IB Chemistry home > Syllabus 2016 > Acids and Bases > Effects of charge density

Syllabus ref: 18.1

The effect of the ionic size on hydrolysis is discussed.

Charge density

This means the charge to size ratio of the ion.

charge density = ionic charge/ionic size

When the ion has a charge of 3+ or when it is very small this charge to size ratio is enough to polarise the water molecules surrounding the ion in solution. This results in a weakening of the O-H bonds within the water molecules allowing hydrogen ions to be released into the solution. Hence the solutions are acidic.

This effect is typified in aluminium salts (the aluminium ion has a charge of 3+) which are very acidic in solution (see below).


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Salts involving ions with a high charge density

Ionic compounds dissociate 100% into ions in solution. These ions become solvated by the water molecules (the water molecules bond to the ions - this is one of the driving forces behind dissolution). The polar water molecules use the lone pairs on the oxygen of the water to coordinate to the positive metal ion. The ions are then enclosed by a 'cage' of water molecules, usually in an octahedral arrangement.

Octahedral arrangement of water molecules around a positive ion (in this case a 3+ ion).

The water molecules are strongly polarised by the high charge density of the 3+ metal ion and have their oxygen - hydrogen bonds weakened. This allows hydrogen ions to escape, making the solution acidic.

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

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The aluminium hexaaqua ion

Aluminium ions are surrounded by six water molecules in an octahedral arrangement. This is called the aluminium hexaaqua ion. The high charge density of the aluminium ion polarises the water molecules and hydrogen ions are released into solution. The solution is so acidic that it releases carbon dioxide from sodium carbonate (this reaction is used in some fire extinguishers to produce foam in conjunction with detergent)

[Al(H2O)6]3+ [Al(OH)(H2O)5]2+ + H+

[Al(OH)(H2O)5]2+ [Al(OH)2(H2O)4]+ + H+


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Transition metal ions

Transition metals have variable oxidation states and the ions that are formed with high charges (high oxidation state) also produce acidic solutions. A good example of this is the iron(III) ion. Salts such as iron(III) sulfate are acidic in solution by hydrolysis.

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

The high charge density of the Fe3+ ion polarises the water molecules, making it easier for the O-H bonds to break. This produces a complex ion with a lower charge density and greater stability.


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