TwoOxidationStatesofVanadiumfall09_000 - Two Oxidation...

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Two Oxidation States of Vanadium 55 Two Oxidation States of Vanadium Objective To determine the standard reduction potential for the reaction, VO 2 + (aq) + 2 H + (aq) + e - VO 2+ (aq) + H 2 O (l) Introduction Reactions that involve a transfer of electrons are called redox reactions. A redox reaction consists of one half reaction in which electrons are lost (oxidation) and one half reaction in which electrons are gained (reduction). Two half reactions and the overall redox reaction are shown below. Cu 2+ (aq) + 2e - Cu (s) reduction Zn (s) Zn 2+ (aq) + 2e - oxidation _____________________________________________________________ Cu 2+ (aq) + Zn (s) Cu (s) + Zn 2+ (aq) Overall redox reaction Reactions involving a transfer of electrons have many practical applications ranging from generating electricity in batteries and fuel cells to coating a material with a thin layer of metal (chrome) to producing energy in the body. An important characteristic of a redox half reaction is its standard electrode potential . First it is important to understand what it meant by an electrode potential. An electrical cell potential can be thought of as the driving force causing electrons to be transferred. A ball will roll down a hill as a result of the difference in gravitational potential energy from the top to the bottom. Similarly, electrons are transferred in a redox reaction as a result of differences in electrical potential energy between oxidation and reduction half reactions. Electrical potential energy difference is measured in volts (V) which has units of joules (J) per coulomb (C). It is a measure of the amount of work (joule) which can be performed for each unit of electrical charge (coulomb). 1 V = 1 J/C
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Two Oxidation States of Vanadium 56 You are familiar with the term volt from voltaic cells commonly known as batteries. Voltaic Cell Voltage (V) Alkaline flashlight battery 1.5 Lead acid car battery ( 6 cells = 12 V) 2.0 Lithium-ion laptop battery 3.7 Electric eel (~5000 cells in a 6ft. eel= 750 V) 0.15 Nerve of a giant squid (across a cell membrane) 0.070 From: Silberberg M., Chemistry, the Molecular Matter of Nature and Change, 4 th Ed., McGraw Hill, N.Y., N.Y., 2006 p. 914. The standard electrode potential is the voltage associated with a given half reaction when all components in their standard states (concentrations of 1 M in solution, temperature = 25°C, and pressure = 1 atm). It is measured in reference to a standard hydrogen electrode which has been defined to have a standard potential of 0.0V. The condition for spontaneity of any reaction is that the free energy must be less than zero ( Δ G<0). The condition for spontaneity in terms of cell potential is that the overall redox reaction has positive cell potential (E>0). The potential of the redox reaction is found by adding the potentials of each half reaction. A zinc-copper voltaic cell will operate spontaneously because the sum of the potentials of the two half reactions is positive. Cu
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TwoOxidationStatesofVanadiumfall09_000 - Two Oxidation...

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