Last updated: June 2026
Use this Cell Potential / EMF Calculator to calculate standard cell potential, Gibbs free energy change and non-standard cell potential using the Nernst equation. It is designed for A-Level and Singapore H2 Chemistry students studying electrochemical cells.
What is cell potential?
Cell potential, also called cell EMF, is the potential difference between the two half-cells of an electrochemical cell. It measures the thermodynamic driving force for electron transfer through the external circuit.
Reduction occurs at the cathode, while oxidation occurs at the anode. However, standard electrode potential tables normally list every half-equation as a reduction. This is why both tabulated values are used as reduction potentials in the subtraction below.
- Choose Standard E⦵cell when both half-cells are under standard conditions.
- Enter the cathode and anode values exactly as they appear in the reduction-potential table.
- Enter n only when you also want ΔG⦵.
- Choose Nernst equation for non-standard conditions, then enter E⦵cell, n, Q and temperature.
- Read the numbered working carefully so that you can reproduce the method in an examination.
Calculating standard cell potential
Standard conditions are usually 298 K, dissolved species at 1 mol dm−3, and gases at the standard pressure specified by the data source. Solids and pure liquids are present in their standard states.
What does the sign of Ecell mean?
| Calculated value | Thermodynamic interpretation |
|---|---|
| Ecell > 0 | The reaction is thermodynamically feasible as written under the stated conditions. |
| Ecell = 0 | The electrochemical system is at equilibrium under the stated conditions. |
| Ecell < 0 | The reaction is not thermodynamically feasible as written under the stated conditions. |
A positive cell potential does not prove that the reaction will be fast. Electrode kinetics, activation energy and surface effects can still make a thermodynamically feasible reaction proceed slowly.
Cell potential and Gibbs free energy
The electrical and thermodynamic descriptions are connected by:
Here, n is the number of moles of electrons transferred in the balanced overall equation and F is the Faraday constant, approximately 96 485 C mol−1. A positive Ecell therefore gives a negative ΔG.
Using the Nernst equation
When concentrations, gas pressures or temperature are not at standard conditions, the reaction quotient Q changes the cell potential. The general Nernst equation is:
At 298 K, the base-10 logarithm form is often written as:
Worked example: the Daniell cell
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
E⦵(Cu2+/Cu) = +0.34 V and E⦵(Zn2+/Zn) = −0.76 V. Copper is reduced at the cathode and zinc is oxidised at the anode.
Since E⦵cell is positive, the overall Daniell-cell reaction is thermodynamically feasible under standard conditions.
Common examination traps
Frequently asked questions
Should I enter an oxidation potential for the anode?
No. Enter the tabulated reduction potential for the anode half-cell. The calculator then applies E⦵cathode − E⦵anode.
Why is the more positive reduction potential usually the cathode?
A more positive reduction potential means a greater thermodynamic tendency to gain electrons. In a feasible galvanic cell, that half-reaction therefore occurs as reduction at the cathode.
Why are standard electrode potentials not multiplied?
Potential difference does not depend on the amount of material reacting. Multiplying a half-equation changes the amount of electrons transferred, but it does not change that half-cell potential.
When should I use the Nernst equation?
Use it when the cell is not under standard conditions and the information allows you to determine Q, temperature and the number of electrons transferred.
Can Ecell be positive while the reaction appears not to occur?
Yes. A positive value describes thermodynamic feasibility. The reaction may still be too slow to observe because of a large activation energy or sluggish electrode kinetics.