 Soha Israr-55 Mohsina Irshad-69

Section: CF3-17 Submission Date: 18th Jan Due Date: 18th Jan

Decomposition Potential

Decomposition potential / (decomposition voltage): When electrolysis is administered, the products of electrolysis accumulate around the electrodes. This causes a change in concentration around the electrodes and an opposing emf (called back emf) is produced. For example, when a potential is applied between two platinum electrodes dipping in dil. the Sulphuric acid solution, at once the electrolysis of water starts evolving hydrogen and oxygen. But electrolysis halts very soon, because of back emf (produced by the adsorption of evolved gases on the surface of two electrodes) is greater than the applied voltage. Now if we increase the applied voltage slowly, the electrolysis proceeds smoothly, when the applied voltage just exceeds the rear emf. Thus, decomposition potential is adequate to back emf. Factors affecting decomposition potential: The magnitude of decomposition Voltage of the electrolyte depends upon: • • • • •

Nature of electrolytes Nature of electrodes Temperature Concentration of electrolytes Nature of product of electrolysis

Definition: The minimum potential which must be applied between the 2 electrodes immersed within the given electrolytic solution to cause continuous electrolytic decomposition is named decomposition potential.

Measurement of Decomposition Potential: The decomposition voltage / Potential can be determined using an electrolytic cell shown in Figure 1.

Figure 1-Electrolytic Cell

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Decomposition Potential

The cell consists of two platinum metal electrodes immersed in the electrolyte. The voltage is varied by moving the contact maker D along with the wire AB and the current passing through the cell is measured using an ammeter (A). If a dilute solution of an acid or base is taken within the cell, at low voltages no reaction is found to occur and there's a slight/little bit increase within the current. On increasing the potential slightly above 1.7V, a sudden evolution of H2 and O2 at the electrodes is observed. this is often amid an abrupt increase within the current. The applied voltage of 1.7V is that the decomposition voltage for dilute acids and bases. When current is plotted against voltage, a curve of the sort shown in Figure-2 is obtained

Figure 2-Curve Showing Decomposition Potential

It is clear from the figure that the rise in current in proportion to the voltage occurs only after a particular potential (decomposition potential). If the voltage is increased further, the current value also increases. Below the decomposition potential the rise in current only marginal indicating that no electrolysis is happening.

Ed = E cathode – E anode The decomposition potential of an electrolyte process is of the same magnitude as the emf set up by the cell resulting from electrolysis. This is illustrated with ZnI2 system figure-3.

Figure 3-ZnI2 Electrolysis

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Decomposition Potential

If a 1 Molar solution of ZnI2 is electrolyzed in an electrolytic cell such as that shown in Figure1, zinc and iodine are liberated at the electrodes. For this concerned electrolysis, the decomposition voltage is found to be around 1.30Volt. The products formed accumulate on the electrodes and result in the setting up of the following cell.

Zn | Zn2+ | I2 | IThis exsert / put a back emf and offers resistance to the flow of current until the applied voltage overcomes the back emf. To put it another way, the magnitude of the back emf should be the same as the Ed. The emf of the above cell is given below

E = ER - EL E = E iodine - E Zinc = 0.54 – (-0.76) = 1.30V Thus, the cell emf (back emf) is a con to the applied voltage. the rear emf arises from the buildup of zinc and iodine on the electrode surface. To pass the current through this cell, the applied voltage must be exceeding the back emf of 1.30V. The cell emf is therefore adequate to the decomposition voltage Ed which is experimentally found to be 4.30V for Zn and iodine cell.

Significance of decomposition Voltage: Decomposition potential plays a very important role in the following Domains •

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Electroplating: The minimum potential required for electroplating of metal under a given set of bath conditions is understood by decomposition potential. Separation of metals: A solution containing copper and zinc ions are often separated by electrolysis. Since the decomposition potential of copper metal is low (1.2V), so at this voltage, only copper metal is deposited. When all copper is deposited, zinc ions get deposited later at a better decomposition potential (2.55V). Refining of metals: A particular metal can be made to get deposited on the cathode by applying a potential equal to its decomposition potential (electrolytic refining).

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