Grade 12

Iron, copper, chromium, zinc and transition metals

Transition-metal chemistry links electron configurations to variable oxidation states, coloured ions, complexes and useful redox reactions. Compare iron, copper, chromium and zinc through characteristic reactions, extraction, corrosion and analysis.

IntuitionA family of changing colours

Many d-block metals can lose different numbers of electrons or bind ligands, so one element may form ions with different charges and colours. These properties make them useful in alloys, catalysts, electroplating and chemical tests.

Select chromium, iron or copper in the scheme. It shows the listed species and their observed colours, with arrows labelled by reagents or conditions that interconvert them; it is a reaction map, not a molecular-scale simulation.

Schoold-block metals: electron structure and properties

The 3d-series atoms have configurations [Ar]3d⁶4s² (Fe), [Ar]3d¹⁰4s¹ (Cu) and [Ar]3d⁵4s¹ (Cr; both Cu and Cr are configuration exceptions). Zn is [Ar]3d¹⁰4s²; Zn²⁺ is 3d¹⁰, so zinc is a d-block element but is not a transition element under the usual definition requiring an incomplete d subshell in an atom or common ion.

Acid shifts chromate (yellow) toward dichromate (orange); base shifts back. Chromium(VI) is a strong oxidant in acid: $E^\circ(\ce{Cr2O7^2-/Cr^3+})=+1.33\,\mathrm{V}$. Chromium(III) hydroxide is amphoteric: it dissolves in excess strong acid and in excess hydroxide to form hydroxo complexes.

In the blast furnace, coke forms carbon monoxide, which reduces hematite. Pig iron contains substantial carbon and impurities; steel is iron with a controlled, usually lower carbon content and selected alloying elements, giving tailored strength and toughness.

Hydroxide precipitates help distinguish the ions: Fe(OH)₂ is pale and rapidly browns in air as it oxidises; Fe(OH)₃ is red-brown. Fe²⁺ also gives Turnbull’s blue with ferricyanide, K₃[Fe(CN)₆], while Fe³⁺ gives a blood-red thiocyanate complex with KSCN.

Rusting is an electrochemical process requiring water and oxygen; the hydrated iron(III) oxides formed are porous and do not protect the underlying metal. Coatings, alloying and galvanic protection slow corrosion.

Copper is refined electrolytically: impure Cu dissolves at the anode and high-purity copper deposits at the cathode; less noble impurities enter solution while noble-metal residues can form anode slime. In limited NH₃, Cu²⁺ first forms light-blue Cu(OH)₂; excess NH₃ yields deep-blue $\ce{[Cu(NH3)4]^2+}$.

Concentrated nitric acid typically produces brown NO₂; dilute nitric acid gives NO, which becomes NO₂ on contact with air. In Fehling’s test, an aldehyde such as glucose reduces blue Cu(II) in alkaline solution to brick-red Cu₂O on warming. Brass is mainly Cu–Zn; bronze is mainly Cu–Sn.

Zinc reacts with non-oxidising acids to release hydrogen. Zn(OH)₂ is amphoteric, dissolving in excess acid or hydroxide. A zinc coating galvanises iron; if the coating is scratched, zinc can still act as a sacrificial anode and oxidise preferentially.

UndergraduateQuantitative links: colour and redox

In an approximately octahedral ligand field, the five d orbitals split into lower $t_{2g}$ and higher $e_g$ levels, separated by $\Delta_o$. Absorption of visible light promotes a d electron across this gap (a d–d transition); the transmitted/reflected complementary light gives the observed colour. The gap depends on metal, oxidation state, geometry and ligands; d⁰ and d¹⁰ ions have no d–d transition of this type.

For the acidic dichromate reduction, the reaction quotient contains $[\ce{H+}]^8$ in the numerator. At 25 °C, $E=E^\circ-(0.05916/6)\log Q$; increasing pH lowers $[\ce{H+}]$, raises Q and lowers the reduction potential. Acid therefore strengthens dichromate’s oxidising drive.

A compact Latimer summary for iron in acidic solution is $\ce{FeO4^2- ->[+2.20 V] Fe^3+ ->[+0.77 V] Fe^2+ ->[-0.44 V] Fe}$. Values are standard reduction potentials for the adjacent couples under stated standard conditions; the diagram helps compare oxidising tendencies but is not a list of isolated species stable at every pH.