when the ions each are 1 M, Zn is consumed. This means 1. 1 2. 1 3. 1 4. 1. More information needed Lecture 24 CH102 A1 (MWF 9:0 am) Monday, March 26, 2018 Cell voltage,, and electrical energy Calculating standard cell voltage, Cell voltage versus spontaneity Next lecture: Continue ch16. Cell voltage versus / : The Nernst equation; Exploring the Nernst equation; Concentration cells: Mixing electric current 1 Cell voltage, E cell, and electrical energy Cell voltage, cell, and electrical energy Electron flow in a voltage is able to provide electrical energy e charge that flows voltage In terms of moles of electrons that flow, e, the amount of charge is charge that flows e where, known as the Faraday constant, is 9648 C/mol. 6 7 1
Cell voltage, E cell, and electrical energy A typical physiological voltage is 0.10 V 0.10 J/C. The corresponding energy due to the transfer of 1.00 mol of electron is Cell voltage, E cell, and electrical energy A typical physiological voltage is 0.10 V 0.10 J/C. The corresponding energy due to the transfer of 1.00 mol of electron is e charge that flows voltage e cell 1.00 mol 9648 C/mol 0.10 J/C 14. kj This is a lot of energy! While each electron contributes a small amount of energy, there are a lot of electrons in a mole! 8 9 The bigger,, the more energy than can be harnessed. Two things determine, : enthalpy change and spontaneity These two things together determine what is called the free energy change, Δ, of the redox process. We will see that is the amount of energy that can be harnessed, excluding any energy associated with pressure volume work. The reason pressure volume work is excluded is that it is typically lost to the surroundings as a consequence of volume change that arise to keep pressure constant. 10 11 2
By our convention that work done on the system corresponds to positive energy change, free energy change is defined with a negative sign e so that negative values of free energy change mean work is available to be done on the surrounding. In general, if 0, that is, if 0, then the redox process is able to provide energy to the surroundings. We know spontaneity is determined by relative to. For now we can simplify things by arranging for 1, typically by making reactants and products be in their standard state. This arrangement defines what we call the standard free energy change, e 12 13 Since is proportional to the Δ, Δ e and because we know how to express a redox process as the sum of its half reactions, we can use Hess s law to express Δ cell as Δ Δ Δ By convention, reduction half reactions have a standard reduction potential, whose value is the cell potential relative to the standard hydrogen electrode, SHE, 2 H 1 M 2 H 2 1 atm, 0 V All other reductions defined relative to SHE Zn 2 1 M 2 Zn, E o 0.763 V Cu 2 1 M 2 Cu, E o 0.340 V etc. 14 1 3
This means we can write for the cathode reduction contribution Δ e and for the anode oxidation contribution and Δ e The reason for the in the anode expression is because oxidation takes place there the reverse of reduction and so the sign of its contribution to free energy change must be reversed. 16 Combining the three expressions Δ e and Δ e Δ Δ Δ Δ e and canceling the common factor e, we get the fundamental expression for cell in terms of reduction potentials, Note that both and are standard reduction potentials. 17 Here is an example: Does Cu 2 oxidize Zn? Cu 2 1 M Zn Cu Zn 2 1 M, 0? The standard reduction potentials are Zn 2 1 M 2 Zn, 0.763 V Cu 2 1 M 2 Cu, 0.340 V The standard cell potential is 0.340 V 0.763 V 1.103 V 0 Cell voltage versus spontaneity Spontaneity is proportional to voltage Spontaneity is proportional to how far away from equilibrium Voltage versus /? So, Zn is oxidized by Cu 2 18 21 4
when the ions each are 1 M, Zn is consumed. This means 1. 1 2. 1 3. 1 4. 1. More information needed Quiz For the redox process when, Zn is consumed. This means 1. 1 2. 1 3. 4.. More information needed 22 23 when 10, Zn is consumed. This means over time the cell voltage will 1. become smaller 2. stay the same 3. become larger 4. More information needed when 10, Zn s is consumed. Compared to the voltage when 1, the voltage when 10 is 1. smaller 2. the same 3. larger 4. More information needed 24 2
Group Quiz For the redox process M X M X when 0.1, M is formed. Compared to the voltage when 1, the magnitude of the voltage when 0.1 is 1. smaller 2. the same 3. larger 4. More information needed 10 26 6