Standard electrode potentials and chemical reactivity
E°(M²⁺/M) becomes less negative across the 3d series, and copper's is positive. Stable d⁰, d³, d⁵ and d¹⁰ ions explain the odd values.
Energy steps for M(s) → M²⁺(aq)
The smaller the net energy needed, the more negative E°(M²⁺/M) and the more easily the metal forms M²⁺ ions.
E°(M²⁺/M) values and what they mean
| Metal | E° (V) | Comment and reason |
|---|---|---|
| Ti, V, Cr | −1.63, −1.18, −0.90 | Become less negative: the sum of the first and second ionisation enthalpies generally rises across the series |
| Mn | −1.18 | More negative than the trend: Mn²⁺ is the stable half-filled d⁵ |
| Fe, Co | −0.44, −0.28 | Continue the general trend |
| Ni | −0.25 | More negative than expected for its position: Ni²⁺ has the most negative hydration enthalpy |
| Cu | +0.34 | Only positive value: high enthalpy of atomisation and high ionisation enthalpies are not balanced by its hydration enthalpy |
| Zn | −0.76 | More negative than expected: Zn²⁺ is the stable full d¹⁰ |
E°(M³⁺/M²⁺) values and what they mean
| Couple | E° (V) | Meaning |
|---|---|---|
| Cr³⁺/Cr²⁺ | −0.41 | Cr²⁺ is easily oxidised to Cr³⁺ (stable d³, half-filled t₂g level): Cr²⁺ is a strong reducing agent |
| V³⁺/V²⁺ | −0.26 | Fairly low, linked to the stability of V²⁺ (half-filled t₂g level) |
| Mn³⁺/Mn²⁺ | +1.57 | Very high: Mn²⁺ (d⁵) is very stable, so Mn³⁺ is a strong oxidising agent |
| Fe³⁺/Fe²⁺ | +0.77 | Comparatively low: Fe³⁺ is also d⁵, so it is quite stable |
| Co³⁺/Co²⁺ | +1.97 | Very high: Co³⁺ is a strong oxidising agent in water |
| Sc | very low | Sc³⁺ has the stable noble-gas configuration, so Sc exists almost only as Sc³⁺ |
Cr²⁺ and Mn³⁺: both d⁴, opposite behaviour
Chemical reactivity
Transition metals vary widely in reactivity. Most first series metals, except copper, are fairly electropositive and dissolve in mineral acids. Titanium and vanadium, though, hardly react with dilute non-oxidising acids at room temperature (they are passive), because a thin oxide layer protects them. The E°(M²⁺/M) values show a decreasing tendency to form M²⁺ across the series. Copper cannot give H₂ with dilute non-oxidising acids; it reacts only with oxidising acids such as nitric acid and hot concentrated sulphuric acid. Ti²⁺, V²⁺ and Cr²⁺ are strong reducing agents and give hydrogen with dilute acid: 2Cr²⁺(aq) + 2H⁺(aq) → 2Cr³⁺(aq) + H₂(g).
Copper pots do not give hydrogen with sour foods like tamarind because E°(Cu²⁺/Cu) is positive, but air can still slowly dissolve copper in acid, so they are tin-coated (kalai). Iron kadais, with a negative E°, add a little iron to food.
2 marks: “Why is E°(Cu²⁺/Cu) positive, unlike the other 3d metals? What does it tell you about copper and dilute HCl?” Copper's high enthalpy of atomisation and ionisation enthalpies are not balanced by the hydration enthalpy of Cu²⁺. So Cu cannot reduce H⁺: it does not give H₂ with dilute HCl.
3 marks: “Explain why E°(Mn³⁺/Mn²⁺) is highly positive, E°(Fe³⁺/Fe²⁺) is comparatively low, and E°(Cr³⁺/Cr²⁺) is negative.” Mn²⁺ is the stable d⁵, so Mn³⁺ is easily reduced. Fe³⁺ is also d⁵, so it resists reduction. Cr³⁺ (d³, half-filled t₂g) is stable, so Cr²⁺ is easily oxidised.
2 marks: “Cr²⁺ is reducing and Mn³⁺ is oxidising, though both are d⁴. Explain.” Cr²⁺ → Cr³⁺ gives d³ (half-filled t₂g, stable). Mn³⁺ → Mn²⁺ gives d⁵ (half-filled, stable). Each moves to its own stable configuration.
Copper is the odd plus one. Ions rush towards d³, d⁵ or d¹⁰: Cr²⁺ gives, Mn³⁺ takes.