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Voltage of a Battery something about _________________________ available energy (_____________________) potential to do work does not need input of energy and entropy of - “wheeeeee!” univ. inc. “wheeeeee!” electrons flow spontaneously e- e e- e-“wheeeeee!” e- from ____________ potential ( energy ) high low to _______________ potential (energy) (like a waterfall Slide 5) the difference in PE for the e- in each environment (“top of the slide” vs “bottom of the slide”) is related to the electrical energy that can be supplied by the battery OR the electrical PE is related to the stability of the e- in an environment eNa Cl Na+ Cl- the electron is more stable in Cl- than in Na Back to the Danielle Cell (Cu/Zn) electrons move from Zn to Cu2+ Zn 2e- Cu2+ Zn2+ Cu the electrons are more stable in Cu than in Zn Potential Energy Zn2+ + 2e- + Cu2+ energy gained as 2e- removed from Zn energy released as 2e- add to Cu2+ difference related to battery’s voltage Zn + Cu2+ Zn2+ + Cu (energies are for the copper and zinc in the environment of the battery) Potential Energy Zn2+ + 2e- + Cu2+ energy gained as 2e- removed from Zn energy released as 2e- add to Cu2+ difference related to battery’s voltage Zn + Cu2+ Zn2+ + Cu to get at need to know the potential energy of an electron in a particular environment energy of ___________ position SUN More familiar… gravitational potential energy attic Gravitational PE of the ball on the 1st floor is less than (it would be) on the 2nd floor and greater than (it would be) in the basement 2nd floor If the ball could get to the center of the Earth, it would have 0 (zero) PE relative to the Earth But, it’s PE has increased relative to the Sun 1st floor basement The point is that you can only speak of PE relative to some position, you cannot speak of absolute (total) PE EARTH The point is that you can only speak of PE relative to some position, you cannot speak of absolute (total) PE The same is true for electrical PE: can only speak of the relative potential energy of an electron in an environment Zn 2e- Cu2+ Zn2+ Cu the electrons are more stable in Cu than in Zn lower electrical PE So, if the electrons are more stable in Cu than in Zn, which of Zn2+ + 2e- Zn Cu2+ + 2e- Cu the reduction of Cu2+ is more likely to happen is more likely to happen ? the relative potential energy of the electron in the metal reflects the relative tendency for a metal to be reduced (to its pure form) - the lower the potential energy of an electron in a metal, the greater the tendency for a metal to be reduced (to its pure form) the relative potential energy of the electron in the metal reflects the relative tendency for a metal to be reduced (to its pure form) - the lower the potential energy of an electron in a metal, the greater the tendency for a metal to be reduced (to its pure form) Reduction Potential – the relative tendency for a reduction to happen - reflects the energy released as electrons are gained by the substance being reduced Zn2+ + 2e- Zn Cu2+ + 2e- Cu Potential Energy Zn2+ + 2e- + Cu2+ energy energy gained released as as 2+ 2e 2e-- removed add to Znfrom Zn energy released as 2e- add to Cu2+ Zn + Cu2+ Zn2+ + Cu The amount of energy released (reflecting the stability of the electron in an environment) is measured by convention relative to the electron(s) being in elemental hydrogen, H2 Potential Energy 2H+ + 2e- + Cu2+ energy released as 2e- added to H+ (go into H2) H2 + Cu2+ energy released as 2e- add to Cu2+ (go into Cu) 2H+ + Cu arbitrarily assigned 0 -10.9 10-20 J/2e- difference in energy between electrons in H2 and in Cu = 10.9 10-20 J/2e- The amount of energy released (reflecting the stability of the electron in an environment) is measured by convention relative to the electron(s) being in elemental hydrogen, H2 Potential Energy 2H+ + 2e- + Zn2+ energy released as 2e- added to H+ (go into H2) H2 + Zn2+ energy released as 2e- add to Zn2+ (go into Zn) 24.4 10-20 J/2e2H+ + Zn arbitrarily assigned 0 difference in energy between electrons in H2 and in Zn = 24.4 10-20 J/2e- Reduction Potential – the relative tendency for a reduction to happen - reflects the energy released as electrons are gained by the substance being reduced RELATIVE to electrons gained by H+ (to produce H2) Reduction Potential Cu2+ + 2e2H+ + 2eZn2+ + 2e- Cu H2 Zn 10.9 10-20 J/2e0 -24.4 10-20 J/2e- Potential Energy 2e- + 2H+ + Cu2+ + Zn2+ 2H+ + Cu2+ + Zn H2 + Zn2+ + Cu2+ 35.3 10-20 J energy released 2H+ + Cu + Zn2+ Standard Units – the energy available from a battery is measured in Volts 1 Volt (V) = 1 Joule (J) 1 Coulomb of charge (C) 96,485 Coulomb Coulomb: mole e- 1 Coulomb 1 Coulomb 1.036410-5 mole e- 6.2411018 e- Reduction Potential Cu2+ + 2e- 2H+ + 2eZn2+ + 2e- Cu H2 Zn 10.9 10-20 J/2e- = x 6.2411018 e- = 0.34 J 1C 0 -24.4 10-20 J/2e- = x 6.2411018 e- = -0.76 J NOTE: the potential energy will depend on the environment standard reduction potentials are for : 25C, 1 atm, and solutes at 1M http://www.google.com/ 1C Standard Reduction Potential E0 Cu2+ + 2e2H+ + 2eZn2+ + 2e- Cu H2 Zn 10.9 10-20 J/2e- = x 6.2411018 e- = 0 -24.4 10-20 J/2e- = x 6.2411018 e- = 0.34 J 1C -0.76 J 1C Potential Energy 2e- + 2H+ + Cu2+ + Zn2+ 2H+ + Cu2+ + Zn H2 + Zn2+ + Cu2+ 0.34V - (-0.76V) = 1.10 Volts (of electrical potential energy) 2H+ + Cu + Zn2+ Battery Voltage Zn(s) + Cu2+(aq) Cu(s) + Zn2+(aq) Zn(s) Zn2+(aq) + 2eCu2+(aq) + 2eCu Battery Voltage = (E0cell) = (net ionic equation) oxidation reduction reduction potential for the reduction reduction potential for the oxidation 0.34 V - (-0.76 V) Determine Voltage for standard lead storage car battery Pb(s) + PbO2(s) + H2SO4(aq) hydrogen fuel cell H2 (g) + O2(g) H2O(l) PbSO4(s) + H2O(l) The Waterfall The Dam The Lake The Turbine The River The Waterfall The Dam OPEN THE DAM!! The Lake The Turbine The River The Waterfall The Dam water at the top of the lake has high PE as it falls due to gravity its PE turns into KE the moving water can do work on the turbine The Lake The Turbine PE – the potential to do work work involves moving cannot “see” the expression of energy until objects are in motion PE KE for work to be done The River Slide 1