Metal-aqua ions

​In a nutshell

A metal-aqua ion consists of a central metal ion with six water ligands co-ordinately bonded to it. Aluminium hydroxide is amphoteric. Test tube reactions can be used to identify metal-aqua ions.

Equations

The general equations for the formation of metal precipitates are shown below.

​$$[M(H_2O)_6]^{3+}(aq) \space + \space H_2O(l) \space \rightleftharpoons \space [M(OH)(H_2O)_5]^{2+}(aq) +H_3O^+(aq)$$

$$[M(OH)(H_2O)_5]^{2+}(aq) \space + \space H_2O(l) \space \rightleftharpoons \space [M(OH)_2(H_2O)_4]^{+}(aq) +H_3O^+(aq)$$

​$$[M(OH)_2(H_2O)_4]^{+}(aq) \space + \space H_2O(l) \space \rightleftharpoons \space M(OH)_3(H_2O)_3(s) +H_3O^+(aq)$$

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Structure

When transition metal compounds are added to water, water ligands co-ordinately bond with the metal ions to form metal-aqua complex ions. A lone pair on the oxygen in water is donated to the metal ion to form the co-ordinate bond.

Usually six water ligands co-ordinate with the central metal ion. The general formula for metal-aqua ions is $$[M(H_2O)_6]^{n+}$$.

Example

A $$[Fe(H_2O)_6]^{3+}$$ ion.

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Acidity

An acidity or a hydrolysis reaction takes place in a solution containing metal-aqua $$2+$$ ions and water. An acidic solution forms because the metal-aqua $$2+$$ ions release $$H^+$$ ions. The solution formed is only weakly acidic as there is partial dissociation of the $$H^+$$ions, only.

Example

​$$[Cu(H_2O)_6]^{2+}(aq) \space + \space H_2O(l) \space \rightarrow \space [Cu(OH)(H_2O)_5]^{+}(aq) +H_3O^+(aq)$$​​

Metal-aqua $$3+$$ ions dissociate more readily than metal-aqua $$2+$$ ions, so their solutions are more acidic. This is because metal $$3+$$ ions have a higher charge density than metal $$2+$$ ions, therefore they are more polarising and are able to attract the lone pair on the oxygen atom in water more strongly. This leads to the weakening of the $$O-H$$ bond and the $$H^+$$ dissociates more readily.

Example

​$$[Cu(H_2O)_6]^{3+}(aq) \space + \space H_2O(l) \space \rightarrow \space [Cu(OH)(H_2O)_5]^{2+}(aq) +H_3O^+(aq)$$​​

Formation of metal precipitates 

Insoluble metal hydroxide precipitates form when $$OH ^{-}$$ ions are added to a solution containing metal-aqua ions. The table below explains how the precipitate forms.

Procedure

Metal-aqua $$2+$$ ions undergo the same process. Although, only two water ligands are deprotonated to form the insoluble, uncharged metal hydroxide precipitate.

​$$[M(H_2O)_6]^{2+}(aq) \space + \space H_2O(l) \space \rightleftharpoons \space [M(OH)(H_2O)_5]^{+}(aq) +H_3O^+(aq)$$

​$$[M(OH)(H_2O)_5]^{+}(aq) \space + \space H_2O(l) \space \rightleftharpoons \space M(OH)_2(H_2O)_4(s) +H_3O^+(aq)$$​​

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Amphoteric metal hydroxides

Metal hydroxide precipitates dissolve in acids. When metal hydroxides are added to an acid, they accept $$H^+$$​ ions and act as a Br$$ø$$nsted-Lowry base. The hydrolysis reaction is reversed and the metal-aqua ions reform.

​$$[M(OH)_3(H_2O)_3](s) \space + 3H_3O^+(aq)\space \rightarrow \space[M(H_2O)_6]^{3+}(aq) \space + \space 3H_2O(l)$$​​

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Some metal hydroxides are amphoteric, which means that they can act as acids and bases. Amphoteric metal hydroxides will dissolve in an excess amount of an acid and base.

