Standard electrode potentials and cell EMF (HL)
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Flip to reveal answersWhat is the standard hydrogen electrode (SHE)?
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Question
What is the standard hydrogen electrode (SHE)?
Answer
The reference half-cell, assigned **E° = 0.00 V**, against which all other standard electrode potentials are measured.
Question
What is a standard electrode potential, E°?
Answer
The voltage of a half-cell measured against the **SHE** under **standard conditions** (298 K, 1 mol dm⁻³, 100 kPa).
Question
What are standard conditions for E°?
Answer
**298 K**, solution concentrations **1 mol dm⁻³**, and gas pressures **100 kPa**.
Question
How are E° half-equations always written?
Answer
As **reductions** — electrons on the left (e.g. Cu²⁺ + 2e⁻ ⇌ Cu).
Question
What does a more POSITIVE E° tell you?
Answer
The species is reduced more readily — it is a **stronger oxidising agent**.
Question
What does a more NEGATIVE E° tell you?
Answer
The species is oxidised more readily — it is a **stronger reducing agent**.
Question
Equation for the standard cell potential?
Answer
$E^{\circ}_{cell} = E^{\circ}_{cathode} - E^{\circ}_{anode}$ — given in the data booklet.
Question
Which half-cell is the cathode?
Answer
The one with the **more positive E°** — it is reduced.
Question
What does a POSITIVE E°cell mean?
Answer
The cell reaction is **spontaneous (feasible)** as written (and ΔG° is negative).
Question
What does a NEGATIVE E°cell mean?
Answer
The reaction is **not spontaneous** in that direction (ΔG° is positive).
Question
Common E°cell sign trap?
Answer
Forgetting that −(negative) **adds**: E°cell = E°(cathode) − E°(anode), so put brackets around each value.
Question
Link between E°cell and Gibbs energy?
Answer
$\Delta G^{\circ} = -nFE^{\circ}_{cell}$ — a positive E°cell makes ΔG° negative, the condition for spontaneity (F = 96 500 C mol⁻¹).
Question
In a spontaneous cell, which is the oxidising agent?
Answer
The species **reduced at the cathode** (it gains electrons); the species oxidised at the anode is the reducing agent.
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Topic 6.2 hub
Electron transfer reactions
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