Metathesis Reactions

 In many aqueous reactions it seems that the reaction involves the ionic compounds swapping their ionic partners. For example, in the reaction involving the ionic compounds silver nitrate and potassium chloride we have:

The silver cation exchanges its nitrate anion partner for the chloride anion. Likewise, the potassium cation exchanges chloride anion for the nitrate anion:

This swapping of ions in aqueous reactions can be symbolically represented as follows:

Note: metathesis is not pronounced "meta-thesis", but rather "meh-TATH-eh-sis" (apparently the Greeks prefer to pronounce it that way)

There is something subtle in the above example that is important to note.

• Take a look at the state of the silver chloride ion pair.
• The AgCl(s) is a solid, and therefore the ions are not dispersed in solution. In fact, they precipitate out of solution.
• The precipitation effectively removes the AgCl ions from the solution, and this is the driving force for the observed metathesis reaction

What are the ways in which ions can be removed from solution and thus drive a metathesis reaction?

• Acid/base neutralization reactions that produce water from H⁺ and OH^- ions are an example of this.
• Being a non-electrolyte (or weak electrolyte) the formation of the molecular compound from the constituent ions is essentially an irreversible process

Metathesis reactions that result in an insoluble precipitate are called precipitation reactions

• Solubility refers to the amount of a substance that can be dissolved in a given quantity of water
• The solubility of an ionic compound determines whether it will precipitate or not
• Any substance with a solubility less than 0.01 moles/L will be considered as essentially insoluble
• The solubility of NaCl is around 10 moles/L (it is highly soluble and won't drive a metathesis reaction). The solubility of PbI₂ is around 0.0012 moles/L (it is essentially insoluble, and can drive a metathesis reaction).

The reaction of KI and Pb(NO₃)₂:

• The PbI₂ is an insoluble ionic compound that will precipitate and drive the metathesis reaction:

Can we predict whether an ionic compound will be soluble or not?

• If an ionic compound is insoluble it means that neighboring ions have an attraction for each other that is greater than the attraction of water for the ions (i.e. water molecules cannot separate, surround and disperse the ions in the ionic solid)
• Unfortunately, there are no clear rules for solubility based on physical properties of ions. However, some general behaviors of certain ions are observed:

Ions can chemically combine to form a nonelectrolyte (or weak electrolyte)

• Even though the nonelectrolyte may be soluble in aqueous solution, its formation is essentially irreversible. Thus, ions are effectively removed from solution by this irreversible process
• The neutralization reaction of HCl and NaOH is an example of this type of reaction:

• The net ionic equation for this neutralization reaction is:

• The formation of the covalent compound (H₂O) from the proton and hydroxide ions is essentially irreversible and drives the metathesis reaction (even though we would consider H₂O to be "highly soluble" in H₂O)

If a possible metathesis reaction involves the formation of a gas (and the gas is not particularly soluble in H₂O) the loss of the gas can drive the metathesis reaction (i.e. the ions react to form a gas, and the gas is lost - therefore, it is an irreversible process)

• Typical gasses that can form from ionic compounds include H₂S (hydrogen sulfide - smells like rotten eggs) and CO₂ (carbon dioxide)

Formation of carbon dioxide from carbonic acid

• The bicarbonate ion (HCO₃^-) can combine with a proton to produce carbonic acid (H₂CO₃):

• Carbonic acid in water is unstable and decomposes to form water and carbon dioxide gas:

• The carbon dioxide gas is lost, and thus the formation of carbonic acid is irreversible and drives the metathesis reaction