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Using the following standard reduction potentials: E°(Sc³+/Sc) = −0.37, E°(Mn³+/Mn²+) = +1.57, E°(Cr²+/Cr) = −0.90, E°(Cu²+/Cu) = +0.34, which statement is not correct?

1. Sc³+ is highly stable because it has the [Ar]3d⁰ 4s⁰ arrangement.
2. Mn³+ is more stable than Mn²+.
3. Cr²+ acts as a reducing agent.
4. Copper does not liberate H2 when treated with dilute H2SO4.

Correct answer: Mn³+ is more stable than Mn²+.

Solution

E(Mn3+/Mn2+) = +1.57 V is large and positive, so Mn3+ is readily reduced to Mn2+; the half-filled d5 Mn2+ is the more stable species. Hence the statement that Mn3+ is more stable than Mn2+ is the incorrect one.

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