electrochemsumadv07 - AP Chemistry Electrochemistry Summary...

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AP Chemistry Electrochemistry Summary Information Electrochemistry deals with: a. using chemical reactions to do work (i.e. provide electricity) (known as voltaic or galvanic cells). b. using electricity to force nonspontaneous reactions (known as electrolytic cells). Voltaic Cells: Recall that oxidation-reduction (redox) reactions involve the transfer of electrons from one chemical to another. Ex. Mg + Cl 2 MgCl 2 In this example magnesium loses two electrons Mg 0 Mg 2+ + 2e - and is considered oxidized Each chlorine gains an electron Cl 2 0 + 2e - 2Cl - and is considered reduced Things to remember: Redox reactions always occur in pairs The charge on any element is zero. The species being reduced is the oxidizing agent: The species being oxidized is the reducing agent. Remember the saying: LEO goes GER L oses E lectrons O xidized G ains E lectrons R educed Question: Determine which element is oxidized and which is reduced in the following reactions: 1. 2AgNO 3 + Cu 2Ag + Cu(NO 3 ) 2 2. F 2 + 2NaCl Cl 2 + 2NaF Answers: Silver is reduced (1 + 0), copper is oxidized (0 2 + ); Fluorine is reduced (0 1 - ), chlorine is oxidized (1 - 0). A more “active” metal will reduce a less active metal (since the more active metal has a greater tendency to lose electrons. Also, in general, most metals will reduce most nonmetals. An activity series lists metals from more active to less active as in the following: Li, K, Ba, Ca, Na, Mg, Al, Zn, Fe, Cd, Ni, Sn, Pb, Cu, Hg, Ag, Au In other words, lithium is the most active metal in this list and can therefore reduce the ion of any of the metals that come after it in the series. (ex. 3Li + Al 3+ 3Li 1+ + Al) By separating the oxidation part of a reaction from the reduction part then connecting them by a wire, useful work can be done. As electrons flow from the oxidation side to the reduction side they can pass through a light bulb, motor, etc., providing power to make them run. This type of setup is known as a voltaic (or galvanic) cell . The difference in potential energy between the two sides (relative to hydrogen) is known as the potential difference and is measured in Volts (J/C) . For instance, the reaction between a silver ion and aluminum metal is 2.46V. These values are determined from a list of standard reduction potentials that gives the relative tendency for an element or ion to be reduced. Examine the following diagram of a typical voltaic cell:
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Electrolytic Cells: In an electrolytic cell the anode (site of oxidation) is considered the positive electrode and the cathode (site of reduction) is considered the negative electrode. Electrolytic cells drive nonspontaneous reactions (i.e. in a direction that would not naturally occur).
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electrochemsumadv07 - AP Chemistry Electrochemistry Summary...

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