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Understanding Standard Cell and Half-Cell Potentials in Electrochemical Cells

This guide explains how to calculate standard cell voltages for electrochemical cells using standard reduction potentials (SRP) of half-cells. For instance, the standard voltage of a zinc/copper cell can be derived by adding the standard oxidation and reduction potentials. The hydrogen reference electrode is defined as 0.0V for measurement consistency. The relationship between half-reactions and voltage is crucial, as a negative E°cell indicates a non-spontaneous reaction. Additionally, concentration changes can affect cell voltage, impacting the equilibrium and battery performance.

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Understanding Standard Cell and Half-Cell Potentials in Electrochemical Cells

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  1. Standard cell potential Standard oxidation potential Standard reduction potential = + 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 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

  2. 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- or Cu2++ 2e- Cu • In reality neither half-reaction alone provides any voltage. • We arbitrarily define the H2 half-reaction to be 0.0V.

  3. 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+.

  4. 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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