Read the Nernst Equation Before Any Non-Standard Cell Problem
Any cell that isn't at standard 1 M concentrations needs the Nernst equation, E = E° - (RT/nF) ln Q — everything else in electrochemistry (spontaneity, EMF ranking, pH-dependence) is this same equation applied to a different Q.
Electrode Potentials & the Nernst Equation is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Building an EMF greater than the standard cell
To increase a Zn/Cu galvanic cell's EMF above standard, decrease the reaction quotient Q — that means lowering the product-side ion concentration (Cu²⁺ here, since it's consumed) or raising the reactant-side concentration (Zn₂⁺), whichever direction the Nernst equation shows pushes E further positive.
Half-cell potentials that depend on pH
When a half-reaction includes H⁺ explicitly, its potential shifts with pH through the Nernst equation's Q term — raising pH by a fixed number of units changes [H⁺] by a fixed power of ten, and the resulting potential shift scales with how many H⁺ appear in that specific half-reaction's balanced equation.
Combining half-reactions and Latimer diagrams
Standard reduction potentials don't average directly when combining half-reactions of different electron counts — convert each to ΔG° (ΔG° = -nFE°), add the ΔG° values algebraically, then convert the combined ΔG° back to E° for the new overall n. This is exactly what a Latimer diagram's unlabelled potential represents.
OlympIQ exam lens
- ✓Any non-standard-concentration cell needs the Nernst equation — no exceptions.
- ✓To raise EMF above standard, decrease Q: lower the concentration of whichever species is a product in the cell reaction.
- ✓A half-reaction's potential shifts with pH only if H⁺ (or OH⁻) appears explicitly in that half-reaction.
- ✓Combine half-cell potentials via ΔG° = -nFE°, not by direct averaging or addition of E° values.
Don't add or average standard reduction potentials directly when combining half-reactions with different numbers of electrons transferred — convert to ΔG° first, since ΔG° (not E°) is additive.
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