Example

Aluminium hydroxide is amphoteric. Aluminium hydroxide acts as a Br$$ø$$nsted-Lowry acid in the presence of a base and a soluble compound forms. Aluminium hydroxide donates $$H^+$$ ions to the $$OH^{-}$$ of the base. ​

$$[Al(OH)_3(H_2O)_3](s) \space + \space OH^{-} (aq) \rightarrow \space[Al(OH)_4]^{-}(aq) \space + \space 3H_2O(l)$$​​

In the presence of an acid, aluminium hydroxide acts as a Br$$ø$$nsted-Lowry base and accepts $$H^+$$ ions from the $$H_3O^+$$ ions present in the solution.

$$[Al(OH)_3(H_2O)_3](s) \space + 3H_3O^+(aq) \space \rightarrow \space[Al(H_2O)_6]^{3+}(aq) \space + \space 3H_2O(l)$$​​

Ammonia to form hydroxide ions

A solution of ammonia can be used to introduce hydroxide ions to a solution, instead of using a strong alkali. When ammonia dissolves in water ammonium ions and hydroxide ions form. Only a small amount of ammonia is required.

$$NH_3(aq) \space + \space H_2O(l) \space\rightleftharpoons \space NH_4^+(aq) \space + \space OH^{-}(aq)$$​​

Sometimes when an excess amount of ammonia is added to a metal hydroxide precipitate, it dissolves. This is because a ligand-exchange reaction occurs. The ammonia ligands displace the water and hydroxide ligands and a soluble, charged complex forms.

Example

​$$[Co(OH)_2(H_2O)_4](s) \space + \space 4NH_3(aq) \space \rightleftharpoons \space [Co(NH_3)_4(H_2O)_2]^{2+}(aq) \space + \space OH^{-}(aq) \space + \space H_3O^+(aq)$$​​

Metal ions and sodium carbonate 

When metal $$2+$$ ions and sodium carbonate react, an insoluble metal carbonate forms.

​$$[M(H_2O)_6]^{2+}(aq) \space + \space CO_3^{2-}(aq) \space \rightleftharpoons \space MCO_3(s) +6H_2O(l)$$​​

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When sodium carbonate is added to a solution containing metal $$3+$$3+ ions the carbonate ions do not displace the water ligands. In a solution containing metal $$3+$$ ions there is a higher concentration of $$H_3O^+$$ as metal $$3+$$ ions are stronger acids. 

The carbonate ions react with the $$H_3O^+$$ ions and removes them from the solution. This shifts the position of the hydrolysis reaction to the right and a $$M(OH)_3(H_2O)_3$$ precipitate forms instead of $$M_2(CO_3)_3$$.

​$$[M(OH)_2(H_2O)_4]^{+}(aq) \space + \space H_2O(l) \space \rightleftharpoons \space M(OH)_3(H_2O)_3(s) +H_3O^+(aq)$$

​$$2H_3O^+(aq) \space + \space CO_3 ^{2-}(aq) \space \rightleftharpoons \space CO_2(g) +3H_2O(l)$$​​

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Identifying metal ions 

Simple test tube reactions can be used to identify whether copper$$(II)$$​ ,iron$$(II)$$​, iron$$(III)$$​ and aluminium$$(III)$$ metal-aqua ions are present in a solution.

​1. ​ Dropping pipette used to add reagents. ​​2. Test tubes containing unknown metal ion solution.​​ ​3. Test tube containing sodium hydroxide solution.​​​ ​4. Test tube containing sodium carbonate solution. 5. Test tube containing ammonia solution.

Procedure

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Reaction with sodium hydroxide 

When sodium hydroxide is added to solutions of all four metal-aqua ions, a precipitate forms. However, upon addition of excess sodium hydroxide, only the aluminium hydroxide precipitate will dissolve, because aluminium hydroxide is amphoteric.

Reaction with sodium carbonate

​When sodium carbonate is added to solutions of all four metal-aqua ions, a precipitate forms. Carbon dioxide will also form in the test tubes containing $$Fe^{3+}$$ and $$Al^{3+}$$ ions and bubbles will be seen. This test can be used to identify between $$Fe^2+$$ and $$Fe^{3+}$$ ions.

Reaction with ammonia 

​​When sodium carbonate is added to solutions of all four metal-aqua ions, a precipitate forms.  However, upon addition of excess ammonia only the copper hydroxide precipitate dissolves as a ligand exchange reaction takes place.

Colours

The table below shows the formulae and colours of all the complex ion solutions and precipitates. 

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