Electrochemical Series Comparator
Pick any two half-reactions to see which is auto-assigned cathode or anode, their rank in the full series, and the resulting E°cell.
📊 What is the Electrochemical Series Comparator?
The Electrochemical Series Comparator lets you pick any two half-reactions and automatically works out which one is reduced at the cathode, which is oxidized at the anode, the resulting standard cell potential, and exactly where both sit within the full electrochemical (reactivity) series, without you needing to already know which one has the higher potential.
This calculator is useful for students learning to read and use a reactivity series table, anyone predicting whether one metal will displace another from solution, chemistry teachers building quick comparison examples, and hobbyists designing simple galvanic cells who want to know how far apart two metals sit in reactivity before picking an electrode pair.
A common source of confusion when first learning electrochemistry is having to manually figure out which of two half-reactions is the cathode before you can even start the E°cell calculation. This comparator removes that step, select any two species in any order and it tells you the correct cathode/anode assignment automatically, based purely on which has the higher standard reduction potential.
Beyond the E°cell result, this comparator also shows both selections' rank within the full series and how many other half-reactions sit between them, giving a quick sense of how far apart in reactivity the two chosen species really are, information a plain E°cell calculation alone does not provide.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Zinc versus copper
Electrode A: Zn2+ + 2e- → Zn (−0.76 V, rank 7). Electrode B: Cu2+ + 2e- → Cu (0.34 V, rank 13).
Example 2 — Magnesium versus silver
Electrode A: Mg2+ + 2e- → Mg (−2.37 V, rank 5). Electrode B: Ag+ + e- → Ag (0.80 V, rank 15).
Example 3 — Lead versus chlorine
Electrode A: Pb2+ + 2e- → Pb (−0.13 V, rank 11). Electrode B: Cl2 + 2e- → 2Cl- (1.36 V, rank 17).
❓ Frequently Asked Questions
🔗 Related Calculators
What does this comparator do differently from a standard cell potential calculator?
A standard cell potential calculator usually requires you to already know which half-reaction is the cathode and which is the anode. This comparator accepts any two half-reactions in any order, automatically determines which is reduced (cathode) and which is oxidized (anode) based on their standard potentials, and shows their relative position in the full series.
How does the comparator decide which electrode is the cathode?
The half-reaction with the higher (more positive) standard reduction potential is always assigned as the cathode, since it is more readily reduced. The other half-reaction becomes the anode, its tabulated reduction potential is used but the reaction itself runs in reverse, as oxidation.
What does the 'species between them' number mean?
It counts how many other half-reactions in the 19-entry table sit strictly between your two selections when the whole table is sorted by standard reduction potential. A larger number means the two species are far apart in reactivity, a smaller number means they are close neighbors in the series.
What is the electrochemical (reactivity) series?
The electrochemical series, also called the activity or reactivity series, ranks elements and ions by their standard reduction potential from most negative (strongest reducing agents, most reactive metals) to most positive (strongest oxidizing agents). It is used to predict which metals displace others from solution and which redox reactions proceed spontaneously.
Why is E°cell always positive in this comparator's result?
Because the comparator always assigns the higher-potential half-reaction as the cathode and the lower as the anode, guaranteeing E°cell = E°higher - E°lower is never negative. This differs from calculators that let you fix roles yourself, where an unfavorable assignment can produce a negative E°cell.
Can this comparator predict whether one metal will displace another from solution?
Yes, qualitatively. If metal A's ion has a higher standard reduction potential than metal B's ion, metal B (the stronger reducing agent) will spontaneously displace metal A's ion from solution, this is the same logic behind the classic zinc-displaces-copper demonstration.
Why does the table only include 19 half-reactions?
This comparator uses a curated table of the half-reactions most commonly taught in general and introductory electrochemistry, covering the alkali and alkaline earth metals, common transition metals, hydrogen, and the halogens. It is not an exhaustive reference table, for less common species consult a full standard reduction potential table.
How is rank different from standard reduction potential itself?
Rank is simply the ordinal position (1st, 2nd, 3rd, and so on) of a half-reaction when the whole table is sorted by E°, while the potential itself is the actual voltage value. Two half-reactions can be adjacent in rank but have very different E° values, or far apart in rank with similar E° values, if the series is unevenly spaced in that region.
Does electrode surface area or concentration change the ranking shown here?
No. Standard reduction potentials, and therefore the ranking in this comparator, are intensive properties defined at standard conditions (25°C, 1 M, 1 atm) and do not depend on the amount, surface area, or geometry of the electrode, only true concentration and temperature changes (handled by the Nernst equation) shift the effective potential.
Is a bigger gap in the series always better for a battery?
Generally, a larger gap between the two chosen half-reactions produces a larger E°cell and more available voltage per cell, but real battery design also weighs cost, weight, safety, and rechargeability, a large E°cell alone does not guarantee a practical battery chemistry.
What happens if I select the same half-reaction for both Electrode A and Electrode B?
The comparator shows a validation error, since comparing a half-reaction to itself is undefined for this calculation, E°cell would be zero and there would be no meaningful cathode/anode assignment to make.