Short on time? This page is the recap. For explanations, the E against ln Q graph and worked examples, use the full study guide.
Recap
- E° is the cell potential at standard conditions, where Q = 1. A real cell usually has Q ≠ 1, so its potential E differs from E°.
- The cell potential is a driving force towards equilibrium. The further from equilibrium, the larger the size of E.
- As a galvanic cell runs, Q rises towards K, so E falls. At Q = K, E = 0: the cell is “dead” (at equilibrium, not necessarily empty).
- A concentration cell has the same half-reaction on both sides, so E° = 0. It still gives a voltage while the concentrations differ. The dilute side is the anode; electrons flow from dilute to concentrated until the concentrations are equal.
- Do not use Le Châtelier’s principle: a working cell is not at equilibrium.
Key relationships
| Relationship | What it tells you |
|---|---|
| E = E° − (RT/nF) ln Q | Q < 1 → E > E°; Q > 1 → E < E° |
| E = 0 when Q = K | equilibrium; no net current |
| E° = (RT/nF) ln K | positive E° ⇔ K > 1 |
| RT/F = 0.0257 V at 298 K | a tenfold change in Q shifts E by only a few hundredths of a volt |
Assumptions behind the reasoning
- Temperature is constant (usually 298 K), so E° and K are fixed.
- Pure solids and liquids are left out of Q.
- The cell starts on the reactant side of equilibrium (Q < K), as every working galvanic cell does.
Mistakes to avoid
- Le Châtelier language (“shifts right”). Say instead: “Q decreases, so the cell is further from equilibrium, so E increases.”
- Changing E°. Concentrations change E, never E°.
- E = 0 means no reactant left. It means Q = K.
- Concentration cell = no voltage. E° = 0, but E ≠ 0 while Q ≠ 1.
- Diluting both half-cells changes nothing. True only if Q stays the same; check the coefficients.
- Calculation without explanation. Explain the change in Q first; a Nernst number only supports it.
Quick self-check
- A cell with E° = +0.50 V has Q = 50. Is E above or below 0.50 V? (Below: Q > 1, so the cell is closer to equilibrium.)
- A galvanic cell’s voltmeter reads 0.00 V. What is true about Q? (Q = K; the cell is at equilibrium.)
- A concentration cell has Ni in 0.10 M Ni²⁺ and Ni in 1.0 M Ni²⁺. Which electrode is the anode, and roughly what is E? (The one in 0.10 M; E = (0.0257/2) ln 10 ≈ 0.030 V.)
Next: practice questions.