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
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.
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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• Winter '09
• Ostrowski
• Electrochemistry, Electric charge, electrode, electrical energy, Cathode, electrode reactions

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