“Red Cat, An Ox”
What happens at each electrode
REDuction at the CAThode, ANode is OXidation. True in both galvanic and electrolytic cells, even though the signs of the electrodes swap.
Chemistry, Class 11 and 12 · Gujarat Board, CBSE, GUJCET, JEE and NEET
Electrochemistry formulas
Electrochemistry joins redox to energy. The Nernst equation and the link between cell potential and Gibbs energy carry the numericals, and conductance supplies the rest. Sign conventions decide most of the marks.
16 formulas, 3 memory hooks, and the 5 mistakes that cost the most marks.
Cells and potentials
| What | Formula | Symbols and notes |
|---|---|---|
| Cell potential | E°cell = E°cathode − E°anode | both as reduction potentials |
| Galvanic cell | spontaneous, E°cell positive | anode is negative, cathode is positive |
| Electrolytic cell | non-spontaneous, needs an external supply | anode is positive, cathode is negative |
| Nernst equation | E = E° − (0.0591/n)log([products]/[reactants]) | at 298 K |
| At equilibrium | E = 0 and E° = (0.0591/n)log K | which links potential to the equilibrium constant |
| Gibbs energy | ΔG° = −nFE°cell | F = 96500 C mol−1 |
Conductance
| What | Formula | Symbols and notes |
|---|---|---|
| Conductance | G = 1/R | unit siemens |
| Conductivity | κ = G × (l/A) | l/A is the cell constant, unit m−1 |
| Molar conductivity | Λm = κ × 1000/C | C in mol per litre |
| On dilution | Λm rises, κ falls | more ions are free but they are further apart |
| Kohlrausch's law | Λ°m = λ°+ + λ°− | limiting molar conductivity adds from the ions |
| Degree of dissociation | α = Λm/Λ°m | for a weak electrolyte |
Electrolysis
| What | Formula | Symbols and notes |
|---|---|---|
| Faraday's first law | m = ZIt | Z is the electrochemical equivalent |
| In terms of equivalents | m = (E × I × t)/96500 | E is the equivalent weight |
| Faraday's second law | m₁/m₂ = E₁/E₂ | for the same charge passed |
| One faraday | 96500 C deposits one gram equivalent | one mole of electrons |
Taught by Darshit Bhatt, founder and lead faculty at Darshit Sir Institute. Instrumentation and Control engineer, with a master's degree and a chartered degree, teaching in Paldi, Ahmedabad since 2006.
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Memory hooks
How to remember this chapter.
“Cathode minus anode”
Calculating cell potential
E°cell = E°cathode − E°anode, with both written as reduction potentials. Never reverse a sign and subtract as well; do one or the other.
“0.0591 over n”
The Nernst equation at room temperature
At 298 K the factor 2.303RT/F is 0.0591 volts. Memorise the number and the equation becomes one line.
Where marks go
The mistakes that cost the most.
- Reversing the sign of the anode potential and then also subtracting it. Do one, not both.
- Assuming the electrode signs are the same in galvanic and electrolytic cells. The chemistry is the same; the signs are opposite.
- Using concentration instead of the correct ratio in the Nernst equation; products over reactants, each to its power.
- Thinking conductivity rises on dilution. Molar conductivity rises, conductivity falls.
- Forgetting that n in ΔG° = −nFE° is the number of electrons in the balanced cell reaction.
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