Flashcards · 9620 · A Level Topic 9

Electrode potentials and electrochemical cells (A Level) flashcards

13 cards on A Level Topic 9, subtopics 3.1.9.1 to 3.1.9.2. Pick a subtopic or study them all.

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Standard electrode potential, E⦵: The EMF of a half-cell measured against the SHE at 298 K, 100 kPa and 1.00 mol dm⁻³ solutions of ions.
Standard hydrogen electrode (SHE): The reference half-cell, defined as 0.00 V.
Salt bridge: Filter paper soaked in an unreactive salt solution (such as KNO₃). Ions move through it; electrons do not.
Make-up of the standard hydrogen electrode: Platinum electrode, H₂ gas at 100 kPa, solution with [H⁺] = 1.00 mol dm⁻³, 298 K. 2H⁺(aq) + 2e⁻ ⇌ H₂(g)
Cell EMF from electrode potentials: EMF = E⦵(right) − E⦵(left) = E⦵(positive electrode) − E⦵(negative electrode). The left-hand (negative) electrode is where oxidation happens.
Conventional representation of a Cd–Au cell, and its EMF (Cd²⁺/Cd −0.40 V, Au³⁺/Au +1.52 V): Cd(s) | Cd²⁺(aq) || Au³⁺(aq) | Au(s) EMF = 1.52 − (−0.40) = +1.92 V
When scaling a half-equation to combine it, what happens to its E⦵?: Nothing. Scale the half-equation, never the E⦵ value.
What can E⦵ values not tell you about a reaction?: Its rate (the activation energy may be too high), or the outcome far from standard conditions.

Non-rechargeable cell, rechargeable cell and fuel cell: Non-rechargeable: reactions cannot be reversed in practice. Rechargeable: an external supply reverses the electrode reactions. Fuel cell: reactants supplied continuously; not recharged electrically.
Electrode reactions in the alkaline hydrogen–oxygen fuel cell: Negative: H₂ + 2OH⁻ → 2H₂O + 2e⁻ Positive: O₂ + 2H₂O + 4e⁻ → 4OH⁻ Overall: 2H₂ + O₂ → 2H₂O
Electrode reactions in the lithium cell during discharge: Positive: Li⁺ + CoO₂ + e⁻ → Li⁺[CoO₂]⁻ (cobalt reduced, +4 → +3). Negative: Li → Li⁺ + e⁻. Charging reverses both reactions.
Benefits and risks of hydrogen fuel cells: Benefit: water is the only product where they run; no CO₂ there. Risks: making hydrogen uses energy, possibly from fossil fuels; hydrogen–air mixtures are explosive; hydrogen is bulky to store.
EMF: The potential difference of a cell when no current flows.