Why does copper have a positive standard reduction potential (E°(Cu2+/Cu) = +0.34 V), whereas all other first-row (3d) transition metals possess negative standard reduction potentials?

Answer and explanation

Correct answer: C. High atomization and ionization enthalpies of copper are not compensated by its hydration enthalpy

  1. Step 1: The standard electrode potential is governed by three thermodynamic parameters: enthalpy of atomization (sublimation, ), sum of ionization enthalpies (), and hydration enthalpy of the gaseous cation ().
  2. Step 2: For copper, the enthalpy of sublimation is unusually high, and the sum of its first two ionization enthalpies is large.
  3. Step 3: The hydration enthalpy released by is insufficient to balance this high energy input.
  4. Step 4: Consequently, the overall for is positive, making positive, which explains why copper does not release from dilute non-oxidizing acids.
Answer
The high sum of enthalpy of atomization and first and second ionization enthalpies of copper is not compensated by its hydration enthalpy
Why not the others: 'Copper cannot form stable coordination complexes with water...' is false because is a stable, well-characterized complex; 'Copper has a completely filled d-subshell that prevents reduction...' is incorrect since has a configuration; 'Copper reacts violently with water...' is factually opposite, as copper is noble and unreactive with water.

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