Does a double replacement reaction involve an exchange of bonds between compounds?

1 Answer
Nov 15, 2015

No, it doesn't.


There are two main types of double replacement reactions: a) formation of insoluble ionic substances and b) formation of a molecular substance from the interaction of positive and negative ions.

In case a) the positive part of reacting substance 1 is capable of forming an insoluble substance by sticking to the negative part of reacting substance 2.
Neither ions in reactant 1 nor reactant 2 are changed to yield new species.

For example:

#BaCl_2(aq) + Na_2SO_4(aq) -> BaSO_4(s)↓ + 2NaCl(aq)#;

there, no ion underwent any change, and neither bonds have got any change or exchanged themselves (they were ionic in the reacting substances 1 & 2, barium chloride and sodium sulphate, and they are ionic also in the product substances, 3 & 4 barium sulfate and sodium chloride.

The only exchange interested the competition between sodium and barium ions to form a stabler and less soluble ionic lattice with sulfate ions.
Being sodium and chloride ions, "spectator ions", this common reaction entailed not really a double exchange, but a single exchange.

In cases b) there is a change in the structure of ionic species caused by the interaction and a consequent partial disappearing of ionic species from the solution to form molecular species as #CO_2# or #H_2O# (in neutralization reactions). The formation of such molecules drives the spontaneity of the reaction towards the products.

As an example of reaction type b) let's take a neutralization reaction:

#HCl(aq) + NaOH(aq) -> NaCl(aq) + H_2O(l)#

Here the positive part of #HCl#, i.e. #H^+"(aq)# ions, combined to form water molecules with negative part of #NaOH#, #OH^-"(aq)# ions, whereas #Na^+(aq)# and #Cl^-"(aq)# remained substantially unchanged (spectator ions).

We can conclude that in this b-type reaction there weren't any exchange, but just a matching of a positive plus negative ions which combined to form a molecule. One of two ionic bonds in the reactants became a covalent bond in one of the two products (water).

Hope this helps to unveil the true nature of so called "double exchange" reactions.