What is the Nernst equation?

The Nernst equation relates the cell potential at non-standard conditions to the standard cell potential: E = E° − (RT/nF)·ln(Q). Here R = 8.314 J/mol·K, T is temperature in Kelvin, n is the number of electrons transferred, F = 96485 C/mol (Faraday's constant), and Q is the reaction quotient.

What is Q (the reaction quotient)?

Q is the ratio of product activities (concentrations raised to stoichiometric powers) to reactant activities at the current moment. For dilute solutions, activity ≈ molar concentration. Q = K at equilibrium.

Why is E = 0 significant?

When E = 0, the cell is at equilibrium (Q = K). No net reaction occurs and no useful electrical work can be extracted. This is the discharged state of a battery.

What is the simplified 25 °C form?

At 298.15 K, RT/F ≈ 0.025693 V. Converting ln to log₁₀ (multiply by 2.3026) gives (0.05916/n)·log₁₀(Q). This is the common textbook simplification valid only at 25 °C.

What units should I use for Q?

Q is dimensionless. For species in solution, use molar concentration (mol/L) as the activity approximation. For gases, use partial pressure in atmospheres (or bar). Pure solids and liquids have activity = 1 and are omitted.

What does a negative cell potential mean?

A negative E means the reaction is non-spontaneous in the forward direction under the given conditions. The reverse reaction would be spontaneous. Increasing Q (more products) drives E negative.

How do I find the standard cell potential E°?

E°cell = E°cathode − E°anode, where both values come from standard reduction potential tables. The cathode is the half-cell where reduction occurs (higher reduction potential); the anode is where oxidation occurs.

How do you use the Nernst equation for potassium (equilibrium potential)?

For an ion at biological temperature (37 °C), use E_K = (61.54 mV / z) · log₁₀([K⁺]_out / [K⁺]_in) with z = +1. With [K⁺]_out = 5 mM and [K⁺]_in = 140 mM, E_K ≈ −89 mV.

Nernst Equation Calculator

Name: Nernst Equation Calculator
Availability: InStock
Author: Nham Vu

Calculate cell potential at non-standard conditions using the Nernst equation E = E° − (RT/nF)·ln(Q). 25 °C simplified form included.

Inputs

E = E° − (RT/nF)·ln(Q)

Standard Cell Potential (E°)

Temperature (T)

Electrons Transferred (n)

mol e⁻

Reaction Quotient (Q)

dimensionless

Enter values on the left and click Calculate to see the result.

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Summary

Calculate cell potential at non-standard conditions using the Nernst equation E = E° − (RT/nF)·ln(Q). 25 °C simplified form included.

How it works

Enter the standard cell potential (E°) in volts — look this up from standard reduction potential tables.
Enter the temperature in Kelvin (K) or switch to Celsius; the tool converts to Kelvin automatically.
Enter the number of electrons transferred (n) in the balanced half-reactions.
Enter the reaction quotient (Q) as a dimensionless number — the ratio of product activities to reactant activities.
Click Calculate; the tool applies E = E° − (RT/nF)·ln(Q) and shows the actual cell potential.
Review the result panel: cell potential, spontaneity verdict, equation breakdown, and the 25 °C simplified form.

Use cases

Calculate the actual voltage of a galvanic cell when ion concentrations differ from 1 M.
Determine whether an electrochemical reaction is spontaneous under given conditions.
Solve electrochemistry problems in general chemistry and physical chemistry courses.
Verify hand-calculated Nernst equation results for lab reports or homework.
Find the equilibrium condition where cell potential equals zero (Q = K_eq).
Analyze concentration cells where E° = 0 but a potential arises from concentration differences.
Predict battery voltage changes as reactants are consumed and Q increases.
Prepare for AP Chemistry, university electrochemistry, or graduate-level physical chemistry exams.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu