Iron is a transition metal in Group 8 of the Periodic Table. An aqueous solution of $$$\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}$$$ contains the complex $$$\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$$$. When solutions of $$$\mathrm{KSCN}(\mathrm{aq})$$$ and $$$\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}(\mathrm{aq})$$$ are mixed, a colour change is observed. The red complex $$$\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{SCN}\right]^{2+}$$$ forms. (i) Define complex. ....................................................................................................................................... . ................................................................................................................................. (ii) State the coordination number of Fe in $$$\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$$$. ................................................................................................................................. (iii) The $$$\mathrm{H}-\mathrm{O}-\mathrm{H}$$$ bond angle in water is $$$104.5^{\circ}$$$. Suggest the $$$\mathrm{H}-\mathrm{O}-\mathrm{H}$$$ bond angle in $$$\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$$$. Explain your answer. ....................................................................................................................................... . ....................................................................................................................................... . ................................................................................................................................. (iv) Explain why iron complexes are coloured. ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ................................................................................................................................. (v) Aqueous solutions of complexes $$$\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$$$ and $$$\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{SCN}\right]^{2+}$$$ are different colours. Explain why these complexes are different colours. ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . .................................................................................................................................

Chemistry

IGCSE&ALevel

CAIE

Exam No：9701_m24_qp_42 Year：2024 Question No：3(b)

Answer:

Knowledge points:

28.2.1 describe and explain the reactions of transition elements with ligands to form complexes, including the complexes of copper(II) and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions

28.2.10 perform calculations involving other redox systems given suitable data

28.2.2 define the term ligand as a species that contains a lone pair of electrons that forms a dative covalent bond to a central metal atom / ion

28.2.3.1 monodentate ligand including as examples

28.2.3.2 bidentate ligand including as examples 1,2-diaminoethane, and the ethanedioate ion

28.2.3.3 polydentate ligand including as an example

28.2.4 define the term complex as a molecule or ion formed by a central metal atom / ion surrounded by one or more ligands

28.2.5 describe the geometry (shape and bond angles) of transition element complexes which are linear, square planar, tetrahedral or octahedral

28.2.6.1 state what is meant by coordination number

28.2.6.2 predict the formula and charge of a complex ion, given the metal ion, its charge or oxidation state, the ligand and its coordination number or geometry

28.2.7 explain qualitatively that ligand exchange can occur, including the complexes of copper(II) ions and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions

28.2.8 predict, using values, the feasibility of redox reactions involving transition elements and their ions

28.2.9.1 acid solution given suitable data

28.2.9.2 acid solution given suitable data

28.2.9.3 given suitable data

28.3.1 define and use the terms degenerate and non-degenerate d orbitals

28.3.2.1 octahedral complexes, two higher and three lower d orbitals

28.3.2.2 tetrahedral complexes, three higher and two lower d orbitals

28.3.3 explain why transition elements form coloured compounds in terms of the frequency of light absorbed as an electron is promoted between two non-degenerate d orbitals

28.3.4 describe, in qualitative terms, the effects of different ligands on Δ E, frequency of light absorbed, and hence the complementary colour that is observed

28.3.5 use the complexes of copper(II) ions and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions as examples of ligand exchange affecting the colour observed

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