Structure and Properties
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Structure and Properties

The bonding and structure of carboxylate salts is somewhat involved. They contain atoms bonded to one another by a variety of bond types: nonpolar covalent bonds, polar covalent bonds and ionic bonds. The distinguishing structural feature is the carboxylate ion. In their pure form carboxylate salts are ionic compounds.

Structure of sodium acetate. [64str06.JPG]

The ones with small carbon chains such as sodium acetate are crystalline ionic network materials. You may recall working with crystals of sodium acetate last term in the supersaturation experiment.

However, if the carbon chain is very long, it gets in the way and an orderly network of ions is not possible. The material is still solid but not crystalline. Soap is an example of a long chained carboxylate salt. Soaps are generally the sodium or potassium salts of fatty acids.

Solubilty in Water

Two factors determine the solubility of carboxylate salts in water.

One is the nature of the carboxylate ion -- its length, shape, amount of branching and so on. The larger and less polar this group is, the less soluble it is in water. Our traditional five-carbon-atom cutoff point for solubility in water is not appropriate for carboxylate ions. The reason is that the carboxylate functional group is not merely polar, it is ionic. This allows for stronger ion-dipole bonds to be formed with the water molecules and pull longer carbon chains into solutions. Soaps, for example, generally have from twelve to eighteen carbon atoms in the carbon chain.

The other factor is the positive ion. Ammonium, potassium and sodium salts are generally soluble in water. Magnesium and calcium salts are generally less soluble. Many others are generally insoluble. For example, a soap containing sodium ion will dissolve in water.
However, if the water contains magnesium, calcium or iron ions (i.e. what we call hard water), these ions will take the place of the sodium ions, combine with the carboxylate ion to make an insoluble compound and precipitate out of solution to form what is commonly called soap scum.

Equations showing the solubility of soap with sodium ions and the precipitation of soap with calcium ion. [64rxn08.JPG]

 

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