In acid solution the Latimer diagram for copper is Cu²⁺ → Cu⁺ (+0.15 V) → Cu (+0.52 V). What happens to Cu⁺ in water under standard conditions?
AIt is stable because both potentials are positive
BIt disproportionates into Cu²⁺ and Cu
CIt is oxidized by water to Cu²⁺ only
DIt comproportionates with Cu²⁺
A species disproportionates when the potential on its right is more positive than the potential on its left. Here 0.52 V > 0.15 V, so 2Cu⁺ → Cu²⁺ + Cu has E° = 0.52 − 0.15 = +0.37 V and is spontaneous.
Question 2 True / False
The standard potential for Fe³⁺ → Fe can be found by adding the potentials for Fe³⁺ → Fe²⁺ and Fe²⁺ → Fe.
TTrue
FFalse
Answer: False
Potentials are intensive and do not add. You add the free energies (−nFE°) and divide by the total number of electrons. Adding the potentials directly gives the wrong answer.
Question 3 Short Answer
Given Fe³⁺/Fe²⁺ = +0.77 V and Fe²⁺/Fe = −0.44 V, calculate E° for Fe³⁺ + 3e⁻ → Fe.
Think about your answer, then reveal below.
Model answer: About −0.04 V
E° = (1 × 0.77 + 2 × (−0.44)) / 3 = (0.77 − 0.88) / 3 ≈ −0.04 V. The electron counts weight each step because ΔG° = −nFE° is the additive quantity.
Question 4 Multiple Choice
On a Frost diagram, a species whose point lies above the straight line joining its two neighboring oxidation states is expected to:
ABe the most stable state of the element
BDisproportionate into those neighbors
CForm by comproportionation of those neighbors
DBe unaffected by pH
If a point lies above the line, the average free energy of the two neighbors is lower than that of the species itself, so splitting into the neighbors is favored. A point below the line means the neighbors would comproportionate to form it.