What is a ligand substitution reaction?

A ligand substitution reaction is a chemical reaction in which one ligand in a complex ion is replaced by another ligand.

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Describe the colour changes, and give the equation, for the reaction between aqueous ammonia (NH3) and [Cu(H2O)6]2+.

A pale blue precipitate of Cu(OH)2 forms initially. When ammonia is added in excess, the precipitate redissolves to form a dark blue solution.

[Cu(H2O)6]2+(aq) + 4NH3(aq) ➔ [Cu(NH3)4(H2O)2]2+(aq) + 4H2O(l)

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Why does the exchange of H2O with NH3 not affect the coordination number around the central metal ion?

Both H2O with NH3 ligands are similar in size.

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Describe the colour change, and give the equation, for the reaction between excess concentrated hydrochloric acid and [Cu(H2O)6]2+.

The solution turns from pale blue to yellow.

[Cu(H2O)6]2+(aq) + 4Cl-(aq) ➔ [CuCl4]2−(aq) + 6H2O(l)

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State and explain how the exchange of H2O with Cl- affects the coordination number around the central metal ion.

The exchange of H2O with Cl- decreases the coordination number (typically from six to four).


This is because Cl- ligands are larger than H2O ligands so fewer Cl- ligands can fit around the central metal ion.

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Describe, and give the equation for, the reaction between excess hydroxide ions and [Cu(H2O)6]2+.

A pale blue precipitate of Cu(H2O)4(OH)2(s) forms from a pale blue solution.

[Cu(H2O)6]2+(aq) + 2OH-(aq) ➔ Cu(H2O)4(OH)2(s) + 2H2O(l)

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Describe the colour changes, and give the equation, for the reaction between aqueous concentrated ammonia (NH3) and [Fe(H2O)6]3+.

A brown precipitate of Fe(H2O)3(OH)3 forms form a yellow solution.

[Fe(H2O)6]3+(aq) + 3NH3(aq) ➔ Fe(H2O)3(OH)3(s) + 3NH4+(aq)

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What is the formula of the precipitate formed when aqueous [M(H2O)6]2+ ions react with hydroxide ions?

[M(H2O)4(OH)2


This could also be written as M(OH)2.

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What is the formula of the precipitate formed when aqueous [M(H2O)6]3+ ions react with hydroxide ions?

[M(H2O)3(OH)3


This could also be written as M(OH)3.

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Explain, with the use of an equation, why aluminum(III) hydroxide dissolves on addition of excess aqueous sodium hydroxide.

Insoluble aluminium(III) hydroxide reacts with hydroxide ions to form soluble aluminium(IV) hydroxide.

Al(H2O)3(OH)3(s) + OH-(aq) ➔ [Al(H2O)2(OH)4]-(aq) + H2O(l)

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Describe the colour changes, and give the equation, for the reaction between aqueous ammonia (NH3) and [Cr(H2O)6]3+.

A dark green precipitate of Cr(OH)3(H2O)3 forms initially. When ammonia is added in excess, the precipitate redissolves to form a purple solution.

[Cr(H2O)6]3+(aq) + 6NH3(aq) ➔ [Cr(NH3)6]3+(aq) + 6H2O(l)

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