iii.electrochemical cells (continued): calculating cell voltages the standard voltage for any...
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III. Electrochemical Cells (continued):Calculating Cell Voltages
The standard voltage for any electrochemical cell can be calculated from the theoretical standard voltage of each of the “half-cells”.
eg: Calculate the standard cell voltage for a zinc // copper cell.
E°cell = E°oxid + E°red
Standard cell
potential
Standard oxidation potential
Standard reduction potential
= +
Note: values on SRP table are for reduction half-reactions.To get standard oxidation potential, change sign of SRP.
E°cell = 0.76V + 0.34V = 1.10V
The Hydrogen Reference Electrode:
All half cell potentials are measured relative to H2.
eg: H2 + Cu2+ Cu + 2H+
If measured voltage under standard conditions is 0.34V, how do we know how much voltage comes from each half-reaction?
H2 2H+ + 2e-
Cu2+ + 2e- Cuor
In reality neither half-reaction alone provides any voltage.
We arbitrarily define the H2 half-reaction to be 0.0V.
E°cell = E°oxid + E°red
0.34V = 0.0V + E°red
The voltage of the reduction of Cu2+ is 0.34V.
A substance with a positive SRP is more easily reduced than H+ and a substance with a negative SRP is not as easily reduced H+.
Notes: Voltage is independent of the number of e-’s
transferred.
A negative value for E°cell means the forward reaction is not spontaneous (the reverse reaction is.)
Increasing reactant concentration or decreasing product concentration will increase the voltage of the cell.
If a cell reaches equilibrium E°cell = 0V (dead battery)
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