Lewis Acids and Bases

Edited by Jamie (ScienceAid Editor), Taylor (ScienceAid Editor), vcdanht

Definitions of Lewis's Acids and Bases

The Brønsted-Lowry definition of acids and bases uses hydrogen. Gilbert Lewis proposed a definition based on electrons. A Lewis acid is an electron-pair acceptor and a Lewis base an electron-pair donor. Using this definition, metal ions H+ are acids, and ligands, water, ammonia and non-metal ions are bases. The Lewis base nature of oxygen and nitrogen is important in many organic mechanisms. Take a look at the reaction below.

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H2 + H+ ==>> H3O+

In this reaction, water is a base (lone pair on the O) and H+ the acid.

Metal-Aqua Ion

When a transition metal is dissolved in water (a metal salt, for example, since metal by itself has too strong bonding] to dissolve it forms a metal-aqua ion where the water acts as a ligand to form a complex. For example, if FeSO,4 (iron (II) sulphate) is dissolved in water the following forms in a green solution.

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The transition metal can be generalized to either M2+ (where M could be Fe, Co, Cu) or M3+ (where M could be Al, V, Cr or Fe; say). And they form the complexes [M(H2O)6]2+ and [M(H2O)26]3+.

The bonding between water and the metal is so strong that when it is evaporated a complex is formed in the solid form. Again using the example of iron (II) sulphate - it can form the complex FeSO4.7H2O (s).

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Acidity of Metal-Aqua Ions

When a metal-aqua ion is placed in water the following equilibria are set up. Note that since the OH- is negatively charged, the overall charge of the complex it is in drops.

[M(H2O)6]2+ + H2O ==>> [M(H2O)5OH]+ + H3O+ [M(H2O)6]3+ + H2O ==>> [M(H2O)5OH]2+ + H3O+

This is called a hydrolysis reaction since one of the H2 ligands is split and an H+ removed. This means an acidic solution is formed. The solution is weakly acidic and the M2+ very weakly acidic. The acidity of the metal-aqua complexes is affected by the change and size of the metal. Together these make the charge density. A small, higher charged ion like Al3+ has a greater polarizing power so makes more acidic solutions. It does this because the ions can hold onto the water more strongly - which weakens the O-H bond.

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Reactions of Metal-Aqua Ions

When metal-aqua ions are reacted with sodium hydroxide, a precipitate forms. This is because the OH- reacts with H3, moving the equilibrium of the: iron with water reaction; to the right. Then another equilibrium is established where [M(H2O)4(OH)2] is formed. If more NaOH is added, hydrolysis will continue. Examples of the reactions and their colours are:

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If these complexes are reacted with ammonia they are exactly the same as above, because OH- is produced in the equilibrium: NH3 + H2O ==>> NH4+ + OH-

When carbonates are added to M2+ they form metal carbonate - MCO3 - precipitates. Metal (III), however are stronger acids so react differently. A metal hydroxide precipitate is formed, and an effervescence of carbon dioxide. Because of this, metal (III) carbonates cannot be prepared so M2(CO3)3 doesn't exist.

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The hydrolytic reactions can also be reversed by using strong acids, so the metal hydroxides can dissolve in acid; also, in excess OH- a soluble anionic complex is formed. The ability to dissolve in acids and bases make metal hydroxides amphoteric. Take the examples of aluminium (III) and chromium (III) hydroxide

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Furthermore, chromate (VI) ions react with acid to form dichromate (VI) ions, this reaction is reversed when reacted with alkaline solution. The color change is from a yellow to orange solution.

2CrO42- + 2H+ ==>> Cr2O72- + H2O Cr2O72- + 2OH- ==>> 2CrO72- + H2O

Referencing this Article

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Lewis Acids and Bases. (2017). In ScienceAid. Retrieved Sep 30, 2023, from https://scienceaid.net/chemistry/inorganic/lewis.html

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Categories : Inorganic

Recent edits by: Taylor (ScienceAid Editor), Jamie (ScienceAid Editor)

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