AQA A-Level Chemistry: Inorganic Chemistry

The transition metals occupy the central block of the periodic table and share a set of characteristic properties not seen in the s-block metals: they form ions with variable oxidation states, many of their aqueous ions and compounds are coloured, and they act as catalysts.

Describe and explain these three characteristic properties of the transition metals, relating each one to the partially filled d sub-shell. In your answer you should explain why many transition-metal ions are coloured (you may refer to a named example), and you should name one transition metal or transition-metal compound used as a catalyst in a named industrial process.

(6 marks)

A technician used a colorimeter to find the concentration of copper(II) ions in a sample of pond water suspected of contamination. Copper(II) sulfate solution is pale blue because of the [Cu(H₂O)₆]²⁺ ion.

A set of standard copper(II) sulfate solutions of known concentration was prepared and the absorbance of each was measured in a 1.00 cm cuvette, giving the calibration data below. The colorimeter was first zeroed using a blank of pure water.

Concentration of Cu²⁺ / mol dm⁻³	Absorbance
0.000	0.00
0.020	0.16
0.040	0.32
0.060	0.48
0.080	0.64
0.100	0.80

(a) The technician chose a red filter for the colorimeter. Explain why a red filter is the correct choice for this pale-blue solution. (2 marks)

(b) The concentrated pond-water sample gave an absorbance of 0.56. The absorbance is directly proportional to the concentration of Cu²⁺ ions. Use the calibration data to determine the concentration of Cu²⁺ ions in the sample. Show your working. (2 marks)

(c) The sample was concentrated by evaporating it to one-tenth of its original volume before the measurement in part (b). Calculate the concentration of Cu²⁺ ions in the original pond water. (2 marks)

A student was given four unlabelled solutions, each containing the aqueous ions of a single metal: one of Fe²⁺, one of Fe³⁺, one of Cu²⁺ and one of Al³⁺ (present as their hexaaqua complexes). To identify them, the student added sodium hydroxide solution dropwise and then in excess, and separately added ammonia solution dropwise and then in excess, to fresh portions of each. The observations are recorded below.

Solution	Add NaOH(aq) dropwise	NaOH(aq) in excess	Add NH₃(aq) dropwise	NH₃(aq) in excess
W	Green precipitate	No change	Green precipitate	No change
X	Pale blue precipitate	No change	Pale blue precipitate	Dissolves → deep blue solution
Y	White precipitate	Dissolves → colourless solution	White precipitate	No change
Z	Orange-brown precipitate	No change	Orange-brown precipitate	No change

(a) Identify the metal ion present in each of solutions W, X, Y and Z. (4 marks)

(b) Write the ionic equation for the formation of the precipitate when NaOH(aq) is first added to solution Y. (1 mark)

When concentrated hydrochloric acid is added drop by drop to a pale-blue aqueous solution of copper(II) sulfate, the chloride ions act as ligands and progressively replace the water ligands around the copper(II) ion. A new, differently coloured copper complex is formed.

A student notes that the chloride ion (Cl⁻) is a much larger ligand than the water molecule.

(a) State the coordination number and shape of the copper complex present before any hydrochloric acid is added, and the coordination number and shape of the new complex formed in concentrated hydrochloric acid. (2 marks)

(b) State the colour change the student would observe and explain, in terms of the size of the ligand, why the coordination number changes. (2 marks)

(c) Write a balanced equation, with state symbols, for the ligand-substitution reaction. (1 mark)

Across Period 3, the oxides change from giant ionic (metal) oxides on the left to molecular covalent (non-metal) oxides on the right. A student adds separate samples of two Period 3 oxides to water:

• sample A is sodium oxide, Na₂O
• sample B is phosphorus(V) oxide, P₄O₁₀

Universal indicator is then added to each resulting mixture.

(a) For each oxide, predict the approximate pH of the solution formed and write a balanced equation for its reaction with water. (3 marks)

(b) Explain, in terms of bonding, why the two oxides give such different pH values. (1 mark)

The hexaaqua ion [Cu(H₂O)₆]²⁺ is an example of a complex ion in which water molecules are bonded to a central copper(II) ion.

Define each of the following terms.

(a) A ligand. (1 mark)

(b) A complex ion. (1 mark)

(c) State what type of bond forms between a ligand and the central metal ion, and explain what is special about how this bond is formed. (1 mark)