OCR A-Level Chemistry: Transition Elements & Aromatic Chemistry

Aqueous solutions of transition-element complexes are usually coloured, and the colour of a particular metal ion often changes when one ligand is replaced by another. For example, when an excess of aqueous ammonia is added to a pale-blue solution containing $[Cu(H_2O)_6]^{2+}$[Cu(H2​O)6​]2+, the solution turns a deep royal-blue as $[Cu(NH_3)_4(H_2O)_2]^{2+}$[Cu(NH3​)4​(H2​O)2​]2+ forms.

Describe and explain the origin of colour in transition-element complexes, and explain why changing the ligand can change the colour. Your answer should refer to the d-orbitals, the electronic transition involved, the link between the energy absorbed and the colour observed, and why a different ligand alters the colour.

(6 marks)

An impure sample of an iron(II) salt is to be analysed by redox titration against acidified potassium manganate(VII). The impurities are inert (they do not react with manganate(VII)).

A student dissolves 1.85 g of the impure salt in dilute sulfuric acid and makes the solution up to exactly 250.0 cm³ in a volumetric flask. 25.0 cm³ portions of this solution are titrated against 0.0200 mol dm⁻³ potassium manganate(VII). The mean titre is 23.40 cm³.

Quantity	Value
Mass of impure salt	1.85 g
Total volume of solution	250.0 cm³
Volume of each portion titrated	25.0 cm³
Concentration of KMnO₄	0.0200 mol dm⁻³
Mean titre	23.40 cm³
$A_r$Ar​(Fe)	55.8

The ionic equation for the reaction is:

$MnO_4^- + 8H^+ + 5Fe^{2+} \rightarrow Mn^{2+} + 4H_2O + 5Fe^{3+}$MnO4−​+8H++5Fe2+→Mn2++4H2​O+5Fe3+

Calculate the percentage by mass of iron in the impure sample. Give your answer to three significant figures.

(6 marks)

When an alkene is hydrogenated, hydrogen adds across the double bond and energy is released. The table gives the enthalpy change of hydrogenation for cyclohexene (one C=C) and the measured value for benzene.

Compound	Number of C=C in the Kekulé structure	Enthalpy change of hydrogenation / kJ mol⁻¹
Cyclohexene	1	−120
Benzene (Kekulé prediction)	3	(to be calculated)
Benzene (measured)	—	−208

(a) Use the cyclohexene value to predict the enthalpy change of hydrogenation that a Kekulé structure of benzene (three separate C=C double bonds) would be expected to have. (1 mark)

(b) Using your answer to (a) and the measured value, deduce what the difference between the two values tells you about the structure and stability of benzene. (2 marks)

(c) State one further piece of physical evidence, other than enthalpy of hydrogenation, that supports the delocalised model of benzene rather than the Kekulé model, and explain how it does so. (2 marks)

A fragrance manufacturer prepares an aromatic ketone by a Friedel-Crafts acylation, reacting benzene with propanoyl chloride ($CH_3CH_2COCl$CH3​CH2​COCl) in the presence of an anhydrous aluminium chloride catalyst. The organic product is 1-phenylpropan-1-one, $C_6H_5COCH_2CH_3$C6​H5​COCH2​CH3​.

(a) Write an equation to show how the electrophile is generated from propanoyl chloride and the aluminium chloride catalyst. (1 mark)

(b) Describe, in words and equations only (no diagrams), the mechanism by which this electrophile reacts with benzene to form 1-phenylpropan-1-one. Your answer should cover the attack on the ring, the intermediate formed, and how the aromatic ring and the catalyst are restored. (3 marks)

(c) Explain why aluminium chloride is described as a catalyst in this reaction even though it appears to be used up when it generates the electrophile. (1 mark)

A technician has two unlabelled solutions, one containing $[Cu(H_2O)_6]^{2+}$[Cu(H2​O)6​]2+ ions and the other containing $[Fe(H_2O)_6]^{2+}$[Fe(H2​O)6​]2+ ions. To tell them apart, a few drops of aqueous sodium hydroxide are added to a sample of each.

(a) State the colour of the precipitate formed when sodium hydroxide is added to each solution, and write an ionic equation (using the aqua-ion formulae) for the precipitation reaction of the copper(II) ion. (3 marks)

(b) When an excess of aqueous ammonia (rather than sodium hydroxide) is added to the copper(II) solution, the precipitate that first forms then dissolves to give a deep-blue solution. State what has happened in this second stage and name the type of reaction. (1 mark)

This question is about transition-metal complexes and aromatic chemistry.

(a) State what is meant by the term ligand. (1 mark)

(b) Benzene can be nitrated using a mixture of two concentrated acids that together generate the nitronium ion ($NO_2^+$NO2+​), the electrophile. Name the two reagents used and write an equation showing how the $NO_2^+$NO2+​ ion is formed from them. (1 mark)

(c) A small piece of sodium is added to liquid phenol (warmed gently so the phenol is molten). State one observation, and name the type of reaction taking place. (1 mark)