Double exchange reactions occur between two composite reactants giving rise to two composite products, see the equation that represents:
AB+CD → AD + BC
For this reaction to take place it is necessary that one of the products (AD or BC), when compared to the reagents, is less ionized, more volatile or insoluble.
1.Less ionized product: it is the least dissociated product, that is, the weakest. Neutralization is an example of a reaction that gives rise to this type of product:
HCl (aq) + NaOH(aq) → NaCl (aq) + H2the(l)
The reaction between hydrochloric acid (HCl) and sodium hydroxide base (NaOH) is considered a double exchange because the product H2O is less ionized (+ weak) than the reactants (HCl and NaOH).
2.most volatile product: the reaction is characterized by the formation of a gas. An example of this type of reaction is the one that occurs between the carbonate salt (CO2-3) and an acid. Process equation:
2h+(here) + CO3-2(here) → CO2(g) + H2O(1)
One of the products formed is carbon dioxide CO
2(g), and as this product is more volatile than the reactants, the reaction is classified as a double exchange.3.Insoluble Product: this product can be obtained from the reaction between lead nitrate [Pb (NO3)2] and sodium iodide (NaI).
[Pb (NO3)2] (here) + 2 NaI(here) → PbI2(s)
Note that the reagents [Pb (NO3)2] and NaI are aqueous solutions that give rise to a solid PbI product2, it is precisely this factor - the formation of a precipitate at the end of the process - that characterizes the double exchange reaction.
Note: This reaction is the easiest to be visualized due to the presence of solid precipitate that settles at the bottom of the container.
By Líria Alves
Graduated in Chemistry
Source: Brazil School - https://brasilescola.uol.com.br/quimica/reacoes-dupla-troca.htm
