Redox Calculator
Electricity from a Difference in Willingness to Grab Electrons
Every element has a measured tendency to either gain or give up electrons, expressed as a standard reduction potential. Pair two half-reactions with different potentials and electrons flow spontaneously from one to the other — that flow is the current in a battery. The size of the voltage is just the gap between the two potentials.
The Formula
Both values are standard reduction potentials, in volts, referenced to the standard hydrogen electrode. A positive E(cell) means the reaction is spontaneous as written (galvanic); a negative value means it needs an external power source to run (electrolytic).
Where This Calculation Matters
- Battery design — choosing electrode materials with a large potential gap maximizes cell voltage.
- Corrosion prediction — two dissimilar metals in contact form a galvanic cell, and the potential difference predicts which one corrodes.
- Electroplating — plating processes rely on driving a normally non-spontaneous reduction using an external voltage.
- Analytical electrochemistry — predicting whether a redox titration or sensor reaction will proceed as intended.
Standard Cell Voltages from Published Reduction Potentials
| Cathode (reduction) | Anode (reduction potential shown) | E(cell) |
|---|---|---|
| Cu2+/Cu (+0.34 V) | Zn2+/Zn (−0.76 V) | 1.10 V |
| Ag+/Ag (+0.80 V) | Cu2+/Cu (+0.34 V) | 0.46 V |
| Cu2+/Cu (+0.34 V) | Fe2+/Fe (−0.44 V) | 0.78 V |
The first row is the classic zinc-copper (Daniell) cell, historically one of the earliest practical batteries, with a well-documented standard voltage of 1.10 V.
How to Use This Calculator
- Enter the Standard Reduction Potential – Cathode (V) for the half-reaction being reduced.
- Enter the Standard Reduction Potential – Anode (V) for the half-reaction being oxidized.
- Select Calculate to get the overall cell potential and whether the reaction is spontaneous.
Related Calculations
Once a redox reaction's stoichiometry is set, the Chemical Equation Balancer can confirm the overall equation balances. For converting between moles of electrons and mass transferred, see the Moles Calculator.