Explain the trend in E°(M²⁺/M) values across the 3d series, and give reasons for the values of Mn, Ni, Cu and Zn.

1. 1 · Name the energy terms. E° depends on atomisation enthalpy, the first and second ionisation enthalpies and the hydration enthalpy.
2. 2 · Give the trend. E° becomes less negative across the series, as the sum of ionisation enthalpies rises.
3. 3 · Explain the exceptions.
   • Mn and Zn: more negative, because Mn²⁺ (d⁵) and Zn²⁺ (d¹⁰) are stable.
   • Ni: more negative, because Ni²⁺ has the most negative hydration enthalpy.
   • Cu: positive (+0.34 V), because high atomisation and ionisation enthalpies are not balanced by hydration.
4. 4 · State the result. Copper does not give H₂ with dilute non-oxidising acids; it needs oxidising acids such as HNO₃.
5. 5 · Close in one line. So stable d⁵ and d¹⁰ ions and hydration enthalpy explain the irregular values.

Scoring tip: Name the configuration (d⁵, d¹⁰) for every exception; the examiner looks for it.