Metal-aqua ions and their acidityAQA A-Level Chemistry: Revision notes
Section 1
Metal-aqua ions
When a metal salt dissolves in water, the metal ion is surrounded by six water molecules. Each water ligand donates a lone pair from its oxygen atom to the metal ion in a coordinate (dative covalent) bond. The ions you need are:
- [M(H₂O)₆]²⁺ for M = Fe and Cu: [Fe(H₂O)₆]²⁺ (pale green) and [Cu(H₂O)₆]²⁺ (blue)
- [M(H₂O)₆]³⁺ for M = Al and Fe: [Al(H₂O)₆]³⁺ (colourless) and [Fe(H₂O)₆]³⁺ (pale violet, but solutions look yellow-brown)
The overall charge is the charge on the metal ion, because water is neutral.
Section 2
Why the solutions are acidic
The metal ion draws electron density away from the oxygen atoms of the water ligands. This polarises and weakens the O–H bonds, so a ligand can lose H⁺ to a water molecule in the solution. The aqua ion acts as a Brønsted–Lowry acid (proton donor):
[Fe(H₂O)₆]³⁺ + H₂O ⇌ [Fe(H₂O)₅(OH)]²⁺ + H₃O⁺
[Cu(H₂O)₆]²⁺ + H₂O ⇌ [Cu(H₂O)₅(OH)]⁺ + H₃O⁺
The H₃O⁺ ions formed make the solution acidic. The overall charge falls by one each time H⁺ is lost.
Do not write OH⁻ as a product of hydrolysis of an aqua ion. The H⁺ is lost from a ligand, giving H₃O⁺ and a hydroxo ligand in the complex.
Section 3
Charge/size ratio
The charge/size ratio is the charge on the ion divided by its ionic radius. A higher ratio means a stronger attraction for electron density and so a greater polarising power.
Worked example: Fe²⁺ has radius 0.078 nm, so the ratio is 2 ÷ 0.078 = 26 e nm⁻¹. Fe³⁺ has radius 0.065 nm, so the ratio is 3 ÷ 0.065 = 46 e nm⁻¹.
Section 4
Why M³⁺ is more acidic than M²⁺
A 3+ ion has a higher charge and a smaller radius than the 2+ ion, so its charge/size ratio is greater. It polarises the O–H bonds in its water ligands more strongly, so H⁺ is released more readily. The hydrolysis equilibrium lies further to the right, [H⁺] is higher and the pH is lower.
So [M(H₂O)₆]³⁺ is more acidic than [M(H₂O)₆]²⁺. For example, 0.1 mol dm⁻³ iron(III) solution has a pH of about 2, compared with about 4 for iron(II).
Use the full chain: higher charge/size ratio, more polarisation of O–H, easier loss of H⁺, higher [H⁺], lower pH.
Section 5
Exam skills
Typical questions ask you to write the hydrolysis equation, to compare two ions, or to calculate a ratio or [H⁺] from a pH. Remember [H⁺] = 10⁻ᵖᴴ. Do not compare Al³⁺ and Fe³⁺ with each other. The specification only requires you to explain that 3+ ions are more acidic than 2+ ions.
Charge alone is not enough for the explanation. You must mention size (charge/size ratio) or charge density.
That's the notes covered.
Carry on to the next subtopic.
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
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).