Chapter 4 · Chemistry
Zinc Fizzes, Copper Stays Silent: Electrode Potentials of the 3d Metals
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Copper is the only 3d metal with a positive E°(M²⁺/M) (+0.34 V), so it gives no H₂ with dilute acids. Turning Cu(s) into Cu²⁺(aq) costs more (atomisation + two ionisations) than hydration pays back: 921 against zinc's 711 kJ/mol. Stable d⁵ and d¹⁰ bend the trend: Mn and Zn dip, Mn³⁺ is a strong oxidant, Fe³⁺ is stable.
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Key points
- More negative E°(M²⁺/M) = the metal forms M²⁺(aq) more easily; metals below 0 V give H₂ with dilute acids.
- Copper alone is positive (+0.34 V): no H₂ from dilute acids, though oxidising acids (nitric, hot conc. sulphuric) dissolve it.
- Energy bill M(s) → M²⁺(aq) = atomisation + IE₁ + IE₂ + hydration: Zn 711, Cu 921 kJ/mol (NCERT Table 4.4).
- Stable d⁵ / d¹⁰: Mn and Zn are more negative than the trend; Mn³⁺ (→ d⁵) is a strong oxidant (+1.57 V); Fe³⁺ is already d⁵ (+0.77 V).
E°(M²⁺/M) across the 3d series
| Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn |
|---|---|---|---|---|---|---|---|---|
| −1.63 | −1.18 | −0.90 | −1.18 | −0.44 | −0.28 | −0.25 | +0.34 | −0.76 |
The energy bill (kJ/mol, NCERT Table 4.4)
| Atomisation | IE₁ | IE₂ | Hydration | Net | |
|---|---|---|---|---|---|
| Zn | 130 | 906 | 1734 | −2059 | 711 |
| Cu | 339 | 745 | 1958 | −2121 | 921 |
Copper's costs (atomisation and the second ionisation) are much higher, and its hydration enthalpy cannot make up for them, so E°(Cu²⁺/Cu) is positive. Across the series the net bill follows the E° order (Mn 643, Zn 711, Fe 743, Ni 798, Cu 921).
Stable d⁵ and d¹⁰
- Mn²⁺ (d⁵) and Zn²⁺ (d¹⁰) are especially stable: E° of Mn and Zn are more negative than the trend (Ni too, because of its high hydration enthalpy).
- E°(Mn³⁺/Mn²⁺) = +1.57 V: Mn³⁺ gains a stable d⁵ by taking one electron, so it is a strong oxidising agent.
- E°(Fe³⁺/Fe²⁺) = +0.77 V, comparatively low: Fe³⁺ is already d⁵.
- Co³⁺/Co²⁺ = +1.97 V: Co³⁺ is also strongly oxidising.
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