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2025 H2 Chem P2 Q3: Iron as a Transition Element Solution

Iron as a Transition Element 2025 A-Level H2 Chemistry · Paper 2 · Q3 · 11 marks

Question

Iron behaves as a typical transition element.

(a)[1]

Explain why iron is described as a transition element.

(b)[1]

Complete Fig. 3.1 to show the arrangement of electrons in \(Fe^{3+}(g)\).

Figure fig_3_1

(c)(i)[2]

The shapes of the five 3d orbitals are shown in Fig. 3.2.

Figure fig_3_2

Draw labelled diagrams to show the energy levels of these five 3d orbitals, A - E, in:
• \(Fe^{3+}\) as an isolated gaseous species
• \(Fe^{3+}\) in the octahedral complex ion \([Fe(H_2O)_6]^{3+}\) present in \(Fe^{3+}(aq)\).

(c)(ii)[3]

State and explain how the energy levels of the 3d orbitals in \(Fe^{3+}(g)\) compares with that of \(Fe^{3+}(aq)\).

(d)(i)[1]

When colourless \(NaSCN(aq)\) is added to \([Fe(H_2O)_6]^{3+}\), one water ligand only is replaced by one thiocyanate anion, \(SCN^-\), to produce a coloured complex ion.

Predict the formula of the complex ion made on addition of NaSCN.

(d)(ii)[3]

Explain why complexes containing \(Fe^{3+}\) are coloured.

Video Solution

Overview

Question 3 focuses on iron as a transition element. It starts with the definition of a transition element and the electron arrangement of the Fe³⁺ ion. The core of the question is the five 3d orbitals: you draw their energy levels for an isolated gaseous Fe³⁺ ion and for Fe³⁺ in the octahedral hexaaqua complex, then explain how the two compare. This is the d-orbital splitting that underlies transition-element chemistry. The question ends with a ligand-exchange reaction, predicting the formula of the complex formed when thiocyanate replaces one water ligand, and an explanation of why iron(III) complexes are coloured in terms of the d-d transition.

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