What type of bond joins a water ligand to the metal ion?

A coordinate (dative covalent) bond, with both electrons from the oxygen lone pair.

Write the hydrolysis of [Fe(H₂O)₆]³⁺.

[Fe(H₂O)₆]³⁺ + H₂O ⇌ [Fe(H₂O)₅(OH)]²⁺ + H₃O⁺

Write the hydrolysis of [Cu(H₂O)₆]²⁺.

[Cu(H₂O)₆]²⁺ + H₂O ⇌ [Cu(H₂O)₅(OH)]⁺ + H₃O⁺

What role does the aqua ion play when it makes a solution acidic?

It is a Brønsted–Lowry acid (proton donor).

What is the charge/size ratio?

The charge on the ion divided by its radius.

Why is [M(H₂O)₆]³⁺ more acidic than [M(H₂O)₆]²⁺?

Higher charge/size ratio, so it polarises the O–H bonds in the ligands more and H⁺ is released more readily.

What happens to the charge of the complex when a ligand loses H⁺?

It falls by one, for example from 3+ to 2+.

How do you find [H⁺] from a pH?

[H⁺] = 10⁻ᵖᴴ mol dm⁻³.

Metal-aqua ions and their acidityAQA A-Level Chemistry: Flashcards

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What is the formula of the copper(II) aqua ion?

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What is the formula of the copper(II) aqua ion?: [Cu(H₂O)₆]²⁺
Which metals form [M(H₂O)₆]²⁺ ions in the specification?: Iron (Fe) and copper (Cu).
Which metals form [M(H₂O)₆]³⁺ ions in the specification?: Aluminium (Al) and iron (Fe).
What type of bond joins a water ligand to the metal ion?: A coordinate (dative covalent) bond, with both electrons from the oxygen lone pair.
Write the hydrolysis of [Fe(H₂O)₆]³⁺.: [Fe(H₂O)₆]³⁺ + H₂O ⇌ [Fe(H₂O)₅(OH)]²⁺ + H₃O⁺
Write the hydrolysis of [Cu(H₂O)₆]²⁺.: [Cu(H₂O)₆]²⁺ + H₂O ⇌ [Cu(H₂O)₅(OH)]⁺ + H₃O⁺
What role does the aqua ion play when it makes a solution acidic?: It is a Brønsted–Lowry acid (proton donor).
What is the charge/size ratio?: The charge on the ion divided by its radius.
Which is more acidic, [M(H₂O)₆]³⁺ or [M(H₂O)₆]²⁺?: [M(H₂O)₆]³⁺.
Why is [M(H₂O)₆]³⁺ more acidic than [M(H₂O)₆]²⁺?: Higher charge/size ratio, so it polarises the O–H bonds in the ligands more and H⁺ is released more readily.
What happens to the charge of the complex when a ligand loses H⁺?: It falls by one, for example from 3+ to 2+.
How do you find [H⁺] from a pH?: [H⁺] = 10⁻ᵖᴴ mol dm⁻³.
Calculate the charge/size ratio of Al³⁺ (radius 0.053 nm).: 3 ÷ 0.053 = 57 e nm⁻¹

Exam questions on Metal-aqua ions and their acidity

A student measures the pH of two aqueous solutions, each of concentration 0.1 mol dm⁻³. Iron(II) nitrate solution has a pH of 3.7 and iron(III) nitrate solution has a pH of 1.9.
Calculate how many times greater the concentration of H⁺ ions is in the iron(III) nitrate solution than in the iron(II) nitrate solution.2 marks
Aluminium chloride and copper(II) sulfate are each dissolved in water to make solutions of the same concentration. The aluminium chloride solution has a lower pH than the copper(II) sulfate solution. The ionic radius of Al³⁺ is 0.053 nm and that of Cu²⁺ is 0.073 nm.
Use the data to explain why the aluminium chloride solution has a lower pH than the copper(II) sulfate solution.2 marks
Crevice corrosion occurs where stagnant water is trapped against steel. Iron dissolves to form iron(II) ions, which are oxidised by dissolved oxygen to iron(III) ions. Engineers find that the liquid trapped in the crevice becomes much more acidic than the surrounding water.
Write an equation to show why a solution containing [Fe(H₂O)₆]³⁺ is acidic, and state the role of the aqua ion in this reaction.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).