Nernst Equation Calculator

Nernst Equation Calculator

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The Nernst Equation

The Nernst Equation is a crucial equation in electrochemistry that relates the reduction potential of an electrochemical reaction (cell potential, E) to its standard electrode potential (E°), temperature, and the activities (often approximated by concentrations or partial pressures) of the chemical species involved.

While the standard cell potential (E°) tells us the voltage under ideal conditions (1 M concentrations, 1 atm pressure, 25°C), the Nernst equation allows us to calculate the actual cell potential (E) under any non-standard conditions encountered in real-world applications.

The general form is: E = E° - (RT / nF) * ln(Q)
At standard temperature (25°C or 298.15K), this simplifies to: E = E° - (0.0592 / n) * log₁₀(Q)

Where:
• E = Cell potential under non-standard conditions (Volts)
• E° = Standard cell potential (Volts)
• R = Ideal Gas Constant (8.314 J/mol·K)
• T = Absolute temperature (Kelvin)
• n = Moles of electrons transferred in the balanced redox reaction
• F = Faraday constant (96485 C/mol)
• Q = Reaction Quotient (ratio of product activities to reactant activities, raised to their stoichiometric coefficients)

Frequently Asked Questions

What is the difference between E and E°?

E° (Standard Cell Potential): This is the cell potential measured under strictly defined standard conditions: 25°C (298.15 K), 1 atm pressure for all gases, and 1 M concentration for all aqueous species. It's a reference value.

E (Cell Potential): This is the actual cell potential measured under *any* set of conditions (different temperatures, pressures, or concentrations). The Nernst equation calculates this value.

How do concentrations affect cell voltage?

Concentrations directly influence the Reaction Quotient (Q). According to the Nernst equation:
• If Q < 1 (more reactants than products relative to equilibrium), the ln(Q) or log₁₀(Q) term is negative. This makes the correction term positive, resulting in E > E° (voltage increases). The reaction is driven forward.
• If Q > 1 (more products than reactants relative to equilibrium), the log term is positive. This makes the correction term negative, resulting in E < E° (voltage decreases). The forward reaction becomes less favorable.
• If Q = 1 (standard conditions), the log term is zero, and E = E°.
• If Q = K (the equilibrium constant), the log term reflects the equilibrium state, and E = 0 (the cell is "dead").

What does spontaneity mean for cell potential?

The sign of the cell potential (E) under the given conditions directly relates to the spontaneity (thermodynamic favorability) of the redox reaction:
• E > 0 (Positive): The reaction is spontaneous as written (favored to proceed forward). This corresponds to a negative Gibbs Free Energy (ΔG = -nFE).
• E < 0 (Negative): The reaction is non-spontaneous as written. The reverse reaction is spontaneous. This corresponds to a positive ΔG.
• E = 0: The reaction is at equilibrium. ΔG = 0.