TAILIEUCHUNG - Electrochemical kinetics

Current Flow in an Electrochemical Cell: Thermodynamic arguments permit the feasibility of overall cell reactions to be predicted, but give no information on rates. To understand the latter it is necessary to consider the effects on various parts of the cell of forcing the cell voltage to assume a value different from that of the equilibrium potential Eeq (V) or electromotive force (emf). In the example of Figure 1, the cell contains hydrochloric acid as aqueous electrolyte and it divides into two compartments by a semipermeable membrane. In one compartment, hydrogen is dissolved and in the other one chlorine gas. | Kinetics S Panero Sapienza University of Rome Rome Italy 2009 Elsevier . All rights reserved. Current Flow in an Electrochemical Cell Thermodynamic arguments permit the feasibility of overall cell reactions to be predicted but give no information on rates. To understand the latter it is necessary to consider the effects on various parts of the cell of forcing the cell voltage to assume a value different from that of the equilibrium potential Eeq V or electromotive force emf . In the example of Figure 1 the cell contains hydrochloric acid as aqueous electrolyte and it divides into two compartments by a semipermeable membrane. In one compartment hydrogen is dissolved and in the other one chlorine gas is fluxed while two platinum foils act as electrodes. Under rest conditions the potential-determining electrode reactions are Cl2 2e 2Cr I and H2 2H 2e- II If the hydrochloric acid has unit mean activity and the equilibrium pressure of gases is 1 atm then the standard thermodynamic tables show that the cell voltage at the equilibrium DE will be E Cl2 Cl - E h h2 1 If the cell operates as a battery by closing the circuit with a resistor electrons flow from the hydrogen electrode to the chlorine one and the equilibrium that was established before the electrodes were connected is destroyed. The potential of the chlorine electrode is reduced as the electrochemical reduction of chlorine takes place at the cathode Cl2 2e Cl Correspondingly the potential of the hydrogen electrode anode is shifted in a positive direction as the reaction IC IC 2e- proceeds. These two effects are summarized in Figure 2 a where it is evident that the overall cell voltage will decrease as the current increases. If instead of operating as a battery the cell is operated in electrolysis mode by the imposing of an external voltage larger than the equilibrium potential the two cell reactions will be driven in the opposite direction. In this process the anode is now the chlorine electrode and the

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