The ions of transition elements form complexes by reacting with ligands - CIE - A-Level Chemistry - Question 2 - 2014 - Paper 1
Question 2
The ions of transition elements form complexes by reacting with ligands.
(a) (i) State what is meant by the terms:
complex,
A complex is a compound/molecule/speci... show full transcript
Worked Solution & Example Answer:The ions of transition elements form complexes by reacting with ligands - CIE - A-Level Chemistry - Question 2 - 2014 - Paper 1
Step 1
State what is meant by the terms: complex
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Answer
A complex is a compound/molecule/species/ion formed by a central metal atom/ion surrounded by/bonded to one or more ligands/groups/molecules/anions.
Step 2
State what is meant by the terms: ligand
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Answer
A ligand is a species that contains a lone pair of electrons that forms a dative bond to a metal atom/ion or a lone pair donor to metal atom/ion.
Step 3
Draw three-dimensional diagrams of their structures
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Answer
The structure of [Cu(H₂O)₆]²⁺ is octahedral, and CuCl₂²⁻ has a tetrahedral shape. These shapes should be depicted accurately in their respective boxes.
Step 4
Suggest the structures of the two isomers
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Answer
The isomers of Pt(NH₃)₂Cl₂ are: 1) cis-isomer where the two Cl ligands are adjacent, and 2) trans-isomer where the Cl ligands are opposite each other. This is similar to geometric isomerism in organic chemistry.
Step 5
Complete the electronic structures of these ions.
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Answer
Cu(II) is [Ar] 3d⁹. Cu(I) is [Ar] 3d¹⁰.
Step 6
Explain why copper(II) salts are usually coloured
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Copper(II) salts are coloured due to the splitting of d orbitals in the presence of ligands, allowing electrons to be promoted to higher energy levels, which absorb specific wavelengths of light.
Step 7
Explain why copper(I) salts are usually white or colourless
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Copper(I) salts are usually colourless because all d orbitals are filled, leading to no electronic transitions that absorb visible light.
Step 8
Calculate the ΔHf° for the reaction Cu(NO₃)₂(s) → CuO(s) + 2NO₂(g) + rac{1}{2}O₂(g)
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Answer
Using the values: ΔHf° = -168.6 + 2 × 33.2 - 157.3 + 302.9 = 212 kJ mol⁻¹.
Step 9
Calculate ΔHf° for the reaction 2CuO(s) → Cu₂O(s) + rac{1}{2}O₂(g)
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ΔHf° = 2 × (-157.3) - (-168.6) = +146.1 kJ mol⁻¹.
Step 10
Suggest temperature conditions for copper(I) oxide production
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A high temperature of oxidation should favour the production of copper(I) oxide since the reaction is endothermic, thus higher temperatures drive the equilibrium towards products.
