331_lecture_notes_chp_2_f08

331_lecture_notes_chp_2_f08 - 2.1 Basic Concepts and...

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2.1 Basic Concepts and Conventions A table of useful names, symbols and units is given below: Current - The current I gives the rate of charge transfer across a circuit due to the flow of electrons. It has the units of coulombs/sec , or amperes (amps). This is analogous to fluid flowrate through a pipe. What is the total charge q (in coulombs) passed by a circuit over a period of time from t 1 to t 2 ? q =
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2.2 current density - It is often useful to normalize the total current with respect to the electrode area across which it is passed. This is called the current density i and has units of amp m -2 , i.e., i I/A where A = electrode area normal to the flow of charge potential or voltage - The voltage across a cell is the driving force pumping electrons through the circuit and is a measure of the energy difference of electrons at the two electrodes of a cell. Potential is analogous to pressure drop in fluid flow. types of electrochemical cells There are two basic types of electrochemical cells: electrolytic - utilizes electrical energy to drive electrochemical reactions, that would not otherwise occur. eg., chloro-alkali cell, electroplating galvanic - spontaneous electrochemical reactions generate a flow of electrons, eg., battery, corrosion, electroless plating
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2.3 types of electrode and electrode reactions Regardless of the type of an electrochemical cell, it must always contain at least one cathode and one anode when current is flowing. cathode - electrode at which a cathodic (reduction or consumption of electrons) reaction occurs eg. 2H + (aq) + 2e - H 2 (g) anode - electrode at which an anodic (oxidation or generation of electrons) reaction occurs eg. Fe 2+ (aq) Fe 3+ (aq) + e - sign conventions Sign conventions used in electrochemical cells can sometimes be confusing since you cannot always associate the cathode and anode with particular signs. The sign of a cathode or anode depends upon whether the cell is electrolytic or galvanic, as shown below: electrode electrolytic cell galvanic cell cathode + anode + The reasons for these sign assignments will become clearer later in the course.
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2.4 mode of operation of an electrolytic cell Both electrolytic and galvanic cells can be operated in one of two ways: galvanostatic mode - cell is operated under conditions whereby current is controlled; however, the voltage across the cell cannot be simultaneously controlled, only monitored. potentiostatic mode - cell is operated under conditions whereby the cell potential is controlled. At the same time, the current cannot be controlled and can only be monitored.
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2.5 What is the Physical Picture of Electrical Potential? Everyone has a physical idea of how fluids move, what pressure drop means, etc. In the case of electricity, electrical potential seems much more abstract. Can we give a physical picture to what it means?
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